C# 协议错误500内部服务器错误

我正在从应用程序调用web服务（.asmx）。当我使用代理类（服务引用）时，它工作正常。如果我使用web请求类调用同一服务，我将收到500个内部服务器代码错误。web异常状态为protocol error。我正在使用asp.net c#测试web服务

我有两种情况的示例代码'

使用webrequest类

            string fileName = @"C:\Departmentpost.xml";

            var req = WebRequest.Create("http://exz.net/webservices/exzwspost.asmx/ProcessDept");


            req.Method = "POST";
            req.ContentType = "text/xml";
            req.Headers.Add("SOAPAction", "https://www.exz.net/ProcessDept");

            using (StreamWriter writer = new StreamWriter(req.GetRequestStream()))
            {
                writer.WriteLine(GetTextFromXMLFile(fileName));
                writer.Close();
            }

            using (var rsp = (HttpWebResponse)req.GetResponse())
            {
                var html = new StreamReader(rsp.GetResponseStream()).ReadToEnd();

            }

            if (req != null) req.GetRequestStream().Close();
        }

服务引用代理类代码

Context.Response.ContentType = @"text/xml"; 
string fileName = @"C:\Departmentpost.xml";
XmlDocument docs= new XmlDocument();   
docs.Load(fileName);  
XmlNode node = docs; 
WebservicePostTest.devservice.WSPostSoapClient post = new devservice.WSPostSoapClient("WSPostSoap2");

             XmlNode Xdoc = post.ProcessDept(node);


            XmlDocument result = new XmlDocument();
            result.AppendChild(result.ImportNode(Xdoc, true));
            Response.Write(result.InnerXml);
            Response.End();