Страницы ошибок asp net core - Не ошибается лишь тот, кто ничего не делает!

Обработка ошибок

Данное руководство устарело. Актуальное руководство: Руководство по ASP.NET Core 7

Последнее обновление: 06.11.2019

Ошибки в приложении можно условно разделить на два типа: исключения, которые возникают в процессе выполнения кода (например, деление на 0), и
стандартные ошибки протокола HTTP (например, ошибка 404).

Обычные исключения могут быть полезны для разработчика в процессе создания приложения, но простые пользователи не должны будут их видеть.

UseDeveloperExceptionPage

Если мы создаем проект ASP.NET Core, например, по типу Empty (да и в других типах проектов), то в классе Startup мы можем найти в начале метода Configure() следующие строки:

if (env.IsDevelopment())
{
	app.UseDeveloperExceptionPage();
}

Если приложение находится в состоянии разработки, то с помощью middleware app.UseDeveloperExceptionPage() приложение перехватывает исключения и
выводит информацию о них разработчику.

Например, изменим класс Startup следующим образом:

public class Startup
{
	public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
	{
		if (env.IsDevelopment())
		{
			app.UseDeveloperExceptionPage();
		}
		app.Run(async (context) =>
		{
			int x = 0;
			int y = 8 / x;
			await context.Response.WriteAsync($"Result = {y}");
		});
	}
}

В middleware app.Run симулируется генерация исключения при делении ноль. И если мы запустим проект, то в браузере мы увидим
информацию об исключении:

Этой информации достаточно, чтобы определить где именно в коде произошло исключение.

Теперь посмотрим, как все это будет выглядеть для простого пользователя. Для этого изменим метод Configure:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
	env.EnvironmentName = "Production";
	if (env.IsDevelopment())
	{
		app.UseDeveloperExceptionPage();
	}
	app.Run(async (context) =>
	{
		int x = 0;
		int y = 8 / x;
		await context.Response.WriteAsync($"Result = {y}");
	});
}

Выражение env.EnvironmentName = "Production"; устанавливает режим развертывания вместо режима разработки. В этом случае выражение if (env.IsDevelopment()) будет возвращать false, и мы увидим в браузере что-то наподобие «HTTP ERROR 500»

UseExceptionHandler

Это не самая лучшая ситуация, и нередко все-таки возникает необходимость дать пользователям некоторую информацию о том, что же все-таки произошло. Либо потребуется как-то обработать данную ситуацию.
Для этих целей можно использовать еще один встроенный middleware в виде метода UseExceptionHandler(). Он перенаправляет
при возникновении исключения на некоторый адрес и позволяет обработать исключение. Например, изменим метод Configure следующим образом:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
	env.EnvironmentName = "Production";
	
	if (env.IsDevelopment())
	{
		app.UseDeveloperExceptionPage();
	}
	else
	{
		app.UseExceptionHandler("/error");
	}
	
	app.Map("/error", ap => ap.Run(async context =>
	{
		await context.Response.WriteAsync("DivideByZeroException occured!");
	}));
	
	app.Run(async (context) =>
	{
		int x = 0;
		int y = 8 / x;
		await context.Response.WriteAsync($"Result = {y}");
	});
}

Метод app.UseExceptionHandler("/error"); перенаправляет при возникновении ошибки на адрес «/error».

Для обработки пути по определенному адресу здесь использовался метод app.Map(). В итоге при возникновении исключения будет срабатывать делегат
из метода app.Map.

Следует учитывать, что оба middleware — app.UseDeveloperExceptionPage() и app.UseExceptionHandler()
следует помещать ближе к началу конвейера middleware.

Обработка ошибок HTTP

В отличие от исключений стандартный функционал проекта ASP.NET Core почти никак не обрабатывает ошибки HTTP, например, в случае если ресурс не найден.
При обращении к несуществующему ресурсу мы увидим в браузере пустую страницу, и только через консоль веб-браузера мы сможем увидеть статусный код.
Но с помощью компонента StatusCodePagesMiddleware можно добавить в проект отправку информации о статусном коде.
Для этого добавим в метод Configure() класса Startup вызов app.UseStatusCodePages():

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
	if (env.IsDevelopment())
	{
		app.UseDeveloperExceptionPage();
	}
	
	// обработка ошибок HTTP
	app.UseStatusCodePages();
	
	app.Map("/hello", ap => ap.Run(async (context) =>
	{
		await context.Response.WriteAsync($"Hello ASP.NET Core");
	}));
}

Здесь мы можем обращаться только по адресу «/hello». При обращении ко всем остальным адресам браузер отобразит базовую информацию об ошибке:

Данный метод позволяет настроить отправляемое пользователю сообщение. В частности, мы можем изменить вызов метода так:

app.UseStatusCodePages("text/plain", "Error. Status code : {0}");

В качестве первого параметра указывается MIME-тип ответа, а в качестве второго — собственно то сообщение, которое увидит пользователь. В сообщение мы можем
передать код ошибки через плейсхолдер «{0}».

Вместо метода app.UseStatusCodePages() мы также можем использовать еще пару других, которые также обрабатываю ошибки HTTP.

С помощью метода app.UseStatusCodePagesWithRedirects() можно выполнить переадресацию на определенный метод, который непосредственно обработает статусный код:

app.UseStatusCodePagesWithRedirects("/error?code={0}");

Здесь будет идти перенаправление по адресу «/error?code={0}». В качестве параметра через плейсхолдер «{0}» будет передаваться статусный код
ошибки.

Но теперь при обращении к несуществующему ресурсу клиент получит статусный код 302 / Found. То есть формально несуществующий ресурс будет существовать, просто статусный код 302
будет указывать, что ресурс перемещен на другое место — по пути «/error/404».

Подобное поведение может быть неудобно, особенно с точки зрения поисковой индексации, и в этом случае мы можем применить другой метод
app.UseStatusCodePagesWithReExecute():

app.UseStatusCodePagesWithReExecute("/error", "?code={0}");

Первый параметр метода указывает на путь перенаправления, а второй задает параметры строки запроса, которые будут передаваться при перенаправлении.
Вместо плейсхолдера {0} опять же будет передаваться статусный код ошибки. Формально мы получим тот же ответ, так как так же будет идти перенаправление на путь «/error?code=404». Но теперь браузер получит оригинальный статусный код 404.

Пример использования:

public void Configure(IApplicationBuilder app)
{
	// обработка ошибок HTTP
	app.UseStatusCodePagesWithReExecute("/error", "?code={0}");

	app.Map("/error", ap => ap.Run(async context =>
	{
		await context.Response.WriteAsync($"Err: {context.Request.Query["code"]}");
	}));

	app.Map("/hello", ap => ap.Run(async (context) =>
	{
		await context.Response.WriteAsync($"Hello ASP.NET Core");
	}));
}

Настройка обработки ошибок в web.config

Еще один способ обработки кодов ошибок представляет собой определение и настройка в файле конфигурации web.config элемента
httpErrors. Этот способ в принципе использовался и в других версиях ASP.NET.
В ASP.NET Core он также доступен, однако имеет очень ограниченное действие. В частности, мы его можем использовать только при развертывании на IIS, а также не можем использовать ряд настроек.

Итак, добавим в корень проекта новый элемент Web Configurarion File, который естественно назовем web.config:

Изменим его следующим образом:

<?xml version="1.0" encoding="utf-8"?>
<configuration>

  <system.webServer>
	<httpErrors errorMode="Custom" existingResponse="Replace">
      <remove statusCode="404"/>
	  <remove statusCode="403"/>
      <error statusCode="404" path="404.html" responseMode="File"/>
      <error statusCode="403" path="403.html" responseMode="File"/>
	</httpErrors>
   
    <handlers>
      <add name="aspNetCore" path="*" verb="*" modules="AspNetCoreModule" resourceType="Unspecified"/>
    </handlers>
    <aspNetCore processPath="%LAUNCHER_PATH%" arguments="%LAUNCHER_ARGS%" stdoutLogEnabled="false" stdoutLogFile=".logsstdout" forwardWindowsAuthToken="false"/>
  </system.webServer>
</configuration>

Также для обработки ошибок добавим в корень проекта новый файл 404.html:

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8" />
    <title>Ошибка 404</title>
</head>
<body>
    <h1>Ошибка 404</h1>
    <h2>Ресурс не найден!</h2>
</body>
</html>

По аналогии можно добавить файл 403.html для ошибки 403.

Итак, элемент httpErrors имеет ряд настроек. Для тестирования настроек локально, необходимо установить атрибут errorMode="Custom".
Если тестирование необязательно, и приложение уже развернуто для использования, то можно установить значение errorMode="DetailedLocalOnly".

Значение existingResponse="Replace" позволит отобразить ошибку по оригинальному запрошенному пути без переадресации.

Внутри элемента httpErrors с помощью отдельных элементов error устанавливается обработка ошибок. Атрибут statusCode
задает статусный код, атрибут path — адрес url, который будет вызываться, а атрибут responseMode указывает, как будет обрабатываться ответ вызванному url.
Атрибут responseMode имеет значение File, что позволяет рассматривать адрес url из атрибута path как статическую страницу и использовать ее в качестве ответа

Настройки элемента httpErrors могут наследоваться с других уровней, например, от файла конфигурации machine.config. И чтобы удалить
все унаследованные настройки, применяется элемент <clear />. Чтобы удалить настройки для отдельных ошибок, применяется элемент
<remove />.

Для тестирования используем следующий класс Startup:

public class Startup
{
	public void Configure(IApplicationBuilder app)
	{
		app.Map("/hello", ap => ap.Run(async (context) =>
		{
			await context.Response.WriteAsync($"Hello ASP.NET Core");
		}));
	}
}

И после обращения к несуществующему ресурсу в приложении отобразится содержимое из файла 404.html.

Источник

title	author	description	monikerRange	ms.author	ms.custom	ms.date	uid
Handle errors in ASP.NET Core	tdykstra	Discover how to handle errors in ASP.NET Core apps.	>= aspnetcore-3.1	riande	mvc	01/18/2023	fundamentals/error-handling

Handle errors in ASP.NET Core

:::moniker range=»>= aspnetcore-7.0″

By Tom Dykstra

This article covers common approaches to handling errors in ASP.NET Core web apps. See xref:web-api/handle-errors for web APIs.

Developer exception page

The Developer Exception Page displays detailed information about unhandled request exceptions. ASP.NET Core apps enable the developer exception page by default when both:

Running in the Development environment.
App created with the current templates, that is, using WebApplication.CreateBuilder. Apps created using the WebHost.CreateDefaultBuilder must enable the developer exception page by calling app.UseDeveloperExceptionPage in Configure.

The developer exception page runs early in the middleware pipeline, so that it can catch unhandled exceptions thrown in middleware that follows.

Detailed exception information shouldn’t be displayed publicly when the app runs in the Production environment. For more information on configuring environments, see xref:fundamentals/environments.

The Developer Exception Page can include the following information about the exception and the request:

Stack trace
Query string parameters, if any
Cookies, if any
Headers

The Developer Exception Page isn’t guaranteed to provide any information. Use Logging for complete error information.

Exception handler page

To configure a custom error handling page for the Production environment, call xref:Microsoft.AspNetCore.Builder.ExceptionHandlerExtensions.UseExceptionHandler%2A. This exception handling middleware:

Catches and logs unhandled exceptions.
Re-executes the request in an alternate pipeline using the path indicated. The request isn’t re-executed if the response has started. The template-generated code re-executes the request using the /Error path.

[!WARNING]
If the alternate pipeline throws an exception of its own, Exception Handling Middleware rethrows the original exception.

Since this middleware can re-execute the request pipeline:

Middlewares need to handle reentrancy with the same request. This normally means either cleaning up their state after calling _next or caching their processing on the HttpContext to avoid redoing it. When dealing with the request body, this either means buffering or caching the results like the Form reader.
For the xref:Microsoft.AspNetCore.Builder.ExceptionHandlerExtensions.UseExceptionHandler(Microsoft.AspNetCore.Builder.IApplicationBuilder,System.String) overload that is used in templates, only the request path is modified, and the route data is cleared. Request data such as headers, method, and items are all reused as-is.
Scoped services remain the same.

In the following example, xref:Microsoft.AspNetCore.Builder.ExceptionHandlerExtensions.UseExceptionHandler%2A adds the exception handling middleware in non-Development environments:

:::code language=»csharp» source=»error-handling/samples/7.x/ErrorHandlingSample/Program.cs» id=»snippet_UseExceptionHandler» highlight=»3,5″:::

The Razor Pages app template provides an Error page (.cshtml) and xref:Microsoft.AspNetCore.Mvc.RazorPages.PageModel class (ErrorModel) in the Pages folder. For an MVC app, the project template includes an Error action method and an Error view for the Home controller.

The exception handling middleware re-executes the request using the original HTTP method. If an error handler endpoint is restricted to a specific set of HTTP methods, it runs only for those HTTP methods. For example, an MVC controller action that uses the [HttpGet] attribute runs only for GET requests. To ensure that all requests reach the custom error handling page, don’t restrict them to a specific set of HTTP methods.

To handle exceptions differently based on the original HTTP method:

For Razor Pages, create multiple handler methods. For example, use OnGet to handle GET exceptions and use OnPost to handle POST exceptions.
For MVC, apply HTTP verb attributes to multiple actions. For example, use [HttpGet] to handle GET exceptions and use [HttpPost] to handle POST exceptions.

To allow unauthenticated users to view the custom error handling page, ensure that it supports anonymous access.

Access the exception

Use xref:Microsoft.AspNetCore.Diagnostics.IExceptionHandlerPathFeature to access the exception and the original request path in an error handler. The following example uses IExceptionHandlerPathFeature to get more information about the exception that was thrown:

:::code language=»csharp» source=»error-handling/samples/7.x/ErrorHandlingSample/Pages/Error.cshtml.cs» id=»snippet_Class» highlight=»15-27″:::

[!WARNING]
Do not serve sensitive error information to clients. Serving errors is a security risk.

Exception handler lambda

An alternative to a custom exception handler page is to provide a lambda to xref:Microsoft.AspNetCore.Builder.ExceptionHandlerExtensions.UseExceptionHandler%2A. Using a lambda allows access to the error before returning the response.

The following code uses a lambda for exception handling:

:::code language=»csharp» source=»error-handling/samples/7.x/ErrorHandlingSample/Snippets/Program.cs» id=»snippet_UseExceptionHandlerInline» highlight=»5-29″:::

[!WARNING]
Do not serve sensitive error information to clients. Serving errors is a security risk.

UseStatusCodePages

By default, an ASP.NET Core app doesn’t provide a status code page for HTTP error status codes, such as 404 — Not Found. When the app sets an HTTP 400-599 error status code that doesn’t have a body, it returns the status code and an empty response body. To enable default text-only handlers for common error status codes, call xref:Microsoft.AspNetCore.Builder.StatusCodePagesExtensions.UseStatusCodePages%2A in Program.cs:

:::code language=»csharp» source=»error-handling/samples/7.x/ErrorHandlingSample/Snippets/Program.cs» id=»snippet_UseStatusCodePages» highlight=»9″:::

Call UseStatusCodePages before request handling middleware. For example, call UseStatusCodePages before the Static File Middleware and the Endpoints Middleware.

When UseStatusCodePages isn’t used, navigating to a URL without an endpoint returns a browser-dependent error message indicating the endpoint can’t be found. When UseStatusCodePages is called, the browser returns the following response:

Status Code: 404; Not Found

UseStatusCodePages isn’t typically used in production because it returns a message that isn’t useful to users.

[!NOTE]
The status code pages middleware does not catch exceptions. To provide a custom error handling page, use the exception handler page.

UseStatusCodePages with format string

To customize the response content type and text, use the overload of xref:Microsoft.AspNetCore.Builder.StatusCodePagesExtensions.UseStatusCodePages%2A that takes a content type and format string:

:::code language=»csharp» source=»error-handling/samples/7.x/ErrorHandlingSample/Snippets/Program.cs» id=»snippet_UseStatusCodePagesContent» highlight=»10″:::

In the preceding code, {0} is a placeholder for the error code.

UseStatusCodePages with a format string isn’t typically used in production because it returns a message that isn’t useful to users.

UseStatusCodePages with lambda

To specify custom error-handling and response-writing code, use the overload of xref:Microsoft.AspNetCore.Builder.StatusCodePagesExtensions.UseStatusCodePages%2A that takes a lambda expression:

:::code language=»csharp» source=»error-handling/samples/7.x/ErrorHandlingSample/Snippets/Program.cs» id=»snippet_UseStatusCodePagesInline» highlight=»9-16″:::

UseStatusCodePages with a lambda isn’t typically used in production because it returns a message that isn’t useful to users.

UseStatusCodePagesWithRedirects

The xref:Microsoft.AspNetCore.Builder.StatusCodePagesExtensions.UseStatusCodePagesWithRedirects%2A extension method:

Sends a 302 — Found status code to the client.
Redirects the client to the error handling endpoint provided in the URL template. The error handling endpoint typically displays error information and returns HTTP 200.

:::code language=»csharp» source=»error-handling/samples/7.x/ErrorHandlingSample/Snippets/Program.cs» id=»snippet_UseStatusCodePagesRedirect» highlight=»9″:::

The URL template can include a {0} placeholder for the status code, as shown in the preceding code. If the URL template starts with ~ (tilde), the ~ is replaced by the app’s PathBase. When specifying an endpoint in the app, create an MVC view or Razor page for the endpoint.

This method is commonly used when the app:

Should redirect the client to a different endpoint, usually in cases where a different app processes the error. For web apps, the client’s browser address bar reflects the redirected endpoint.
Shouldn’t preserve and return the original status code with the initial redirect response.

UseStatusCodePagesWithReExecute

The xref:Microsoft.AspNetCore.Builder.StatusCodePagesExtensions.UseStatusCodePagesWithReExecute%2A extension method:

Generates the response body by re-executing the request pipeline using an alternate path.
Does not alter the status code before or after re-executing the pipeline.

The new pipeline execution may alter the response’s status code, as the new pipeline has full control of the status code. If the new pipeline does not alter the status code, the original status code will be sent to the client.

:::code language=»csharp» source=»error-handling/samples/7.x/ErrorHandlingSample/Snippets/Program.cs» id=»snippet_UseStatusCodePagesReExecute» highlight=»9″:::

If an endpoint within the app is specified, create an MVC view or Razor page for the endpoint.

This method is commonly used when the app should:

Process the request without redirecting to a different endpoint. For web apps, the client’s browser address bar reflects the originally requested endpoint.
Preserve and return the original status code with the response.

The URL template must start with / and may include a placeholder {0} for the status code. To pass the status code as a query-string parameter, pass a second argument into UseStatusCodePagesWithReExecute. For example:

:::code language=»csharp» source=»error-handling/samples/7.x/ErrorHandlingSample/Snippets/Program.cs» id=»snippet_UseStatusCodePagesReExecuteQueryString»:::

The endpoint that processes the error can get the original URL that generated the error, as shown in the following example:

:::code language=»csharp» source=»error-handling/samples/7.x/ErrorHandlingSample/Pages/StatusCode.cshtml.cs» id=»snippet_Class» highlight=»12-21″:::

Since this middleware can re-execute the request pipeline:

Middlewares need to handle reentrancy with the same request. This normally means either cleaning up their state after calling _next or caching their processing on the HttpContext to avoid redoing it. When dealing with the request body, this either means buffering or caching the results like the Form reader.
Scoped services remain the same.

Disable status code pages

To disable status code pages for an MVC controller or action method, use the [SkipStatusCodePages] attribute.

To disable status code pages for specific requests in a Razor Pages handler method or in an MVC controller, use xref:Microsoft.AspNetCore.Diagnostics.IStatusCodePagesFeature:

:::code language=»csharp» source=»error-handling/samples/7.x/ErrorHandlingSample/Snippets/Pages/Index.cshtml.cs» id=»snippet_OnGet»:::

Exception-handling code

Code in exception handling pages can also throw exceptions. Production error pages should be tested thoroughly and take extra care to avoid throwing exceptions of their own.

Response headers

Once the headers for a response are sent:

The app can’t change the response’s status code.
Any exception pages or handlers can’t run. The response must be completed or the connection aborted.

Server exception handling

In addition to the exception handling logic in an app, the HTTP server implementation can handle some exceptions. If the server catches an exception before response headers are sent, the server sends a 500 - Internal Server Error response without a response body. If the server catches an exception after response headers are sent, the server closes the connection. Requests that aren’t handled by the app are handled by the server. Any exception that occurs when the server is handling the request is handled by the server’s exception handling. The app’s custom error pages, exception handling middleware, and filters don’t affect this behavior.

Startup exception handling

Only the hosting layer can handle exceptions that take place during app startup. The host can be configured to capture startup errors and capture detailed errors.

The hosting layer can show an error page for a captured startup error only if the error occurs after host address/port binding. If binding fails:

The hosting layer logs a critical exception.
The dotnet process crashes.
No error page is displayed when the HTTP server is Kestrel.

When running on IIS (or Azure App Service) or IIS Express, a 502.5 — Process Failure is returned by the ASP.NET Core Module if the process can’t start. For more information, see xref:test/troubleshoot-azure-iis.

Database error page

The Database developer page exception filter xref:Microsoft.Extensions.DependencyInjection.DatabaseDeveloperPageExceptionFilterServiceExtensions.AddDatabaseDeveloperPageExceptionFilter%2A captures database-related exceptions that can be resolved by using Entity Framework Core migrations. When these exceptions occur, an HTML response is generated with details of possible actions to resolve the issue. This page is enabled only in the Development environment. The following code adds the Database developer page exception filter:

:::code language=»csharp» source=»error-handling/samples/7.x/ErrorHandlingSample/Program.cs» id=»snippet_AddDatabaseDeveloperPageExceptionFilter» highlight=»3″:::

Exception filters

In MVC apps, exception filters can be configured globally or on a per-controller or per-action basis. In Razor Pages apps, they can be configured globally or per page model. These filters handle any unhandled exceptions that occur during the execution of a controller action or another filter. For more information, see xref:mvc/controllers/filters#exception-filters.

Exception filters are useful for trapping exceptions that occur within MVC actions, but they’re not as flexible as the built-in exception handling middleware, xref:Microsoft.AspNetCore.Builder.ExceptionHandlerExtensions.UseExceptionHandler%2A. We recommend using UseExceptionHandler, unless you need to perform error handling differently based on which MVC action is chosen.

Model state errors

For information about how to handle model state errors, see Model binding and Model validation.

Problem details

[!INCLUDE]

The following code configures the app to generate a problem details response for all HTTP client and server error responses that don’t have a body content yet:

:::code language=»csharp» source=»error-handling/samples/7.x/ErrorHandlingSample/Snippets/Program.cs» id=»snippet_AddProblemDetails» highlight=»3″:::

The next section shows how to customize the problem details response body.

Customize problem details

The automatic creation of a ProblemDetails can be customized using any of the following options:

Use ProblemDetailsOptions.CustomizeProblemDetails
Use a custom IProblemDetailsWriter
Call the IProblemDetailsService in a middleware

`CustomizeProblemDetails` operation

The generated problem details can be customized using xref:Microsoft.AspNetCore.Http.ProblemDetailsOptions.CustomizeProblemDetails, and the customizations are applied to all auto-generated problem details.

The following code uses xref:Microsoft.AspNetCore.Http.ProblemDetailsOptions to set xref:Microsoft.AspNetCore.Http.ProblemDetailsOptions.CustomizeProblemDetails:

:::code language=»csharp» source=»error-handling/samples/7.x/ErrorHandlingSample/Snippets/Program.cs» id=»snippet_CustomizeProblemDetails» highlight=»3-5″:::

For example, an HTTP Status 400 Bad Request endpoint result produces the following problem details response body:

{
  "type": "https://tools.ietf.org/html/rfc9110#section-15.5.1",
  "title": "Bad Request",
  "status": 400,
  "nodeId": "my-machine-name"
}

Custom `IProblemDetailsWriter`

An xref:Microsoft.AspNetCore.Http.IProblemDetailsWriter implementation can be created for advanced customizations.

:::code language=»csharp» source=»~/../AspNetCore.Docs.Samples/fundamentals/middleware/problem-details-service/SampleProblemDetailsWriter.cs» :::

Note: When using a custom IProblemDetailsWriter, the custom IProblemDetailsWriter must be registered before calling xref:Microsoft.Extensions.DependencyInjection.MvcServiceCollectionExtensions.AddRazorPages%2A, xref:Microsoft.Extensions.DependencyInjection.MvcServiceCollectionExtensions.AddControllers%2A, or xref:Microsoft.Extensions.DependencyInjection.MvcServiceCollectionExtensions.AddControllersWithViews%2A:

:::code language=»csharp» source=»~/../AspNetCore.Docs.Samples/fundamentals/middleware/problem-details-service/Program.cs» id=»snippet_sampleproblemdetailswriter» :::

Problem details from Middleware

An alternative approach to using xref:Microsoft.AspNetCore.Http.ProblemDetailsOptions with xref:Microsoft.AspNetCore.Http.ProblemDetailsOptions.CustomizeProblemDetails is to set the xref:Microsoft.AspNetCore.Http.ProblemDetailsContext.ProblemDetails in middleware. A problem details response can be written by calling IProblemDetailsService.WriteAsync:

:::code language=»csharp» source=»~/../AspNetCore.Docs.Samples/fundamentals/middleware/problem-details-service/Program.cs» id=»snippet_middleware» highlight=»5,19-40″:::

In the preceding code, the minimal API endpoints /divide and /squareroot return the expected custom problem response on error input.

The API controller endpoints return the default problem response on error input, not the custom problem response. The default problem response is returned because the API controller has written to the response stream, Problem details for error status codes, before IProblemDetailsService.WriteAsync is called and the response is not written again.

The following ValuesController returns xref:Microsoft.AspNetCore.Mvc.BadRequestResult, which writes to the response stream and therefore prevents the custom problem response from being returned.

:::code language=»csharp» source=»~/../AspNetCore.Docs.Samples/fundamentals/middleware/problem-details-service/Controllers/ValuesController.cs» id=»snippet» highlight=»9-17,27-35″:::

The following Values3Controller returns ControllerBase.Problem so the expected custom problem result is returned:

:::code language=»csharp» source=»~/../AspNetCore.Docs.Samples/fundamentals/middleware/problem-details-service/Controllers/ValuesController.cs» id=»snippet3″ highlight=»16-21″:::

Produce a ProblemDetails payload for exceptions

Consider the following app:

:::code language=»csharp» source=»~/../AspNetCore.Docs.Samples/fundamentals/middleware/problem-details-service/Program.cs» id=»snippet_apishort» highlight=»4,8″:::

In non-development environments, when an exception occurs, the following is a standard ProblemDetails response that is returned to the client:

{
"type":"https://tools.ietf.org/html/rfc7231#section-6.6.1",
"title":"An error occurred while processing your request.",
"status":500,"traceId":"00-b644<snip>-00"
}

For most apps, the preceding code is all that’s needed for exceptions. However, the following section shows how to get more detailed problem responses.

:::code language=»csharp» source=»~/../AspNetCore.Docs.Samples/fundamentals/middleware/problem-details-service/Program.cs» id=»snippet_lambda» :::

[!WARNING]
Do not serve sensitive error information to clients. Serving errors is a security risk.

An alternative approach to generate problem details is to use the third-party NuGet package Hellang.Middleware.ProblemDetails that can be used to map exceptions and client errors to problem details.

Additional resources

View or download sample code (how to download)
xref:test/troubleshoot-azure-iis
xref:host-and-deploy/azure-iis-errors-reference

:::moniker-end

:::moniker range=»= aspnetcore-6.0″

By Tom Dykstra

This article covers common approaches to handling errors in ASP.NET Core web apps. See xref:web-api/handle-errors for web APIs.

Developer exception page

The Developer Exception Page displays detailed information about unhandled request exceptions. ASP.NET Core apps enable the developer exception page by default when both:

Running in the Development environment.
App created with the current templates, that is, using WebApplication.CreateBuilder. Apps created using the WebHost.CreateDefaultBuilder must enable the developer exception page by calling app.UseDeveloperExceptionPage in Configure.

The developer exception page runs early in the middleware pipeline, so that it can catch unhandled exceptions thrown in middleware that follows.

Detailed exception information shouldn’t be displayed publicly when the app runs in the Production environment. For more information on configuring environments, see xref:fundamentals/environments.

The Developer Exception Page can include the following information about the exception and the request:

Stack trace
Query string parameters, if any
Cookies, if any
Headers

The Developer Exception Page isn’t guaranteed to provide any information. Use Logging for complete error information.

Exception handler page

To configure a custom error handling page for the Production environment, call xref:Microsoft.AspNetCore.Builder.ExceptionHandlerExtensions.UseExceptionHandler%2A. This exception handling middleware:

Catches and logs unhandled exceptions.
Re-executes the request in an alternate pipeline using the path indicated. The request isn’t re-executed if the response has started. The template-generated code re-executes the request using the /Error path.

[!WARNING]
If the alternate pipeline throws an exception of its own, Exception Handling Middleware rethrows the original exception.

In the following example, xref:Microsoft.AspNetCore.Builder.ExceptionHandlerExtensions.UseExceptionHandler%2A adds the exception handling middleware in non-Development environments:

:::code language=»csharp» source=»error-handling/samples/6.x/ErrorHandlingSample/Program.cs» id=»snippet_UseExceptionHandler» highlight=»3,5″:::

To handle exceptions differently based on the original HTTP method:

For Razor Pages, create multiple handler methods. For example, use OnGet to handle GET exceptions and use OnPost to handle POST exceptions.
For MVC, apply HTTP verb attributes to multiple actions. For example, use [HttpGet] to handle GET exceptions and use [HttpPost] to handle POST exceptions.

To allow unauthenticated users to view the custom error handling page, ensure that it supports anonymous access.

Access the exception

:::code language=»csharp» source=»error-handling/samples/6.x/ErrorHandlingSample/Pages/Error.cshtml.cs» id=»snippet_Class» highlight=»15-27″:::

[!WARNING]
Do not serve sensitive error information to clients. Serving errors is a security risk.

Exception handler lambda

The following code uses a lambda for exception handling:

:::code language=»csharp» source=»error-handling/samples/6.x/ErrorHandlingSample/Snippets/Program.cs» id=»snippet_UseExceptionHandlerInline» highlight=»5-29″:::

[!WARNING]
Do not serve sensitive error information to clients. Serving errors is a security risk.

UseStatusCodePages

:::code language=»csharp» source=»error-handling/samples/6.x/ErrorHandlingSample/Snippets/Program.cs» id=»snippet_UseStatusCodePages» highlight=»9″:::

Call UseStatusCodePages before request handling middleware. For example, call UseStatusCodePages before the Static File Middleware and the Endpoints Middleware.

Status Code: 404; Not Found

UseStatusCodePages isn’t typically used in production because it returns a message that isn’t useful to users.

[!NOTE]
The status code pages middleware does not catch exceptions. To provide a custom error handling page, use the exception handler page.

UseStatusCodePages with format string

To customize the response content type and text, use the overload of xref:Microsoft.AspNetCore.Builder.StatusCodePagesExtensions.UseStatusCodePages%2A that takes a content type and format string:

:::code language=»csharp» source=»error-handling/samples/6.x/ErrorHandlingSample/Snippets/Program.cs» id=»snippet_UseStatusCodePagesContent» highlight=»10″:::

In the preceding code, {0} is a placeholder for the error code.

UseStatusCodePages with a format string isn’t typically used in production because it returns a message that isn’t useful to users.

UseStatusCodePages with lambda

To specify custom error-handling and response-writing code, use the overload of xref:Microsoft.AspNetCore.Builder.StatusCodePagesExtensions.UseStatusCodePages%2A that takes a lambda expression:

:::code language=»csharp» source=»error-handling/samples/6.x/ErrorHandlingSample/Snippets/Program.cs» id=»snippet_UseStatusCodePagesInline» highlight=»9-16″:::

UseStatusCodePages with a lambda isn’t typically used in production because it returns a message that isn’t useful to users.

UseStatusCodePagesWithRedirects

The xref:Microsoft.AspNetCore.Builder.StatusCodePagesExtensions.UseStatusCodePagesWithRedirects%2A extension method:

Sends a 302 — Found status code to the client.
Redirects the client to the error handling endpoint provided in the URL template. The error handling endpoint typically displays error information and returns HTTP 200.

:::code language=»csharp» source=»error-handling/samples/6.x/ErrorHandlingSample/Snippets/Program.cs» id=»snippet_UseStatusCodePagesRedirect» highlight=»9″:::

This method is commonly used when the app:

Should redirect the client to a different endpoint, usually in cases where a different app processes the error. For web apps, the client’s browser address bar reflects the redirected endpoint.
Shouldn’t preserve and return the original status code with the initial redirect response.

UseStatusCodePagesWithReExecute

The xref:Microsoft.AspNetCore.Builder.StatusCodePagesExtensions.UseStatusCodePagesWithReExecute%2A extension method:

Returns the original status code to the client.
Generates the response body by re-executing the request pipeline using an alternate path.

:::code language=»csharp» source=»error-handling/samples/6.x/ErrorHandlingSample/Snippets/Program.cs» id=»snippet_UseStatusCodePagesReExecute» highlight=»9″:::

If an endpoint within the app is specified, create an MVC view or Razor page for the endpoint.

This method is commonly used when the app should:

Process the request without redirecting to a different endpoint. For web apps, the client’s browser address bar reflects the originally requested endpoint.
Preserve and return the original status code with the response.

:::code language=»csharp» source=»error-handling/samples/6.x/ErrorHandlingSample/Snippets/Program.cs» id=»snippet_UseStatusCodePagesReExecuteQueryString»:::

The endpoint that processes the error can get the original URL that generated the error, as shown in the following example:

:::code language=»csharp» source=»error-handling/samples/6.x/ErrorHandlingSample/Pages/StatusCode.cshtml.cs» id=»snippet_Class» highlight=»12-21″:::

Disable status code pages

To disable status code pages for an MVC controller or action method, use the [SkipStatusCodePages] attribute.

To disable status code pages for specific requests in a Razor Pages handler method or in an MVC controller, use xref:Microsoft.AspNetCore.Diagnostics.IStatusCodePagesFeature:

:::code language=»csharp» source=»error-handling/samples/6.x/ErrorHandlingSample/Snippets/Pages/Index.cshtml.cs» id=»snippet_OnGet»:::

Exception-handling code

Code in exception handling pages can also throw exceptions. Production error pages should be tested thoroughly and take extra care to avoid throwing exceptions of their own.

Response headers

Once the headers for a response are sent:

The app can’t change the response’s status code.
Any exception pages or handlers can’t run. The response must be completed or the connection aborted.

Server exception handling

Startup exception handling

Only the hosting layer can handle exceptions that take place during app startup. The host can be configured to capture startup errors and capture detailed errors.

The hosting layer can show an error page for a captured startup error only if the error occurs after host address/port binding. If binding fails:

The hosting layer logs a critical exception.
The dotnet process crashes.
No error page is displayed when the HTTP server is Kestrel.

Database error page

:::code language=»csharp» source=»error-handling/samples/6.x/ErrorHandlingSample/Program.cs» id=»snippet_AddDatabaseDeveloperPageExceptionFilter» highlight=»3″:::

Exception filters

Model state errors

For information about how to handle model state errors, see Model binding and Model validation.

Additional resources

View or download sample code (how to download)
xref:test/troubleshoot-azure-iis
xref:host-and-deploy/azure-iis-errors-reference

:::moniker-end

:::moniker range=»>= aspnetcore-5.0 < aspnetcore-6.0″

By Kirk Larkin, Tom Dykstra, and Steve Smith

This article covers common approaches to handling errors in ASP.NET Core web apps. See xref:web-api/handle-errors for web APIs.

View or download sample code. (How to download.) The network tab on the F12 browser developer tools is useful when testing the sample app.

Developer Exception Page

The Developer Exception Page displays detailed information about unhandled request exceptions. The ASP.NET Core templates generate the following code:

:::code language=»csharp» source=»error-handling/samples/5.x/ErrorHandlingSample/Startup.cs» id=»snippet» highlight=»3-6″:::

The preceding highlighted code enables the developer exception page when the app is running in the Development environment.

The templates place xref:Microsoft.AspNetCore.Builder.DeveloperExceptionPageExtensions.UseDeveloperExceptionPage%2A early in the middleware pipeline so that it can catch unhandled exceptions thrown in middleware that follows.

The preceding code enables the Developer Exception Page only when the app runs in the Development environment. Detailed exception information shouldn’t be displayed publicly when the app runs in the Production environment. For more information on configuring environments, see xref:fundamentals/environments.

The Developer Exception Page can include the following information about the exception and the request:

Stack trace
Query string parameters if any
Cookies if any
Headers

The Developer Exception Page isn’t guaranteed to provide any information. Use Logging for complete error information.

Exception handler page

To configure a custom error handling page for the Production environment, call xref:Microsoft.AspNetCore.Builder.ExceptionHandlerExtensions.UseExceptionHandler%2A. This exception handling middleware:

Catches and logs unhandled exceptions.
Re-executes the request in an alternate pipeline using the path indicated. The request isn’t re-executed if the response has started. The template-generated code re-executes the request using the /Error path.

[!WARNING]
If the alternate pipeline throws an exception of its own, Exception Handling Middleware rethrows the original exception.

In the following example, xref:Microsoft.AspNetCore.Builder.ExceptionHandlerExtensions.UseExceptionHandler%2A adds the exception handling middleware in non-Development environments:

:::code language=»csharp» source=»error-handling/samples/5.x/ErrorHandlingSample/Startup.cs» id=»snippet_DevPageAndHandlerPage» highlight=»5-9″:::

To handle exceptions differently based on the original HTTP method:

For Razor Pages, create multiple handler methods. For example, use OnGet to handle GET exceptions and use OnPost to handle POST exceptions.
For MVC, apply HTTP verb attributes to multiple actions. For example, use [HttpGet] to handle GET exceptions and use [HttpPost] to handle POST exceptions.

To allow unauthenticated users to view the custom error handling page, ensure that it supports anonymous access.

Access the exception

Use xref:Microsoft.AspNetCore.Diagnostics.IExceptionHandlerPathFeature to access the exception and the original request path in an error handler. The following code adds ExceptionMessage to the default Pages/Error.cshtml.cs generated by the ASP.NET Core templates:

:::code language=»csharp» source=»error-handling/samples/5.x/ErrorHandlingSample/Pages/Error.cshtml.cs» id=»snippet»:::

[!WARNING]
Do not serve sensitive error information to clients. Serving errors is a security risk.

To test the exception in the sample app:

Set the environment to production.
Remove the comments from webBuilder.UseStartup<Startup>(); in Program.cs.
Select Trigger an exception on the home page.

Exception handler lambda

The following code uses a lambda for exception handling:

:::code language=»csharp» source=»error-handling/samples/5.x/ErrorHandlingSample/StartupLambda.cs» id=»snippet»:::

[!WARNING]
Do not serve sensitive error information from xref:Microsoft.AspNetCore.Diagnostics.IExceptionHandlerFeature or xref:Microsoft.AspNetCore.Diagnostics.IExceptionHandlerPathFeature to clients. Serving errors is a security risk.

To test the exception handling lambda in the sample app:

Set the environment to production.
Remove the comments from webBuilder.UseStartup<StartupLambda>(); in Program.cs.
Select Trigger an exception on the home page.

UseStatusCodePages

By default, an ASP.NET Core app doesn’t provide a status code page for HTTP error status codes, such as 404 — Not Found. When the app sets an HTTP 400-599 error status code that doesn’t have a body, it returns the status code and an empty response body. To provide status code pages, use the status code pages middleware. To enable default text-only handlers for common error status codes, call xref:Microsoft.AspNetCore.Builder.StatusCodePagesExtensions.UseStatusCodePages%2A in the Startup.Configure method:

:::code language=»csharp» source=»error-handling/samples/5.x/ErrorHandlingSample/StartupUseStatusCodePages.cs» id=»snippet» highlight=»13″:::

Call UseStatusCodePages before request handling middleware. For example, call UseStatusCodePages before the Static File Middleware and the Endpoints Middleware.

When UseStatusCodePages isn’t used, navigating to a URL without an endpoint returns a browser-dependent error message indicating the endpoint can’t be found. For example, navigating to Home/Privacy2. When UseStatusCodePages is called, the browser returns:

Status Code: 404; Not Found

UseStatusCodePages isn’t typically used in production because it returns a message that isn’t useful to users.

To test UseStatusCodePages in the sample app:

Set the environment to production.
Remove the comments from webBuilder.UseStartup<StartupUseStatusCodePages>(); in Program.cs.
Select the links on the home page on the home page.

[!NOTE]
The status code pages middleware does not catch exceptions. To provide a custom error handling page, use the exception handler page.

UseStatusCodePages with format string

To customize the response content type and text, use the overload of xref:Microsoft.AspNetCore.Builder.StatusCodePagesExtensions.UseStatusCodePages%2A that takes a content type and format string:

:::code language=»csharp» source=»error-handling/samples/5.x/ErrorHandlingSample/StartupFormat.cs» id=»snippet» highlight=»13-14″:::

In the preceding code, {0} is a placeholder for the error code.

UseStatusCodePages with a format string isn’t typically used in production because it returns a message that isn’t useful to users.

To test UseStatusCodePages in the sample app, remove the comments from webBuilder.UseStartup<StartupFormat>(); in Program.cs.

UseStatusCodePages with lambda

To specify custom error-handling and response-writing code, use the overload of xref:Microsoft.AspNetCore.Builder.StatusCodePagesExtensions.UseStatusCodePages%2A that takes a lambda expression:

:::code language=»csharp» source=»error-handling/samples/5.x/ErrorHandlingSample/StartupStatusLambda.cs» id=»snippet» highlight=»13-20″:::

UseStatusCodePages with a lambda isn’t typically used in production because it returns a message that isn’t useful to users.

To test UseStatusCodePages in the sample app, remove the comments from webBuilder.UseStartup<StartupStatusLambda>(); in Program.cs.

UseStatusCodePagesWithRedirects

The xref:Microsoft.AspNetCore.Builder.StatusCodePagesExtensions.UseStatusCodePagesWithRedirects%2A extension method:

Sends a 302 — Found status code to the client.
Redirects the client to the error handling endpoint provided in the URL template. The error handling endpoint typically displays error information and returns HTTP 200.

:::code language=»csharp» source=»error-handling/samples/5.x/ErrorHandlingSample/StartupSCredirect.cs» id=»snippet» highlight=»13″:::

This method is commonly used when the app:

Should redirect the client to a different endpoint, usually in cases where a different app processes the error. For web apps, the client’s browser address bar reflects the redirected endpoint.
Shouldn’t preserve and return the original status code with the initial redirect response.

To test UseStatusCodePages in the sample app, remove the comments from webBuilder.UseStartup<StartupSCredirect>(); in Program.cs.

UseStatusCodePagesWithReExecute

The xref:Microsoft.AspNetCore.Builder.StatusCodePagesExtensions.UseStatusCodePagesWithReExecute%2A extension method:

Returns the original status code to the client.
Generates the response body by re-executing the request pipeline using an alternate path.

:::code language=»csharp» source=»error-handling/samples/5.x/ErrorHandlingSample/StartupSCreX.cs» id=»snippet» highlight=»13″:::

If an endpoint within the app is specified, create an MVC view or Razor page for the endpoint. Ensure UseStatusCodePagesWithReExecute is placed before UseRouting so the request can be rerouted to the status page. For a Razor Pages example, see Pages/MyStatusCode2.cshtml in the sample app.

This method is commonly used when the app should:

Process the request without redirecting to a different endpoint. For web apps, the client’s browser address bar reflects the originally requested endpoint.
Preserve and return the original status code with the response.

The URL and query string templates may include a placeholder {0} for the status code. The URL template must start with /.

The endpoint that processes the error can get the original URL that generated the error, as shown in the following example:

:::code language=»csharp» source=»error-handling/samples/5.x/ErrorHandlingSample/Pages/MyStatusCode2.cshtml.cs» id=»snippet»:::

For a Razor Pages example, see Pages/MyStatusCode2.cshtml in the sample app.

To test UseStatusCodePages in the sample app, remove the comments from webBuilder.UseStartup<StartupSCreX>(); in Program.cs.

Disable status code pages

To disable status code pages for an MVC controller or action method, use the [SkipStatusCodePages] attribute.

To disable status code pages for specific requests in a Razor Pages handler method or in an MVC controller, use xref:Microsoft.AspNetCore.Diagnostics.IStatusCodePagesFeature:

:::code language=»csharp» source=»error-handling/samples/5.x/ErrorHandlingSample/Pages/Privacy.cshtml.cs» id=»snippet»:::

Exception-handling code

Code in exception handling pages can also throw exceptions. Production error pages should be tested thoroughly and take extra care to avoid throwing exceptions of their own.

Response headers

Once the headers for a response are sent:

The app can’t change the response’s status code.
Any exception pages or handlers can’t run. The response must be completed or the connection aborted.

Server exception handling

Startup exception handling

Only the hosting layer can handle exceptions that take place during app startup. The host can be configured to capture startup errors and capture detailed errors.

The hosting layer can show an error page for a captured startup error only if the error occurs after host address/port binding. If binding fails:

The hosting layer logs a critical exception.
The dotnet process crashes.
No error page is displayed when the HTTP server is Kestrel.

Database error page

The Database developer page exception filter AddDatabaseDeveloperPageExceptionFilter captures database-related exceptions that can be resolved by using Entity Framework Core migrations. When these exceptions occur, an HTML response is generated with details of possible actions to resolve the issue. This page is enabled only in the Development environment. The following code was generated by the ASP.NET Core Razor Pages templates when individual user accounts were specified:

:::code language=»csharp» source=»error-handling/samples/5.x/StartupDBexFilter.cs» id=»snippet» highlight=»6″:::

Exception filters

Exception filters are useful for trapping exceptions that occur within MVC actions, but they’re not as flexible as the built-in exception handling middleware, UseExceptionHandler. We recommend using UseExceptionHandler, unless you need to perform error handling differently based on which MVC action is chosen.

:::code language=»csharp» source=»error-handling/samples/5.x/ErrorHandlingSample/Startup.cs» id=»snippet» highlight=»9″:::

Model state errors

For information about how to handle model state errors, see Model binding and Model validation.

Additional resources

xref:test/troubleshoot-azure-iis
xref:host-and-deploy/azure-iis-errors-reference

:::moniker-end

:::moniker range=»< aspnetcore-5.0″

By Tom Dykstra, and Steve Smith

This article covers common approaches to handling errors in ASP.NET Core web apps. See xref:web-api/handle-errors for web APIs.

View or download sample code. (How to download.)

Developer Exception Page

The Developer Exception Page displays detailed information about request exceptions. The ASP.NET Core templates generate the following code:

:::code language=»csharp» source=»error-handling/samples/2.x/ErrorHandlingSample/Startup.cs» id=»snippet_DevPageAndHandlerPage» highlight=»1-4″:::

The preceding code enables the developer exception page when the app is running in the Development environment.

The templates place xref:Microsoft.AspNetCore.Builder.DeveloperExceptionPageExtensions.UseDeveloperExceptionPage%2A before any middleware so exceptions are caught in the middleware that follows.

The preceding code enables the Developer Exception Page only when the app is running in the Development environment. Detailed exception information should not be displayed publicly when the app runs in production. For more information on configuring environments, see xref:fundamentals/environments.

The Developer Exception Page includes the following information about the exception and the request:

Stack trace
Query string parameters if any
Cookies if any
Headers

Exception handler page

To configure a custom error handling page for the Production environment, use the Exception Handling Middleware. The middleware:

Catches and logs exceptions.
Re-executes the request in an alternate pipeline for the page or controller indicated. The request isn’t re-executed if the response has started. The template generated code re-executes the request to /Error.

In the following example, xref:Microsoft.AspNetCore.Builder.ExceptionHandlerExtensions.UseExceptionHandler%2A adds the Exception Handling Middleware in non-Development environments:

:::code language=»csharp» source=»error-handling/samples/2.x/ErrorHandlingSample/Startup.cs» id=»snippet_DevPageAndHandlerPage» highlight=»5-9″:::

The Razor Pages app template provides an Error page (.cshtml) and xref:Microsoft.AspNetCore.Mvc.RazorPages.PageModel class (ErrorModel) in the Pages folder. For an MVC app, the project template includes an Error action method and an Error view in the Home controller.

Don’t mark the error handler action method with HTTP method attributes, such as HttpGet. Explicit verbs prevent some requests from reaching the method. Allow anonymous access to the method if unauthenticated users should see the error view.

Access the exception

Use xref:Microsoft.AspNetCore.Diagnostics.IExceptionHandlerPathFeature to access the exception and the original request path in an error handler controller or page:

:::code language=»csharp» source=»error-handling/samples/2.x/ErrorHandlingSample/Pages/MyFolder/Error.cshtml.cs» id=»snippet_ExceptionHandlerPathFeature»:::

[!WARNING]
Do not serve sensitive error information to clients. Serving errors is a security risk.

To trigger the preceding exception handling page, set the environment to productions and force an exception.

Exception handler lambda

Here’s an example of using a lambda for exception handling:

:::code language=»csharp» source=»error-handling/samples/2.x/ErrorHandlingSample/Startup.cs» id=»snippet_HandlerPageLambda»:::

In the preceding code, await context.Response.WriteAsync(new string(' ', 512)); is added so the Internet Explorer browser displays the error message rather than an IE error message. For more information, see this GitHub issue.

[!WARNING]
Do not serve sensitive error information from xref:Microsoft.AspNetCore.Diagnostics.IExceptionHandlerFeature or xref:Microsoft.AspNetCore.Diagnostics.IExceptionHandlerPathFeature to clients. Serving errors is a security risk.

To see the result of the exception handling lambda in the sample app, use the ProdEnvironment and ErrorHandlerLambda preprocessor directives, and select Trigger an exception on the home page.

UseStatusCodePages

By default, an ASP.NET Core app doesn’t provide a status code page for HTTP status codes, such as 404 — Not Found. The app returns a status code and an empty response body. To provide status code pages, use Status Code Pages middleware.

The middleware is made available by the Microsoft.AspNetCore.Diagnostics package.

To enable default text-only handlers for common error status codes, call xref:Microsoft.AspNetCore.Builder.StatusCodePagesExtensions.UseStatusCodePages%2A in the Startup.Configure method:

:::code language=»csharp» source=»error-handling/samples/2.x/ErrorHandlingSample/Startup.cs» id=»snippet_StatusCodePages»:::

Call UseStatusCodePages before request handling middleware (for example, Static File Middleware and MVC Middleware).

When UseStatusCodePages isn’t used, navigating to a URL without an endpoint returns a browser dependent error message indicating the endpoint can’t be found. For example, navigating to Home/Privacy2. When UseStatusCodePages is called, the browser returns:

Status Code: 404; Not Found

UseStatusCodePages with format string

To customize the response content type and text, use the overload of xref:Microsoft.AspNetCore.Builder.StatusCodePagesExtensions.UseStatusCodePages%2A that takes a content type and format string:

:::code language=»csharp» source=»error-handling/samples/2.x/ErrorHandlingSample/Startup.cs» id=»snippet_StatusCodePagesFormatString»:::

UseStatusCodePages with lambda

To specify custom error-handling and response-writing code, use the overload of xref:Microsoft.AspNetCore.Builder.StatusCodePagesExtensions.UseStatusCodePages%2A that takes a lambda expression:

:::code language=»csharp» source=»error-handling/samples/2.x/ErrorHandlingSample/Startup.cs» id=»snippet_StatusCodePagesLambda»:::

UseStatusCodePagesWithRedirects

The xref:Microsoft.AspNetCore.Builder.StatusCodePagesExtensions.UseStatusCodePagesWithRedirects%2A extension method:

Sends a 302 — Found status code to the client.
Redirects the client to the location provided in the URL template.

:::code language=»csharp» source=»error-handling/samples/2.x/ErrorHandlingSample/Startup.cs» id=»snippet_StatusCodePagesWithRedirect»:::

The URL template can include a {0} placeholder for the status code, as shown in the example. If the URL template starts with ~ (tilde), the ~ is replaced by the app’s PathBase. If you point to an endpoint within the app, create an MVC view or Razor page for the endpoint. For a Razor Pages example, see Pages/StatusCode.cshtml in the sample app.

This method is commonly used when the app:

Should redirect the client to a different endpoint, usually in cases where a different app processes the error. For web apps, the client’s browser address bar reflects the redirected endpoint.
Shouldn’t preserve and return the original status code with the initial redirect response.

UseStatusCodePagesWithReExecute

The xref:Microsoft.AspNetCore.Builder.StatusCodePagesExtensions.UseStatusCodePagesWithReExecute%2A extension method:

Returns the original status code to the client.
Generates the response body by re-executing the request pipeline using an alternate path.

:::code language=»csharp» source=»error-handling/samples/2.x/ErrorHandlingSample/Startup.cs» id=»snippet_StatusCodePagesWithReExecute»:::

If you point to an endpoint within the app, create an MVC view or Razor page for the endpoint. Ensure UseStatusCodePagesWithReExecute is placed before UseRouting so the request can be rerouted to the status page. For a Razor Pages example, see Pages/StatusCode.cshtml in the sample app.

This method is commonly used when the app should:

Process the request without redirecting to a different endpoint. For web apps, the client’s browser address bar reflects the originally requested endpoint.
Preserve and return the original status code with the response.

The URL and query string templates may include a placeholder ({0}) for the status code. The URL template must start with a slash (/). When using a placeholder in the path, confirm that the endpoint (page or controller) can process the path segment. For example, a Razor Page for errors should accept the optional path segment value with the @page directive:

The endpoint that processes the error can get the original URL that generated the error, as shown in the following example:

:::code language=»csharp» source=»error-handling/samples/2.x/ErrorHandlingSample/Pages/StatusCode.cshtml.cs» id=»snippet_StatusCodeReExecute»:::

Disable status code pages

To disable status code pages for an MVC controller or action method, use the [SkipStatusCodePages] attribute.

To disable status code pages for specific requests in a Razor Pages handler method or in an MVC controller, use xref:Microsoft.AspNetCore.Diagnostics.IStatusCodePagesFeature:

var statusCodePagesFeature = HttpContext.Features.Get<IStatusCodePagesFeature>();

if (statusCodePagesFeature != null)
{
    statusCodePagesFeature.Enabled = false;
}

Exception-handling code

Code in exception handling pages can throw exceptions. It’s often a good idea for production error pages to consist of purely static content.

Response headers

Once the headers for a response are sent:

The app can’t change the response’s status code.
Any exception pages or handlers can’t run. The response must be completed or the connection aborted.

Server exception handling

In addition to the exception handling logic in your app, the HTTP server implementation can handle some exceptions. If the server catches an exception before response headers are sent, the server sends a 500 — Internal Server Error response without a response body. If the server catches an exception after response headers are sent, the server closes the connection. Requests that aren’t handled by your app are handled by the server. Any exception that occurs when the server is handling the request is handled by the server’s exception handling. The app’s custom error pages, exception handling middleware, and filters don’t affect this behavior.

Startup exception handling

Only the hosting layer can handle exceptions that take place during app startup. The host can be configured to capture startup errors and capture detailed errors.

The hosting layer can show an error page for a captured startup error only if the error occurs after host address/port binding. If binding fails:

The hosting layer logs a critical exception.
The dotnet process crashes.
No error page is displayed when the HTTP server is Kestrel.

Database error page

Database Error Page Middleware captures database-related exceptions that can be resolved by using Entity Framework migrations. When these exceptions occur, an HTML response with details of possible actions to resolve the issue is generated. This page should be enabled only in the Development environment. Enable the page by adding code to Startup.Configure:

if (env.IsDevelopment())
{
    app.UseDatabaseErrorPage();
}

xref:Microsoft.AspNetCore.Builder.DatabaseErrorPageExtensions.UseDatabaseErrorPage%2A requires the Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore NuGet package.

Exception filters

In MVC apps, exception filters can be configured globally or on a per-controller or per-action basis. In Razor Pages apps, they can be configured globally or per page model. These filters handle any unhandled exception that occurs during the execution of a controller action or another filter. For more information, see xref:mvc/controllers/filters#exception-filters.

[!TIP]
Exception filters are useful for trapping exceptions that occur within MVC actions, but they’re not as flexible as the Exception Handling Middleware. We recommend using the middleware. Use filters only where you need to perform error handling differently based on which MVC action is chosen.

Model state errors

For information about how to handle model state errors, see Model binding and Model validation.

Additional resources

xref:test/troubleshoot-azure-iis
xref:host-and-deploy/azure-iis-errors-reference

:::moniker-end

Источник

Anyone that has used the full .net MVC framework has spent many an hour trying to rejig the web.config and custom MVC filters to get custom error pages going. Often it would lead you on a wild goose chase around Stack Overflow finding answers that went something along the lines of “just do this one super easy thing and it will work”… It never worked.

.net Core has completely re-invented how custom errors work. Partly because with no web.config, any XML configuration is out the window, and partly because the new “middleware pipeline” mostly does away with the plethora of MVC filters you had to use in the past.

Developer Exception Page

The developer exception page is more or less the error page you used to see in full .net framework if you had custom errors off. That is, you could see the stack trace of the error and other important info to help you debug the issue.

By default, new ASP.net core templates come with this turned on when creating a new project. You can check this by looking at the Configure method of your startup.cs file. It should look pretty close to the following.

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
{
	if (env.IsDevelopment())
	{
		app.UseDeveloperExceptionPage();
	}

	app.UseMvcWithDefaultRoute();
}

Note that checking if the environment is development is so important! Just like the CustomErrors tag in the full framework, you only want to leak out your stacktrace and other sensitive info if you are debugging this locally or you specifically need to see what’s going on for testing purposes. Under no circumstances should you just turn on the “UseDeveloperExceptionPage” middleware without first making sure you are working locally (Or some other specific condition).

Another important thing to note is that as always, the ordering of your middleware matters. Ensure that you are adding the Developer Exception Page middleware before you are going into MVC. Without it, MVC (Or any other middleware in your pipeline), can short circuit the process without it ever reaching the Developer Exception page code.

If your code encounters an exception now, you should see something similar to the following :

As we can see, we get the full stack as well as being able to see any query we sent, cookies and other headers. Again, not things we want to start leaking out all over the place.

Exception Handler Page

ASP.net core comes with a catch all middleware that handles all exceptions and will redirect the user to a particular error page. This is pretty similar to the default redirect in the CustomErrors attribute in web.config or the HandleError attribute in full framework MVC. An important note is that this is an “exception” handler. Other status code errors (404 for example) do not get caught and redirected using this middleware.

The pattern is usually to show the developer error page when in the development environment, otherwise to redirect to the error page. So it might look a bit like this :

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
{
	if (env.IsDevelopment())
	{
		app.UseDeveloperExceptionPage();
	}
	else
	{
		app.UseExceptionHandler("/home/error");
	}

	app.UseMvcWithDefaultRoute();
}

You would then need to create an action to handle the error. One very important thing to note is that the ExceptionHandler will be called with the same HTTP Verb as the original request. e.g. If the exception happened on a Post request, then your handler at /home/error should be able to accept Posts.

What this means in practice is that you should not decorate your action with any particular HTTP verb, just allow it to accept them all.

Statuscode Pages

ASP.net core comes with an inbuilt middleware that allows you to capture other types of HTTP status codes (Other than say 500), and be able to show them certain content based on the status code.

There are actually two different ways to get this going. The first is :

app.UseStatusCodePagesWithRedirects("/error/{0}");

Using “StatusCodePagesWithRedirects” you redirect the user to the status page. The issue with this is that the client is returned a 302 and not the original status code (For example a 404). Another issue is that if the exception is somewhere in your pipeline you are essentially restarting the pipeline again (And it could throw the same issue up).

The second option is :

app.UseStatusCodePagesWithReExecute("/error/{0}");

Using ReExecute, your original response code is returned but the content of the response is from your specified handler. This means that 404’s will be treated as such by spiders and browsers, but it still allows you to display some custom content (Such as a nice 404 page for a user).

Custom Middleware

Remember that you can always create custom middleware to handle any exception/status code in your pipeline. Here is an example of a very simple middleware to handle a 404 status code.

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
{
	app.Use(async (context, next) =>
	{
		await next.Invoke();

		//After going down the pipeline check if we 404'd. 
		if (context.Response.StatusCode == StatusCodes.Status404NotFound)
		{
			await context.Response.WriteAsync("Woops! We 404'd");
		}
	});

	app.UseMvcWithDefaultRoute();
}

On our way back out (Remember, the code after the “next” is code to be run on the way out of the pipeline), we check if the status code was 404, if it is then we return a nice little message letting people know we 404’d.

If you want more info on how to create a custom middleware to handle exceptions (Including how to write a nice class to wrap it), check out our tutorial on writing custom middleware in asp.net core.

Источник

Update for beta8:
In beta8 Microsoft changed the name to UseDeveloperExceptionPage. So if you want to use the ErrorPage, call:

app.UseDeveloperExceptionPage();

Here is the link to the related Github issue.
The ErrorPageOptions are the same as in beta6/7.

You can use

app.UseErrorPage(ErrorPageOptions.ShowAll)

until beta5 of Asp.Net Mvc.

As of beta6, ErrorPageOptions.ShowAll has been removed. You can now use the version without parameters

app.UseErrorPage();

or create an ErrorPageOptions object and specify how many lines around the error you want to display by setting SourceCodeLineCount.

app.UseErrorPage(new ErrorPageOptions() {SourceCodeLineCount = 100});

Additional Information

They removed multiple properties of ErrorPageOptions in this commit.

Before:

public class ErrorPageOptions
{
    private bool _defaultVisibility;

    private bool? _showExceptionDetails;
    private bool? _showSourceCode;
    private bool? _showQuery;
    private bool? _showCookies;
    private bool? _showHeaders;
    private bool? _showEnvironment;
    ...
}

After:

public class ErrorPageOptions
{
    public int SourceCodeLineCount { get; set; }
    public IFileProvider FileProvider { get; set; }
    ...
}

So now you can only set how many lines of source code are printed.

Источник

Just recently I realized this blog had a small issue.
While there was a custom error page for 404s, it was done with a redirect, and the view was returned with a 200 OK.
This meant that in my telemetry, I would see the PageNotFound entries, but not what page they tried to access.

So I set out to fix this issue along with some other related things.
This is how I implemented custom error pages for 404s and exceptions on this blog.

Custom error pages

So what is a custom error page?
It is a nice-looking view that is meant to be used when something goes wrong in a production environment.

In development you would use:

app.UseDeveloperExceptionPage();

Now when an exception occurs, you will see a view that describes what went wrong and where.

But it should not be used in production as it could lead to security problems.
Users might also consider your site less stable, and never visit again.

So we want to show them something that says something like:

Oops! Something went wrong. We are very sorry, please try again in a moment. If the error persists…

In case of a 404 status code, by default ASP.NET Core just returns a white page.

Instead we would like to show something like:

Hey that thing you tried to access does not exist. Please check the URL is correct.

Exception handler middleware

Let’s start by handling exceptions properly.

In our application pipeline configuration, we will add the exception handler middleware:

if(!env.IsDevelopment())
{
    app.UseExceptionHandler("/error/500");
}

So what does this middleware do?

Well, put simply it:

Calls the next middleware with a try-catch block
If an exception is caught, the next middleware is called again with the request path set to what you gave as an argument

This re-execution means the original URL is preserved in the browser.

The controller and action that is implemented looks like this:

[Route("error")]
public class ErrorController : Controller
{
    private readonly TelemetryClient _telemetryClient;

    public ErrorController(TelemetryClient telemetryClient)
    {
        _telemetryClient = telemetryClient;
    }

    [Route("500")]
    public IActionResult AppError()
    {
        var exceptionHandlerPathFeature = HttpContext.Features.Get<IExceptionHandlerPathFeature>();
        _telemetryClient.TrackException(exceptionHandlerPathFeature.Error);
        _telemetryClient.TrackEvent("Error.ServerError", new Dictionary<string, string>
        {
            ["originalPath"] = exceptionHandlerPathFeature.Path,
            ["error"] = exceptionHandlerPathFeature.Error.Message
        });
        return View();
    }
}

I took a look at the source code of the exception handler middleware,
and found that it sets this IExceptionHandlerPathFeature on the context before re-executing the request.

Here we access it to get the relative URL the user tried to access and the exception that occurred.
We use Application Insights to track the exception.

The view is pretty simple:

@{
    ViewBag.Title = "Error occurred";
}

<h1>We have a problem</h1>

<p>Sorry, an error occurred while executing your request.</p>

Now when an exception is thrown:

The exception is logged in Application Insights
User gets a nice-looking view instead of a stack trace
The original URL is preserved in the browser so the user can try to refresh
The response comes back with a 500 status code so it is tracked as a failed request in Application Insights

Handling 404s

We also want to handle 404 status codes gracefully.

In this blog, it could happen because the URL did not map to a controller action.
Or it might have, but we could not find something in the database.

404s are handled with a small middleware that is placed before the MVC middleware:

app.Use(async (ctx, next) =>
{
    await next();

    if(ctx.Response.StatusCode == 404 && !ctx.Response.HasStarted)
    {
        //Re-execute the request so the user gets the error page
        string originalPath = ctx.Request.Path.Value;
        ctx.Items["originalPath"] = originalPath;
        ctx.Request.Path = "/error/404";
        await next();
    }
});

Re-executing a request is not hard as you can see.
We just call await next(); again.

Here we grab the original URL and put it in the HttpContext.Items collection.

This is the action that then handles the error in the ErrorController introduced earlier:

[Route("404")]
public IActionResult PageNotFound()
{
    string originalPath = "unknown";
    if (HttpContext.Items.ContainsKey("originalPath"))
    {
        originalPath = HttpContext.Items["originalPath"] as string;
    }
    _telemetryClient.TrackEvent("Error.PageNotFound", new Dictionary<string, string>
    {
        ["originalPath"] = originalPath
    });
    return View();
}

We track the event with a custom event in Application Insights, and include the original URL in the properties.

The view is quite simple again:

@{
    ViewBag.Title = "404";
}

<h1>404 - Page not found</h1>

<p>Oops, better check that URL.</p>

Go ahead, try accessing this link to see how it looks like: Test 404.

If you open the browser DevTools, you can again see we get the right status code: 404.

And the original URL is preserved in the browser.

Conclusions

Thanks for reading!

Feel free to add a comment if you have questions or improvement suggestions.

Official error handling docs can be found here: docs.asp.net.

Источник

Обработка ошибок

UseDeveloperExceptionPage

UseExceptionHandler

Обработка ошибок HTTP

Настройка обработки ошибок в web.config

Handle errors in ASP.NET Core

Developer exception page

Exception handler page

Access the exception

Exception handler lambda

UseStatusCodePages

UseStatusCodePages with format string

UseStatusCodePages with lambda

UseStatusCodePagesWithRedirects

UseStatusCodePagesWithReExecute

Disable status code pages

Exception-handling code

Response headers

Server exception handling

Startup exception handling

Database error page

Exception filters

Model state errors

Problem details

Customize problem details

CustomizeProblemDetails operation

Custom IProblemDetailsWriter

Problem details from Middleware

Produce a ProblemDetails payload for exceptions

Additional resources

Developer exception page

Exception handler page

Access the exception

Exception handler lambda

UseStatusCodePages

UseStatusCodePages with format string

UseStatusCodePages with lambda

UseStatusCodePagesWithRedirects

UseStatusCodePagesWithReExecute

Disable status code pages

Exception-handling code

Response headers

Server exception handling

Startup exception handling

Database error page

Exception filters

Model state errors

Additional resources

Developer Exception Page

Exception handler page

Access the exception

Exception handler lambda

UseStatusCodePages

UseStatusCodePages with format string

UseStatusCodePages with lambda

UseStatusCodePagesWithRedirects

UseStatusCodePagesWithReExecute

Disable status code pages

Exception-handling code

Response headers

Server exception handling

Startup exception handling

Database error page

Exception filters

Model state errors

Additional resources

Developer Exception Page

Exception handler page

Access the exception

Exception handler lambda

UseStatusCodePages

UseStatusCodePages with format string

UseStatusCodePages with lambda

UseStatusCodePagesWithRedirects

UseStatusCodePagesWithReExecute

Disable status code pages

Exception-handling code

Response headers

Server exception handling

Startup exception handling

`CustomizeProblemDetails` operation

Custom `IProblemDetailsWriter`