Understanding `UseExceptionHandler` in ASP.NET Core Web API

Question

I have an exception handling middleware that looks like this:

public class ExceptionHandlerMiddleware : IExceptionHandler
{
    public async ValueTask<bool> TryHandleAsync(HttpContext httpContext, Exception exception, CancellationToken cancellationToken)
    {
        httpContext.Response.Clear();
        httpContext.Response.StatusCode = GetStatusCode(exception);

        await httpContext.Response.WriteAsJsonAsync
        (
            new ProblemDetails
            {
                Title = "An error occurred",
                Detail = GetErrorMessage(exception)
            },
            JsonSerializerOptions.Default,
            "application/problem+json",
            cancellationToken
        );

        return true;
    }

    /* ... Other stuff ... */
}

In my Startup.cs, this is the code that calls UseExceptionHandler (as recommended by .NET documentation):

app.UseExceptionHandler(appError =>
{
    appError.Run(async context =>
    {
        var exceptionHandler = context.RequestServices.GetRequiredService<IExceptionHandler>();
        var exceptionFeature = context.Features.Get<IExceptionHandlerFeature>();

        if (exceptionFeature != null)
        {
            await exceptionHandler.TryHandleAsync(context, exceptionFeature.Error, default);
        }
    });
});

However, I don't understand why this lambda method is required. A breakpoint inside it will never be hit, and the code works exactly the same way if I change it to this:

app.UseExceptionHandler(_ => { });

My questions:

What is the point of having a lambda method when it never seems to run?
Why does app.UseExceptionHandler(); (without even an empty lambda method) throw a runtime error, even though the compiler allows it?

Michał Turczyn · Accepted Answer · 2024-08-03 17:10:57Z

3

As documentation linked by you states:

If an exception handler handles a request, it can return true to stop processing. If an exception isn't handled by any exception handler, then control falls back to the default behavior and options from the middleware.

You are returning always true, so you are marking the exception as handled. If you would return false your lambda passed in UseExceptionHandler would be executed.

edited Aug 3, 2024 at 17:10

answered Aug 2, 2024 at 5:48

Michał Turczyn

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