如何在C＃中使用基本身份验证在github上创建要点

我使用C＃.net 4.0，我想在github上使用我的用户名和密码（使用基本身份验证）创建一个公共要点。http://developer.github.com/v3/auth/#basic-authentication

如果使用正确的密码，则会收到以下响应： The remote server returned an error: (404) Not Found.

如果我使用了错误的密码，则会收到响应： The remote server returned an error: (401) Unauthorized.

我如何在用户名下验证和发布公共要点？

http://developer.github.com/v3/gists/#create-a-gist。我尝试发送的json：

{
    "description": "the description for this gist",
    "public": true,
    "files": {
        "file1.txt": {
            "content": "my new content "
        }
    }
}

C＃.NET 4.0代码

private void button_createGist_Click(object sender, EventArgs e)
    {
        String jsonMessage = "{ \"description\": \"the description for this gist\",  \"public\": true,"
                   + "\"files\": {   \"file1.txt\": {"
                   + "\"content\":\"my new content \"  } }}";

        String _url = "https://api.github.com/gists/";

        HttpWebRequest req = WebRequest.Create(new Uri(_url)) as HttpWebRequest;

        String userName = "myUserName";
        String userPassword = "pass123";  
        string authInfo = userName + ":" + userPassword;
        authInfo = Convert.ToBase64String(Encoding.Default.GetBytes(authInfo));

        req.Method = "POST";
        req.ContentType = "application/json";
        // req.Headers.Add(string.Format("Authorization: token {0}", oauthToken));
         //req.Headers.Add("Authorization", "Basic " + Convert.ToBase64String(new ASCIIEncoding().GetBytes(username + ":" + password)));
        req.Headers.Add("Authorization", "Basic " + authInfo);
        StreamWriter writer = new StreamWriter(req.GetRequestStream());

        System.Diagnostics.Debug.WriteLine(jsonMessage);
        writer.Write(jsonMessage);
        writer.Close();

        string result = null;
        using (HttpWebResponse resp = req.GetResponse() as HttpWebResponse)//Exception here
        {
            StreamReader reader =
                new StreamReader(resp.GetResponseStream());
            result = reader.ReadToEnd();
            System.Diagnostics.Debug.WriteLine(result);
        }
    }