System.Net.WebClient "Unable to write data to the transport connection"

I am trying to do a plain multipart form upload using System.Net.WebClient and Basic Authentication.

I have had some trouble with this and have been using Fiddler2 for debugging some 401 errors I have been having with the service.

I have arrived at the code below, which succeeds while Fiddler2 is running, but fails when it does not, with the following error:

"Unable to write data to the transport connection: An existing connection was forcibly closed by the remote host."

Code is as follows:

    var wc = new MyWebClient();
    var cc = new System.Net.CredentialCache();
    cc.Add(new Uri(uri), "Basic", new System.Net.NetworkCredential(user, pass));
    wc.Credentials = cc;
    System.Net.ServicePointManager.Expect100Continue = false;
    wc.UploadFile(uri +  folder, file);
    wc.DownloadString(uri + folder).Dump();
    return;

class MyWebClient : System.Net.WebClient
{
    protected override System.Net.WebReq开发者_如何转开发uest GetWebRequest(Uri address)
    {
        System.Net.WebRequest request = base.GetWebRequest(address);
        if (request is System.Net.HttpWebRequest)
        {
            (request as System.Net.HttpWebRequest).KeepAlive = false;
        }
        return request;
    }
}

You are disabling keep alive while you are using client twice. I believe that is your problem.

What happens if you remove the download bit, only keeping upload?

UPDATE

I know that using non-basic/digest involves a round trip and will not work with keep-alive set to false but not sure about basic.