How to modify HttpContext.Request.Form in asp.net core

Question

I have an HttpContext.Request object that has data in the Form that is wrong and I want to fix it up and send the correct HttpContext on its way. HttpContext.Request.Form is readonly, but if it wasn't I would have simply done the following; HttpContext.Request.Form["a"] = "the correct value for a";

So, where is the best place in the pipeline to do this. Is it possible to make the HttpContext.Request.Form write accessable via reflection?

Answer 1

This was easier than I thought. I am doing this in my middleware which is there to correct bad form data that came in.

public async Task Invoke(HttpContext context)
{
    ....
    NameValueCollection fcnvc = context.Request.Form.AsNameValueCollection();
    fcnvc.Set("a", "the correct value of a");
    fcnvc.Set("b", "a value the client forgot to post");
    Dictionary<string, StringValues> dictValues = new Dictionary<string, StringValues>();
    foreach (var key in fcnvc.AllKeys)
    {
      dictValues.Add(key, fcnvc.Get(key));
    }
    var fc = new FormCollection(dictValues);
    context.Request.Form = fc;
    ....
    await _next.Invoke(context);
}

Interestingly the FormCollection is readonly, but the HttpContext.Request object is not thus allowing me to replace the entire Form.