Modify middleware response

Question

My requirement: write a middleware that filters all \"bad words\" out of a response that comes from another subsequent middleware (e.g. Mvc).

The problem: streaming

Answer 1

Unfortunately I'm not allowed to comment since my score is too low. So just wanted to post my extension of the excellent top solution, and a modification for .NET Core 3.0+

First of all

context.Request.EnableRewind();

has been changed to

context.Request.EnableBuffering();

in .NET Core 3.0+

And here's how I read/write the body content:

First a filter, so we just modify the content types we're interested in

private static readonly IEnumerable validContentTypes = new HashSet() { "text/html", "application/json", "application/javascript" };

It's a solution for transforming nuggeted texts like [[[Translate me]]] into its translation. This way I can just mark up everything that needs to be translated, read the po-file we've gotten from the translator, and then do the translation replacement in the output stream - regardless if the nuggeted texts is in a razor view, javascript or ... whatever. Kind of like the TurquoiseOwl i18n package does, but in .NET Core, which that excellent package unfortunately doesn't support.

...

if (modifyResponse)
{
    //as we replaced the Response.Body with a MemoryStream instance before,
    //here we can read/write Response.Body
    //containing the data written by middlewares down the pipeline

    var contentType = context.Response.ContentType?.ToLower();
    contentType = contentType?.Split(';', StringSplitOptions.RemoveEmptyEntries).FirstOrDefault();   // Filter out text/html from "text/html; charset=utf-8"

    if (validContentTypes.Contains(contentType))
    {
        using (var streamReader = new StreamReader(context.Response.Body))
        {
            // Read the body
            context.Response.Body.Seek(0, SeekOrigin.Begin);
            var responseBody = await streamReader.ReadToEndAsync();

            // Replace [[[Bananas]]] with translated texts - or Bananas if a translation is missing
            responseBody = NuggetReplacer.ReplaceNuggets(poCatalog, responseBody);

            // Create a new stream with the modified body, and reset the content length to match the new stream
            var requestContent = new StringContent(responseBody, Encoding.UTF8, contentType);
            context.Response.Body = await requestContent.ReadAsStreamAsync();//modified stream
            context.Response.ContentLength = context.Response.Body.Length;
        }
    }

    //finally, write modified data to originBody and set it back as Response.Body value
    ReturnBody(context.Response, originBody);
}
...

private void ReturnBody(HttpResponse response, Stream originBody)
{
    response.Body.Seek(0, SeekOrigin.Begin);
    response.Body.CopyToAsync(originBody);
    response.Body = originBody;
}