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Error Handling in ASP.NET Core

By Steve Smith

When errors occur in your ASP.NET app, you can handle them in a variety of ways, as described in this article.

Sections

  • Configuring an Exception Handling Page
  • Using the Developer Exception Page
  • Configuring Status Code Pages
  • Limitations of Exception Handling During Client-Server Interaction
  • Server Exception Handling
  • Startup Exception Handling
  • ASP.NET MVC Error Handling

View or download sample code

Configuring an Exception Handling Page

You configure the pipeline for each request in the Startup class’s Configure() method (learn more about Application Startup ). You can add a simple exception page, meant only for use during development, very easily. All that’s required is to add a dependency on Microsoft.AspNet.Diagnostics to the project and then add one line to Configure() in Startup.cs :

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

The above code includes a check to ensure the environment is development before adding the call to UseDeveloperExceptionPage . This is a good practice, since you typically do not want to share detailed exception information about your application publicly while it is in production. Learn more about configuring environments .

The sample application includes a simple mechanism for creating an exception:

ic static void HomePage(IApplicationBuilder app)  app.Run(async (context) => {     if (context.Request.Query.ContainsKey("throw"))     { 

If a request includes a non-empty querystring parameter for the variable throw (e.g. a path of /?throw=true ), an exception will be thrown. If the environment is set to Development , the developer exception page is displayed:

Error Handling in ASP.NET Core

When not in development, it’s a good idea to configure an exception handler path using the UseExceptionHandler middleware:

app.UseExceptionHandler("/Error");

Using the Developer Exception Page

The developer exception page displays useful diagnostics information when an unhandled exception occurs within the web processing pipeline. The page includes several tabs with information about the exception that was triggered and the request that was made. The first tab includes a stack trace:

Error Handling in ASP.NET Core

The next tab shows the query string parameters, if any:

Error Handling in ASP.NET Core

In this case, you can see the value of the throw parameter that was passed to this request. This request didn’t have any cookies, but if it did, they would appear on the Cookies tab. You can see the headers that were passed in the last tab:

Error Handling in ASP.NET Core

Configuring Status Code Pages

By default, your app will not provide a rich status code page for HTTP status codes such as 500 (Internal Server Error) or 404 (Not Found). You can configure the StatusCodePagesMiddleware adding this line to the Configure method:

app.UseStatusCodePages();

By default, this middleware adds very simple, text-only handlers for common status codes. For example, the following is the result of a 404 Not Found status code:

Error Handling in ASP.NET Core

The middleware supports several different extension methods. You can pass it a custom lamba expression:

app.UseStatusCodePages(context =>   context.HttpContext.Response.SendAsync("Handler, status code: " +   context.HttpContext.Response.StatusCode, "text/plain"));

Alternately, you can simply pass it a content type and a format string:

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

The middleware can handle redirects (with either relative or absolute URL paths), passing the status code as part of the URL:

app.UseStatusCodePagesWithRedirects("~/errors/{0}");

In the above case, the client browser will see a 302 / Found status and will redirect to the URL provided.

Alternately, the middleware can re-execute the request from a new path format string:

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

The UseStatusCodePagesWithReExecute method will still return the original status code to the browser, but will also execute the handler given at the path specified.

If you need to disable status code pages for certain requests, you can do so using the following code:

var statusCodePagesFeature = context.Features.Get<IStatusCodePagesFeature>(); if (statusCodePagesFeature != null) {   statusCodePagesFeature.Enabled = false; }

Limitations of Exception Handling During Client-Server Interaction

Web apps have certain limitations to their exception handling capabilities because of the nature of disconnected HTTP requests and responses. Keep these in mind as you design your app’s exception handling behavior.

  1. Once the headers for a response have been sent, you cannot change the response’s status code, nor can any exception pages or handlers run. The response must be completed or the connection aborted.
  2. If the client disconnects mid-response, you cannot send them the rest of the content of that response.
  3. There is always the possibility of an exception occuring one layer below your exception handling layer.
  4. Don’t forget, exception handling pages can have exceptions, too. It’s often a good idea for production error pages to consist of purely static content.

Following the above recommendations will help ensure your app remains responsive and is able to gracefully handle exceptions that may occur.

Server Exception Handling

In addition to the exception handling logic in your app, the server hosting your app will perform some exception handling. If the server catches an exception before the headers have been sent it will send a 500 Internal Server Error response with no body. If it catches an exception after the headers have been sent it must close the connection. Requests that are not handled by your app will be handled by the server, and any exception that occurs will be handled by the server’s exception handling. Any custom error pages or exception handling middleware or filters you have configured for your app will not affect this behavior.

Startup Exception Handling

One of the trickiest places to handle exceptions in your app is during its startup. Only the hosting layer can handle exceptions that take place during app startup. Exceptions that occur in your app’s startup can also impact server behavior. For example, to enable SSL in Kestrel, one must configure the server with app.UseKestrelHttps() . If an exception happens before this line in Startup , then by default hosting will catch the exception, start the server, and display an error page on the non-SSL port. If an exception happens after that line executes, then the error page will be served over HTTPS instead.

ASP.NET MVC Error Handling

MVC apps have some additional options when it comes to handling errors, such as configuring exception filters and performing model validation.

Exception Filters

Exception filters can be configured globally or on a per-controller or per-action basis in an MVC app. These filters handle any unhandled exception that occurs during the execution of a controller action or another filter, and are not called otherwise. Exception filters are detailed in filters .

Tip

Exception filters are good for trapping exceptions that occur within MVC actions, but they’re not as flexible as error handling middleware. Prefer middleware for the general case, and use filters only where you need to do error handling differently based on which MVC action was chosen.

Handling Model State Errors

Model validation occurs prior to each controller action being invoked, and it is the action method’s responsibility to inspect ModelState.IsValid and react appropriately. In many cases, the appropriate reaction is to return some kind of error response, ideally detailing the reason why model validation failed.

Some apps will choose to follow a standard convention for dealing with model validation errors, in which case a filter may be an appropriate place to implement such a policy. You should test how your actions behave with valid and invalid model states (learn more about testing controller logic ).

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