c#之使用 httpwebrequest 调用 Web 服务方法之无法连接到远程服务器错误

我正在调用我的 Web 服务的特定方法，但是当我创建一个新的 StreamWriter 对象时，我收到了上述错误。我不确定为什么会这样，因为我以前从未这样做过。我用来执行此操作的代码是使用此链接编写的 - Invoking an ASP.NET web service method via an http request作为引用。这是片段-

var httpWebRequest = (HttpWebRequest)WebRequest.Create("http://localhost/Helpers/Symbology.asmx"); 
httpWebRequest.Headers.Add("SOAPAction", "\"http://tempuri.org/GetRelatedSymbols\""); 
httpWebRequest.ContentType = "text/json"; 
httpWebRequest.Method = "POST"; 
JavaScriptSerializer serializer = new JavaScriptSerializer(); 
 
*using (var streamWriter = new StreamWriter(httpWebRequest.GetRequestStream()))*//error occurs here 
{ 
    string json = serializer.Serialize(relatedSymbols); 
    streamWriter.Write(json); 
} 
var httpResponse = (HttpWebResponse)httpWebRequest.GetResponse(); 
string responseText = String.Empty; 
using (var streamReader = new StreamReader(httpResponse.GetResponseStream())) 
{ 
     responseText = streamReader.ReadToEnd(); 
} 

这是我的网络服务代码片段 -

[WebService(Namespace = "http://tempuri.org/")] 
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)] 
//[System.ComponentModel.ToolboxItem(false)] 
// To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line.  
// [System.Web.Script.Services.ScriptService] 
public class Symbology: System.Web.Services.WebService 
{ 
 
    [WebMethod] 
    public List<string> GetRelatedSymbols() 
    { 
        try 
        { 
             //code here 
        } 
        catch(){} 
    } 
} 

任何人都可以帮助指出我可能做错了什么吗？

请您参考如下方法:

当我运行网络服务器时，我总是需要连接这样的 URL http://localhost:PORTNUMBER/ ...