utilReplaceInRequest does not replace strings in HTTP request body in FiddlerCore

Question

utilReplaceInRequest does not seem to be working for me in FiddlerCore.

I'm uploading a file to a cloud service (Microsoft Azure). I can see that the body of the associated HTTP request contains the file contents. I need to replace some of those contents (e.g. in the HTTP request body) before the file is uploaded.

I've created a Visual Studio C# program and integrated with FiddlerCore. I'm using the utilReplaceInRequest method as per Fiddler's documentation:

                 outcome = session.utilReplaceInRequest(original_str, replace_str);
                Console.WriteLine("result: " + outcome + "\n");

What is happening is utilReplaceInRequest "appears" to work in that the outcome variable is set to True the multiple times I'm calling utilReplaceInRequest. (I need to replace multiple strings in the body, so I call utilReplaceInRequest multiple times). But when I check the actual file contents in the cloud service, they are the original strings. Nothing has been replaced. I even tried invoking session.GetRequestBodyAsString() after calling utilReplaceInRequest in FiddlerCore, and see that the strings have indeed been "changed" in the body. But that "body" is just not being sent to the cloud service.

But interestingly enough--when I do the same thing in Fiddler itself (customize the Rules in the "static function OnBeforeRequest(oSession: Session) { }" function), it works, and the modified HTTP body is being sent to the cloud service!

Is there some configuration option that I need to set (perhaps in my C# program) to make FiddlerCore work like Fiddler?

Thank you.

Stan

Answer 1

You can try setting:

session.bBufferResponse = true;

as described in the FiddlerCore demos here.