Why am I running out of bytes for the stream while performing an HTTP POST?

This is driving me nuts:

    WebRequest request = WebRequest.Create(url);
    request.Method = "POST";
    request.ContentType = "application/x-www-form-urlencoded";
    request.ContentLength = Encoding.UTF8.GetByteCount(data);
    Stream reqs = request.GetRequestStream();
    StreamWriter stOut = new StreamWriter(reqs, Encoding.UTF8);
    stOut.Write(data);
    stOut.Flush();

I get an exception that I've run out of bytes in the stream...but I've used the same encoding t开发者_C百科o get the byte count!

Using ASCII this doesn't fail. Is this because of the UTF-8 BOM that Windows likes to add in?

This is probably the BOM; try using an explicit encoding without a BOM:

Encoding enc = new UTF8Encoding(false);
...
request.ContentLength = enc.GetByteCount(data);
...
StreamWriter stOut = new StreamWriter(reqs, enc);

Even easier; switch to WebClient instead and of trying to handle it all yourself; it is very easy to post a form with this:

    using (var client = new WebClient())
    {
        var data = new NameValueCollection();
        data["foo"] = "123";
        data["bar"] = "456";
        byte[] resp = client.UploadValues(address, data);
    }

Or with the code from here:

  byte[] resp = client.Post(address, new {foo = 123, bar = 546});

You can also try something like this:

byte[] bytes = Encoding.UTF8.GetBytes(data);

request.ContentLength = bytes.Length;
request.GetRequestStream().Write(bytes, 0, bytes.Length);

Don't forget to actually URL-encode the data, like you promised in the ContentType. It's a one-liner:

byte[] bytes = System.Web.HttpUtility.UrlEncodeToBytes(data, Encoding.UTF8);