Unable to send modified HttpResponse back to Response.Body - asp.net-core

Question

I am creating a proxy using middleware in ASP.NET Core 2.1 that makes 3rd party API (OData endpoint) call to
Get data
Do some changes
Send response to Response.Body
I took a reference from here
Below is the code snippet that works fine as whatever response I am getting from API, I am sending it further
using (var responseMessage = await _httpClient.SendAsync(targetRequestMessage, HttpCompletionOption.ResponseHeadersRead, context.RequestAborted))
{
context.Response.StatusCode = (int)responseMessage.StatusCode;
CopyFromTargetResponseHeaders(context, responseMessage);
await responseMessage.Content.CopyToAsync(context.Response.Body);
}
However, If I modify the response here, for example, like this, it does not work and it shows blank page without any error.
using (var responseMessage = await _httpClient.SendAsync(targetRequestMessage, HttpCompletionOption.ResponseHeadersRead, context.RequestAborted))
{
context.Response.StatusCode = (int)responseMessage.StatusCode;
CopyFromTargetResponseHeaders(context, responseMessage);
var output = new StringContent("some sample string or may be JSON", Encoding.UTF8, "application/json");
await output.CopyToAsync(context.Response.Body);
}
It looks like we are not allowed to make any change in the response received from API call. Can anyone please tell me how can send modified content back to Response.Body?

Answer 1

I am able to solve the problem by updating "Content-Length" response header before rendering modified response to context.Response.Body something like this:
context.Response.Headers.Remove("Content-Length");
context.Response.Headers.Add("Content-Length", modifiedResponseStream.Length.ToString());

Answer 2

You might run into a System.InvalidOperationException: Response Content-Length mismatch: too few bytes written or similar exception (which you should see in the Output window). So do not use the Content-Length and maybe Content-Type headers from the response, because they probably don't match with the Content-Length and Content-Type of your modified content, e.g.:
private void CopyFromTargetResponseHeaders(HttpContext context, HttpResponseMessage responseMessage)
{
...
foreach (var header in responseMessage.Content.Headers)
{
// do not use the content headers from the response because the content will be modified
// context.Response.Headers[header.Key] = header.Value.ToArray();
}
...
}