（400）动态调用asmx webmethod时出现错误请求

Question

当我尝试从asmx webservice获得响应时，我收到了（400）错误的请求。直到现在，我能够运行用户提供的服务URL并在下拉列表中获取所有webmethods。然后，我为所选的webmethod创建一个SOAP消息。当用户单击“执行”按钮时，我尝试使用SOAP消息调用webmethod。这是c＃代码：

protected void btnExecute_Click(object sender, EventArgs e)
{
    HttpWebRequest req = (HttpWebRequest)WebRequest.CreateDefault(new Uri(txtWSDL.Text));
    req.ContentType = "text/xml; charset=utf-8";
    req.Method = "POST";
    req.Accept = "text/xml";
    req.Headers.Add("SOAPAction", "http://tempuri.org/" + ddlMethods.SelectedItem.Text);
    StreamWriter stm = new StreamWriter(req.GetRequestStream(), Encoding.UTF8);
    stm.Write(txtFormattedSoap.Text);
    stm.Flush();
    stm.Close();

    WebResponse resp = req.GetResponse();  //I get error in this line.
}

请告诉我在哪里/哪些地方出错或者我错过了什么。