Rewriting ASP.NET Core response duplicates the content instead of replacing the original stream

Question

I have an ASP.NET Core application with a custom middleware designed to replace some external CDN links. The middleware correctly updates the response body with the new data, but keeps appending it to the old version. It effectively shows two web pages (the original followed by the modified page).

This is the code for the middleware:

public async Task InvokeAsync(HttpContext context)
{
    using var buffer = new MemoryStream();
    var originalStream = context.Response.Body;
    context.Response.Body = buffer;

    await _next.Invoke(context);

    buffer.Seek(0, SeekOrigin.Begin);

    // Rewrite the body content
    var bufferReader = new StreamReader(buffer);
    string body = await bufferReader.ReadToEndAsync();
    var modifiedBody = RewriteCdnUrls(body);

    // Replace the old content
    await context.Response.WriteAsync(modifiedBody);
    context.Response.Body.Seek(0, SeekOrigin.Begin);

    await context.Response.Body.CopyToAsync(originalStream);
    context.Response.Body = originalStream;
}

I must be missing something fairly obvious but I just can't seem to see it at the moment.

Answer 1

You need to Clear the response

public async Task InvokeAsync(HttpContext context)
{
    using var buffer = new MemoryStream();
    var originalStream = context.Response.Body;
    context.Response.Body = buffer;

    await _next.Invoke(context);

    buffer.Seek(0, SeekOrigin.Begin);

    // Rewrite the body content
    var bufferReader = new StreamReader(buffer);
    string body = await bufferReader.ReadToEndAsync();
    var modifiedBody = RewriteCdnUrls(body);

    // this line below
    context.Response.Clear(); 
    // this line above

    // Replace the old content
    await context.Response.Body.CopyToAsync(originalStream);
    context.Response.Body = originalStream;
}