Modify static file response in ASP.NET Core

Question

I serve a bunch of static files in my app with app.UseStaticFiles(). I\'d like to inject some additional markup into the response for a particular HTML file bef

Answer 1

Yes, the default stream provided is read only because the data is only buffered for a short moment and flushed to the client. Hence you can't rewind or read it.

Your second attempt doesn't work because the original stream is never processed. You replaced the response body stream entirely with MemoryStream and thrown away the original request, so there is never something written to it and the client waits forever.

You must not forget, that the stream to the client is within the original stream, you can't just replace it with something else.

After calling await next() you must read the data from the MemoryStream and then write it to the original stream.

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

    // Here you must read the MemoryStream, modify it, then write the 
    // result into "originalStream"
});

Remark

But be aware, that this solution will buffer the whole response into the servers memory, so if you send large files this will significantly degenerate the performance of your ASP.NET Core application, especially if you serve files which are several megabytes in size and cause the garbage collection to be triggered more often.

And this wouldn't only affect your static files, but also all your regular requests, because the MVC Middleware is called after the static files middleware.

If you really want to modify a single (or a list of files) on each request, I'd rather suggest you doing this inside a controller and route certain files there. Remember, if the given file is not found by the static files middleware, it will call the next one in chain until it comes to the mvc middleware.

Just setup a route there that will match a specific file or folder and route it to a controller. Read the file in the controller and write it to the response stream or just return the new steam (using return File(stream, contentType);