ASPNET CORE Middleware strange exception

Question

I've created a middleware delegate in a brand new blazor-server project, to transform my html responses. See code below.


            app.Use(async (context, next) =>
            {
                var originalStream = context.Response.Body;// currently holds the original stream                    
                var newStream = new MemoryStream();
                context.Response.Body = newStream;

                await next(); 

                string contentType1 = context.Response.ContentType?.Split(';', StringSplitOptions.RemoveEmptyEntries)[0];
                string contentType2 = "text/html";

                if (contentType1 == "text/html") // EXCEPTION THROWN, SEE IMAGE 1
                //if (contentType2 == "text/html") // EXCEPTION NOT THROWN, SEE IMAGE 2
                {
                    newStream.Seek(0, SeekOrigin.Begin);
                    var originalContent = new StreamReader(newStream).ReadToEnd();
                    var updatedContent = originalContent.Replace("Hello", "HOLA");

                    context.Response.Body = originalStream;
                    await context.Response.WriteAsync(updatedContent);
                }
            });

But when I compare a string extracted from the Response.ContentType, I'm getting an exception (image 1). But when comparing another string with the same value, I'm not getting it (image 2) In both cases the page is rendered with the replacement. Any help on this one?

Image 1

Image 2

Did you try to disable the BrowserLink? Here's how to do that https://learn.microsoft.com/en-us/aspnet/core/client-side/using-browserlink?view=aspnetcore-3.1 — Nouman
– Nouman, Commented Mar 5, 2020 at 17:55

Juan · Accepted Answer · 2020-03-06 10:36:20Z

0

Actually, the Exception occurs when the response content type is NOT "text/html" (of course, in other requests)

The real cause is that I'm assigning the body a new stream but never restoring the originalStream, which is wrong.

But even assigning it afterwards does not work. This is the final solution I've managed to work, please, see the comments en the IF statements.

            app.Use(async (context, next) =>
            {
                var originalStream = context.Response.Body;           
                var newStream = new MemoryStream();
                context.Response.Body = newStream;

                await next();
                string contentType = context.Response.ContentType?.Split(';', StringSplitOptions.RemoveEmptyEntries)[0];
                //if (contentType == "text/html") //THIS IF -at this level- is still throwing the exceptions for NO text/hteml content type.
                //{

                    newStream.Seek(0, SeekOrigin.Begin);
                    var originalContent = new StreamReader(newStream).ReadToEnd();

                    if (contentType == "text/html") //THIS IF at these level, makes the replacement if needed, but it does not avoid the "Streams operations"
                    {
                        originalContent = originalContent.Replace("Hello", "HOLA");
                    }

                    var memoryStreamModified = GenerateStreamFromString(originalContent);
                    await memoryStreamModified.CopyToAsync(originalStream).ConfigureAwait(false);
                //}
                context.Response.Body = originalStream;
            });

edited Mar 6, 2020 at 10:36

answered Mar 6, 2020 at 9:16

Juan

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1 Comment

Juan Over a year ago

BTW, I was trying to use the solutions proposed in here: stackoverflow.com/questions/44508028/modify-middleware-response/…

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