How to return Json from WCF Service?

Question

I have the piece of code below of a template Ajax enabled WCF service. What can i do to make it return JSon instead of XML? thanks.

using System; 
using System.Linq;
using System.Runtime.Serialization;
using System.ServiceModel;
using System.ServiceModel.Activation;

[ServiceContract(Namespace = "WCFServiceEight")]
[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
public class CostService
{
    // Add [WebGet] attribute to use HTTP GET
    [OperationContract]
    [WebGet]
    public double CostOfSandwiches(int quantity)
    {
        return 1.25 * quantity;
    }
}

Answer 1

Have you tried:

[WebGet(ResponseFormat= WebMessageFormat.Json)]

Answer 2

If you want to use the POST verb as in $.ajax({ type: "POST", ...) you will need to markup your method with [WebInvoke(Method="POST"].

Since you marked it up with [WebGet] (which is equivalent to [WebInvoke(Method="GET")]) you should call the service using the GET verb, e.g.:

$.ajax({ type: "GET", ...) or use $.get(url, data, ...) (see jQuery.get for more info).

And you'll need to set the ResponseFormat to Json, as tomasr already pointed out.