Intercept 401 error in ASP.net Core 2.0 pipeline

Question

In my ASP.net Core 2.0 pipeline, I have the following two configuration entries:

app.UseIdentityServer();
app.UseMvc(...);

I wasn't sure which of these generates 401 responses, so I added some custom middleware to see if it was possible to intercept them at various stages:

app.Use(async (http, next) =>
{
    if (http.Response.StatusCode == StatusCodes.Status401Unauthorized))
    {
        Console.WriteLine("intercept!");
    }
});

This code would fire after if placed after UseIdentityServer(), but not with the 401, and would not fire at all if placed after UseMvc() in a 401 scenario. That is, UseMvc() appears to be ending the pipeline.

Since in one case the error hasn't happened, and in the other it's never reached, how can I go about intercepting these 401s? (And potentially rewriting them)

I also tried a try-catch around the pipeline continuation, placed very early in the pipeline, but this didn't fire either:

app.Use(async (http, next) =>
{
    try
    {
        await next();
    }
    catch (Exception ex)
    {
        Console.WriteLine("intercept!");
    }
});

Any ideas on how I can put that intercept in place?

Nish26 Nish26 · Accepted Answer · 2018-03-28T18:07:35

Use this middleware before anything else :

app.Use(async (context, next) =>
{
    await next.Invoke();

    if(context.Response.StatusCode == StatusCodes.Status404NotFound)
    {
        await context.Response.WriteAsync("You seem to have mistyped the url");
    }              
});

There won't be any exception for Not found. Default value of StatusCode for a context response is 404. If no middleware modifies the response , 404 will be returned to the client. In this case, we have set up a middleware that allows other middlewares to process the request first by awaiting next.Invoke()..Now, while returning back it checks if StatusCode is still 404 and writes a "you seem to have mistyped the url" message.

Intercept 401 error in ASP.net Core 2.0 pipeline

1 Answers