Trigger exception handler (with status code) from middleware

Question

I'm trying to trigger an exception (handled by /Error) from a middleware in ASP.NET Core 3.1.

Exception handler is registered in Startup.cs (as shown below) without app.UseDeveloperExceptionPage(), and works well otherwise.

app.UseExceptionHandler("/Error");
app.UseStatusCodePagesWithReExecute("/Error", "?statusCode={0}");

But how do I trigger an exception (with an HTTP status code) from a middleware's Invoke method?

Update (solution): I mainly wanted to raise an exception from a custom middleware, but also pass a status code to the exception handler (/Error). I got it working with this code in the middleware:

public async Task Invoke(HttpContext context)
{
    if (context?.Request?.Path.Value.StartsWith("/error"))
    {
        // allow processing of exception handler
        await _next(context);
        return;
    }

    // skip processing, and set status code for use in exception handler
    context.SetEndpoint(endpoint: null);
    context.Response.StatusCode = 503;
}

Brando Zhang · Accepted Answer · 2021-01-15 08:55:45Z

1

According to the document, 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 the 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.

If you directly throw the exception at the application start, it will not goes to the exception handler.

Like below middleware:

        app.Use(async (context, next) =>
        {
  
                throw new ArgumentException("aaa");
     
            await next();
        });

If you throw the exception is thrown after the application has completely started, it will go to the exception handler like below example. If you type home/privacy in your url, you will find it goes to the exception handler page.

     app.Use(async (context, next) =>
        {
            if (context.Request.Path == "/Home/Privacy")
            {
                throw new ArgumentException("aaa");
            }
            await next();
        });

Result:

edited Jan 15, 2021 at 8:55

answered Jan 15, 2021 at 8:45

Brando Zhang

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1 Comment

Nick Over a year ago

Thanks. I mainly wanted to raise an exception but also pass a status code to the exception handler. I got it working with a slightly different approach (solution in update above).

Nick · Accepted Answer · 2021-01-20 06:19:25Z

0

I mainly wanted to raise an exception from a custom middleware, but also pass a status code to the exception handler (/Error). I got it working with this code in the middleware:

public async Task Invoke(HttpContext context)
{
    if (context?.Request?.Path.Value.StartsWith("/error"))
    {
        // allow processing of exception handler
        await _next(context);
        return;
    }

    // skip processing, and set status code for use in exception handler
    context.SetEndpoint(endpoint: null);
    context.Response.StatusCode = 503;
}

answered Jan 20, 2021 at 6:19

Nick

1,4342 gold badges19 silver badges42 bronze badges

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