ASP.NET Web API OperationCanceledException when browser cancels the request

This is a bug in ASP.NET Web API 2 and unfortunately, I don't think there's a workaround that will always succeed. We filed a bug to fix it on our side.

Ultimately, the problem is that we return a cancelled task to ASP.NET in this case, and ASP.NET treats a cancelled task like an unhandled exception (it logs the problem in the Application event log).

In the meantime, you could try something like the code below. It adds a top-level message handler that removes the content when the cancellation token fires. If the response has no content, the bug shouldn't be triggered. There's still a small possibility it could happen, because the client could disconnect right after the message handler checks the cancellation token but before the higher-level Web API code does the same check. But I think it will help in most cases.

David

config.MessageHandlers.Add(new CancelledTaskBugWorkaroundMessageHandler());class CancelledTaskBugWorkaroundMessageHandler : DelegatingHandler{    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)    {        HttpResponseMessage response = await base.SendAsync(request, cancellationToken);        // Try to suppress response content when the cancellation token has fired; ASP.NET will log to the Application event log if there's content in this case.        if (cancellationToken.IsCancellationRequested)        {            return new HttpResponseMessage(HttpStatusCode.InternalServerError);        }        return response;    }}

When implementing an exception logger for WebApi, it is recommend to extend the System.Web.Http.ExceptionHandling.ExceptionLogger class rather than creating an ExceptionFilter. The WebApi internals will not call the Log method of ExceptionLoggers for canceled requests (however, exception filters will get them). This is by design.

HttpConfiguration.Services.Add(typeof(IExceptionLogger), myWebApiExceptionLogger); 

Here's an other workaround for this issue. Just add a custom OWIN middleware at the beginning of the OWIN pipeline that catches the OperationCanceledException:

#if !DEBUGapp.Use(async (ctx, next) =>{    try    {        await next();    }    catch (OperationCanceledException)    {    }});#endif