--- title: Handle errors in ASP.NET Core author: tdykstra description: Discover how to handle errors in ASP.NET Core apps. monikerRange: '>= aspnetcore-2.1' ms.author: tdykstra ms.custom: mvc ms.date: 05/03/2019 uid: fundamentals/error-handling --- # Handle errors in ASP.NET Core By [Tom Dykstra](https://github.com/tdykstra/), [Luke Latham](https://github.com/guardrex), and [Steve Smith](https://ardalis.com/) This article covers common approaches to handling errors in ASP.NET Core apps. [View or download sample code](https://github.com/aspnet/AspNetCore.Docs/tree/master/aspnetcore/fundamentals/error-handling/samples). ([How to download](xref:index#how-to-download-a-sample).) The article includes instructions about how to set preprocessor directives (`#if`, `#endif`, `#define`) in the sample app to enable different scenarios. ## Developer Exception Page The *Developer Exception Page* displays detailed information about request exceptions. The page is made available by the [Microsoft.AspNetCore.Diagnostics](https://www.nuget.org/packages/Microsoft.AspNetCore.Diagnostics/) package, which is in the [Microsoft.AspNetCore.App metapackage](xref:fundamentals/metapackage-app). Add code to the `Startup.Configure` method to enable the page when the app is running in the Development [environment](xref:fundamentals/environments): [!code-csharp[](error-handling/samples/2.x/ErrorHandlingSample/Startup.cs?name=snippet_DevPageAndHandlerPage&highlight=1-4)] Place the call to before any middleware that you want to catch exceptions. > [!WARNING] > Enable the Developer Exception Page **only when the app is running in the Development environment**. You don't want to share detailed exception information publicly when the app runs in production. For more information on configuring environments, see . The page includes the following information about the exception and the request: * Stack trace * Query string parameters (if any) * Cookies (if any) * Headers To see the Developer Exception Page in the [sample app](https://github.com/aspnet/AspNetCore.Docs/tree/master/aspnetcore/fundamentals/error-handling/samples), use the `DevEnvironment` preprocessor directive and select **Trigger an exception** on the home page. ## 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. In the following example, adds the Exception Handling Middleware in non-Development environments: [!code-csharp[](error-handling/samples/2.x/ErrorHandlingSample/Startup.cs?name=snippet_DevPageAndHandlerPage&highlight=5-9)] The Razor Pages app template provides an Error page (*.cshtml*) and class (`ErrorModel`) in the *Pages* folder. For an MVC app, the project template includes an Error action method and an Error view. Here's the action method: ```csharp [AllowAnonymous] public IActionResult Error() { return View(new ErrorViewModel { RequestId = Activity.Current?.Id ?? HttpContext.TraceIdentifier }); } ``` Don't decorate 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 so that unauthenticated users are able to receive the error view. ### Access the exception Use to access the exception and the original request path in an error handler controller or page: [!code-csharp[](error-handling/samples/2.x/ErrorHandlingSample/Pages/Error.cshtml.cs?name=snippet_ExceptionHandlerPathFeature&3,7)] > [!WARNING] > Do **not** serve sensitive error information to clients. Serving errors is a security risk. To see the exception handling page in the [sample app](https://github.com/aspnet/AspNetCore.Docs/tree/master/aspnetcore/fundamentals/error-handling/samples), use the `ProdEnvironment` and `ErrorHandlerPage` preprocessor directives, and select **Trigger an exception** on the home page. ## Exception handler lambda An alternative to a [custom exception handler page](#exception-handler-page) is to provide a lambda to . 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[](error-handling/samples/2.x/ErrorHandlingSample/Startup.cs?name=snippet_HandlerPageLambda)] > [!WARNING] > Do **not** serve sensitive error information from or to clients. Serving errors is a security risk. To see the result of the exception handling lambda in the [sample app](https://github.com/aspnet/AspNetCore.Docs/tree/master/aspnetcore/fundamentals/error-handling/samples), 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](https://www.nuget.org/packages/Microsoft.AspNetCore.Diagnostics/) package, which is in the [Microsoft.AspNetCore.App metapackage](xref:fundamentals/metapackage-app). To enable default text-only handlers for common error status codes, call in the `Startup.Configure` method: [!code-csharp[](error-handling/samples/2.x/ErrorHandlingSample/Startup.cs?name=snippet_StatusCodePages)] Call `UseStatusCodePages` before request handling middleware (for example, Static File Middleware and MVC Middleware). Here's an example of text displayed by the default handlers: ``` Status Code: 404; Not Found ``` To see one of the various status code page formats in the [sample app](https://github.com/aspnet/AspNetCore.Docs/tree/master/aspnetcore/fundamentals/error-handling/samples), use one of the preprocessor directives that begin with `StatusCodePages`, and select **Trigger a 404** on the home page. ## UseStatusCodePages with format string To customize the response content type and text, use the overload of that takes a content type and format string: [!code-csharp[](error-handling/samples/2.x/ErrorHandlingSample/Startup.cs?name=snippet_StatusCodePagesFormatString)] ## UseStatusCodePages with lambda To specify custom error-handling and response-writing code, use the overload of that takes a lambda expression: [!code-csharp[](error-handling/samples/2.x/ErrorHandlingSample/Startup.cs?name=snippet_StatusCodePagesLambda)] ## UseStatusCodePagesWithRedirect The extension method: * Sends a *302 - Found* status code to the client. * Redirects the client to the location provided in the URL template. [!code-csharp[](error-handling/samples/2.x/ErrorHandlingSample/Startup.cs?name=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 a tilde (~), the tilde 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](https://github.com/aspnet/AspNetCore.Docs/tree/master/aspnetcore/fundamentals/error-handling/samples). 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 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[](error-handling/samples/2.x/ErrorHandlingSample/Startup.cs?name=snippet_StatusCodePagesWithReExecute)] 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](https://github.com/aspnet/AspNetCore.Docs/tree/master/aspnetcore/fundamentals/error-handling/samples). 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: ```cshtml @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[](error-handling/samples/2.x/ErrorHandlingSample/Pages/StatusCode.cshtml.cs?name=snippet_StatusCodeReExecute)] ## Disable status code pages Status code pages can be disabled for specific requests in a Razor Pages handler method or in an MVC controller. To disable status code pages, use the : ```csharp var statusCodePagesFeature = HttpContext.Features.Get(); 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](xref:fundamentals/servers/index) 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](xref:fundamentals/host/web-host#capture-startup-errors) and [capture detailed errors](xref:fundamentals/host/web-host#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](xref:fundamentals/servers/kestrel). When running on [IIS](/iis) or [IIS Express](/iis/extensions/introduction-to-iis-express/iis-express-overview), a *502.5 - Process Failure* is returned by the [ASP.NET Core Module](xref:host-and-deploy/aspnet-core-module) if the process can't start. For more information, see . For information on troubleshooting startup issues with Azure App Service, see . ## Database error page The [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`: ```csharp if (env.IsDevelopment()) { app.UseDatabaseErrorPage(); } ``` ## 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 . > [!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](xref:mvc/models/model-binding) and [Model validation](xref:mvc/models/validation). ## Additional resources * * *