Modifying a response with FiddlerCore

Environment

Product	FiddlerCore
Product Version	5.0.1

Description

Modifying a Response with FiddlerCore

To modify an HTTP response, ensure the session is set to buffered mode.

Use some of the early events of the response, for example, the ResponseHeadersAvailable to access the session object and set the bBufferResponse property to true. If the property is not set to true, in the BeforeResponse event, you'll receive a copy of the response, while it will be streamed to the client as well. So your modifications will not be applied to the response that the client will receive.

private static void FiddlerApplication_ResponseHeadersAvailable(Session oSession)
{

    if (oSession.fullUrl.Contains("example.com"))
    {
        /*
        Set this to true, so in BeforeResponse, you'll be able to modify the body.
        If the value is false (default one), the response you'll work with within the BeforeResponse handler will be just a copy. 
        The original one will already be streamed to the client, and all of your modifications will not be visible there.
        */
        oSession.bBufferResponse = true;
    }
}

Then you can handle the BeforeResponse event and modify the session's response as per your requirements.

private static void FiddlerApplication_BeforeResponse(Session oSession)
{
    if (oSession.fullUrl.Contains("example.com") && oSession.HTTPMethodIs("GET"))
    {
        oSession.bBufferResponse = true;
        oSession.utilDecodeResponse();

        // Remove any compression or chunking
        oSession.utilDecodeResponse();
        var oBody = System.Text.Encoding.UTF8.GetString(oSession.responseBodyBytes);

        // Modify the body as you want
        oBody = "Replaced body";

        // Set the response body to the div-less string
        oSession.utilSetResponseBody(oBody);
    }
}