WCF offers two options for ResponseFormat attribute in WebGet annotation in ServiceContract.

[ServiceContract]
public interface IService1
{
    [OperationContract]
    [WebGet(UriTemplate = "greet/{value}", BodyStyle = WebMessageBodyStyle.Bare)]
    string GetData(string value);

    [OperationContract]
    [WebGet(UriTemplate = "foo", BodyStyle = WebMessageBodyStyle.Bare, ResponseFormat = WebMessageFormat.Json)]
    string Foo();

The options for ResponseForamt are WebMessageFormat.Json and WebMessageFormat.Xml. Is it possible to write my own web message format? I would like that when client calls foo() method he gets raw string - without json or xml wrappers.