如何为WCF ServiceContract设置默认的RequestFormat？

[ServiceContract]
public interface IMyService
{
    [OperationContract]
    [WebGet(RequestFormat = WebMessageFormat.Json)]
    string GetData();
}

public class MyService : IMyService
{
    public string GetData()
    {
        return "Hello, WCF!";
    }
}

 <services>
   <service name="MyService">
     <endpoint address="" binding="webHttpBinding" contract="IMyService">
       <identity>
          <dns value="localhost" />
        </identity>
      </endpoint>
     <endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange" />
    </service>
  </services>
  <behaviors>
   <serviceBehaviors>
      <behavior>
       <serviceMetadata httpGetEnabled="true" />
       <serviceDebug includeExceptionDetailInFaults="false" />
      </behavior>
    </serviceBehaviors>
  </behaviors>
</system.serviceModel>

var factory = new WebChannelFactory<IMyService>(new Uri("http://localhost/MyService.svc"));
var service = factory.CreateChannel();
var result = service.GetData();
Console.WriteLine(result);