Web服务拒绝接受JSON参数和回复

我试图调用从C＃应用程序的Web服务（的.asmx），JSON格式。

I'm trying to call a web service (.asmx) from a c# application, in json format.

当我指定请求方法为GET，并且不指定的contentType。

When I specify request method as GET, and don't specify contentType.

（REQ是HttpWebRequest的）

(req is HttpWebRequest)

req.Method = "GET";

一切都工作得很好，但我得到XML响应。结果
当我指定的内容类型：

Everything works well, but I get XML response.
When I specify content Type:

req.ContentType = "application/json; charset=utf-8";  

我得到

500内部服务器错误。

500 internal server error.

当我改变请求方法：

req.Method = "POST";  

我只能调用参数的方法，它返回正确的json，但如果我尝试调用一个方法的与参数，我再得到500错误。

I can call parameterless methods only, which returns correctly json, but if I try calling a method with parameters, I get 500 error again.

Web服务code：

The web service code:

    [WebMethod(EnableSession =true)]
    [ScriptMethod(ResponseFormat = ResponseFormat.Json)]
    public string SimplestWebService()
    {         
        return "hello";
    }  

和使用参数：

    [WebMethod(EnableSession = true)]
    [ScriptMethod(ResponseFormat = ResponseFormat.Json)]
    public string Echo(string aString)
    {       
        return aString;
    }

任何想法将大大AP preciated。

Any ideas will be much appreciated.

补充：也许我不写POST请求权（现在是我要送它的头，就像一个GET请求）。是否有人可以指导我一下吗？

Added: Maybe I'm not writing the POST request right (now I'm sending it in the header, just like a GET request). Can someone please guide me on that?

模式又说：该网站确实被标记为脚本：

Mode added: The web site is indeed marked as script:

[ScriptService]
public class MyAPI : System.Web.Services.WebService  

这是我建立我的POST请求（我真的倾向于认为这就是问题所在）：

And here is how I build my POST request (I really tend to believe that's the problem):

HttpWebRequest req = (HttpWebRequest)WebRequest.Create(methodUrl.ToString());
req.Method = "POST";
req.Headers.Add("aString","oren");
req.ContentLength = 0;    
...
req.ContentType = "application/json; charset=utf-8";
req.Accept = "application/json; charset=utf-8";
using (HttpWebResponse res = (HttpWebResponse)req.GetResponse())
{
  StreamReader sr = new StreamReader(res.GetResponseStream());
  result.Append(sr.ReadToEnd());
}  
...

也试过：

req.Method = "POST";          
string postData = "aString=kjkjk";
req.ContentType = @"application/json; charset=utf-8";
req.Accept = @"application/json; charset=utf-8";   
ASCIIEncoding encoding = new ASCIIEncoding();
byte[] byte1 = encoding.GetBytes(postData);
req.ContentLength = byte1.Length;
Stream newStream = req.GetRequestStream();
newStream.Write(byte1, 0, byte1.Length);
newStream.Close();           

最后两个注意事项：结果
1.此Web服务在XML中使用浏览器的工作原理。结果
2.问计JSON，在code 从未达到的Web服务一个破发点。所以这可能是一个IIS（我使用的是IIS 6.1）的问题。我试过MIME类型推荐here.

Two last notes:
1. This web service works in XML using the browser.
2. Asking for json, the code never reaches a break point at the web service. So this is possibly a IIS (I'm using IIS 6.1) issue. I've tried the MIME type recommendation here.

非常感谢。