Web service refuses to receive parameters and reply in JSON

Question

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

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

(req is HttpWebRequest)

req.Method = "GET";

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

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

I get

500 internal server error.

When I change request method:

req.Method = "POST";  

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

The web service code:

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

And With parameters:

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

Any ideas will be much appreciated.

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?

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

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

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());
}  
...

Also tried:

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();           

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.

Thanks a lot.

Answer 1

Update 2:

I'm using fiddler2 to look at a successful POST request (from jquery to my asmx service), and I see a difference that might be worth checking:

Accept: application/json, text/javascript, */*

There's some other differences as well, but that one is the only one that leaps out as a possible smoking gun.

Updated Answer (in response to updated question):

It looks like you're using the contentStream correctly. However, the data you're pushing onto it ("aString=kjkjk") isn't valid JSON. As noted below, your data will likely need to be in valid JSON format:

{'aString':'kjkjk'}

Original Answer:

I believe you're setting the "aString" content incorrectly. For POST-method requests, the "payload" data isn't applied to the header, it is the content of the request itself. Therefore, you need to stream it onto the request object; see this MSDN reference for an example. Don't forget to also set the request's content-length.

Since you're implementing the service via JSON (and have specified JSON as the content-type), I believe you'll need to convert a JSON string to raw bytes, and that's what you'll push it onto the stream. Your JSON string should look something like this:

{'aString':'This is the text I want to echo.'}

You might need to do some fine-tuning (I'm not at a place where I can easily put together an exact code sample), but this should get you going in the right direction.