c# - HttpWebReqest headers not sent depending on order when RequestStream was populated -

i'm writing .net 2 code , forced using it's tools. i'm,trying send put , attach headers (i.e. user-agent). code:

void main() {     var req = (httpwebrequest)webrequest.create("http://requestb.in/xan9icxa");     req.method = "put";     req.contenttype = "application/json";     // fillrequestbody("{'c':'d'}", req);     req.useragent = "linqpad/4";     fillrequestbody("{'a':'b'}", req);     var resp = (httpwebresponse) req.getresponse();     //resp.dump(); }  private static void fillrequestbody(string contentasstring, webrequest request) {   var databytes = encoding.utf8.getbytes(contentasstring);   request.contentlength = databytes.length;   using (var requeststream = request.getrequeststream())       requeststream.write(databytes, 0, databytes.length); } 

that runs fine , i'm getting headers expected:

user-agent: linqpad/4 content-length: 9 connection: close content-type: application/json 

but when run fillrequestbody("{'c':'d'}", req); instead of fillrequestbody("{'a':'b'}", req); (which tried , spent time trying figure out why) - user agent not sent:

content-type: application/json connection: close content-length: 9 

the difference order of call when i'm filling request stream data: before setting user-agent header after it.

my question why happening - there documentation says filling body of request has last thing before sending http message?