ExceptionHandlerMiddleware will throw original exception if exception handler cannot be found

[JunTaoLuo]

ExceptionHandlerMiddleware will throw original exception if exception handler cannot be found

Currently, the ExceptionHandlerMiddleware will execute the configured exception handler when an exception has occurred. If the exception handler, configured via ExceptionHandlerOptions.ExceptionHandlingPath for example, cannot be found, a 404 response will be produced. The 404 error is especially misleading since it makes it seem like an user error and obscures the fact that an exception occurred on the server.

To resolve this, the default behavior of ExceptionHandlerMiddleware will now throw the original exception if the exception handler could not be found. As a result, a 500 response will be produced by the server and it will be more obvious to examine the server logs when debugging the error that occurred.

Version introduced

ASP.NET Core 5.0 RC 1

Old behavior

The ExceptionHandlerMiddleware will currently produce a 404 error response when the configured exception handler could not be found when the middleware executes.

New behavior

By default, the ExceptionHandlerMiddleware will now throw the original exception when the configured exception handler could not be found (i.e. the exception handler returns a 404) when the middleware executes. To allow 404 responses to be returned by the exception handler, please configure the option AllowStatusCode404Response on ExceptionHandlerOptions to true. When this option is set to true, the current behavior is maintained.

Reason for change

We have often seen ExceptionHandlerMiddleware with misconfigured handlers producing 404 responses leading to confusion since the 404 error doesn't make it obvious an exception occurred on the server. This change is made so that a 500 error will be produced instead to make it more obvious that it's not caused by user error but rather an exception was encountered on the server.

Recommended action

There is no API change due to this breaking change so all existing apps will continue to compile and run. The exception thrown will be handled by the server. For example, the exception will be converted to a 500 error response by Kestrel HTTP server or HTTP.SYS server.

For backwards compatibility using the current behavior, where a 404 response from the exception handler is allowed, please set AllowStatusCode404Response on ExceptionHandlerOptions to true. See "New behavior" section for details.

Category

ASP.NET

Affected APIs

"Not detectable via API analysis"

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Issue metadata

• Issue type: breaking-change

See discussion at: dotnet/aspnetcore#25288