Do not throw a null reference from request.GetDisplayUrl()

[filipw]

This is based on a customer bug. It is possible for a middleware and other participants in the request pipeline to modify things like properties of the request (i.e. PathBase). Then, when request.GetDisplayUrl() is called on the same request, there are no null checks on the request properties needed to compute the URL, which in extreme cases causes it to throw an unhandled exception.

One example of running into this, is a following middleware.

            app.Use((context, next) =>
            {
                context.Request.PathBase = new PathString(context.Request.Headers["x-forwarded-prefix"]);
                return next();
            });

When issuing a request without x-forwarded-prefix that then calls into request.GetDisplayUrl(), we get a null ref.

While in this case it can be argued that the middleware should validate what it is actually doing, in general I don't think a framework method like request.GetDisplayName() should ever throw a null reference.

[Tratcher]

Thanks