AspNetCore.Docs/aspnetcore/fundamentals/error-handling.md

title	author	description	monikerRange	ms.author	ms.custom	ms.date	no-loc	uid
Handle errors in ASP.NET Core	rick-anderson	Discover how to handle errors in ASP.NET Core apps.	>= aspnetcore-2.1	riande	mvc	12/05/2019	Home Privacy Kestrel appsettings.json ASP.NET Core Identity cookie Cookie Blazor Blazor Server Blazor WebAssembly Identity Let's Encrypt Razor SignalR	fundamentals/error-handling

Handle errors in ASP.NET Core

::: moniker range=">= aspnetcore-5.0"

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-csharp]

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 should not 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 is not 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-csharp]

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 code adds ExceptionMessage to the default Pages/Error.cshtml.cs generated by the ASP.NET Core templates:

[!code-csharp]

[!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

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-csharp]

[!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 encounters 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-csharp]

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-csharp]

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-csharp]

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-csharp]

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. For a Razor Pages example, see Pages/MyStatusCode.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.

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-csharp]

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-csharp]

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-csharp]

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 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-csharp]

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, UseExceptionHandler. We recommend using UseExceptionHandler, unless you need to perform error handling differently based on which MVC action is chosen.

[!code-csharp]

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-csharp]

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-csharp]

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-csharp]

[!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

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.

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

[!code-csharp]

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-csharp]

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-csharp]

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-csharp]

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-csharp]

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-csharp]

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:

@page "{code?}"

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

[!code-csharp]

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.

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.

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

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