[ADVANCED-DOTNET] Problems with GetRequestStream() from HttpWebRequest

[Manuel Aschwanden]

Hi all
I try to program something like the Achilles 0.27 Proxy and so far I
managed already quite a lot. But now I have a serious problem that needs
your help to be solved.
The code is the following
public void AddPostData(HttpWebRequest request)
  {
   if (_postData != null && _postData.Length > 0)
   {
    // Set the length of the post data
    //MA:PostData.Length is is not allowed to
be changed if we are in the intercept mode


//    if(!SSLProxy.InterceptModeIsOn)
//    {
     request.ContentLength =
_postData.Length;
//    }
    // Copy all post data to the request
    Stream requestStream = null;
    try
    {
     request.Pipelined=false;
     requestStream =
request.GetRequestStream();
    requestStream.Write(_postData, 0,
_postData.Length);
    }

/////in the main program the header of the HttpWebRequest instance that is
handed over to request is manipuled (this means I use .Headers.clear() and
Headers.add(,)). When I now watch what happens when I step through the
commands, I see that before the execution of the request.GetRequestStream()
the local variables like request.Expect have the new (manipulated) value.
But after the execution of request.GetRequestStream() some variables like
request.Expect change. Some observed changes are:
request.DelegationFix={System.Net.DelegationFix}
request.HaveWritten=true  //was before false
request.Expect="new,100-continue" //was before "new"

Since the requestStream has now this "wrong" values, the header
manipulation is not possible as wanted.
I hope someone can tell me what I have to do that request is not changed
when I call request.GetRequestStream();
Thank you in advance.
Manuel