wcf - dynamic return type xml or json

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C#

WCF

how to make wcf service method which returns the data in both xml and json,
in short how to enable wcf service method which return dynamic output either xml or json.

i made one method that meet my requirement but i know its not true method to do this.

my code:

INTERFACE :

[OperationContract]
        [WebInvoke(Method = "GET", BodyStyle = WebMessageBodyStyle.Wrapped,
            UriTemplate = "/ReturnTime?ReturnType={ReturnType}")]
        string ReturnTime(String ReturnType);

METHOD :

public string ReturnTime(String ReturnType)
        {
            if (ReturnType != "" && ReturnType != null)
            {
                if (ReturnType.ToUpper() == "STRING" || ReturnType.ToUpper() == "JSON" || ReturnType.ToUpper() == "XML")
                {
                    if (ReturnType.ToUpper() == "JSON")
                        return "[{\"Current Time is \":" + DateTime.Now.ToString() + "}]";
                    else if (ReturnType.ToUpper() == "XML")
                        return "<CurrentTimeIs>" + DateTime.Now.ToString() + "</CurrentTimeIs>";
                    else if (ReturnType.ToUpper() == "STRING")
                        return "Current time is : " + DateTime.Now.ToString();
                    else
                        return "<CurrentTimeIs>" + DateTime.Now.ToString() + "</CurrentTimeIs>";
                }
                else
                {
                    return "<ErrorMessage><Message>" + ReturnType + " as returntype is not available. Please use JSON,XML OR STRING.</Message></ErrorMessage>";
                }
            }
            else
            {
                return "<ErrorMessage><Message>Please provide Returntype either JSON,XML OR STRING.</Message></ErrorMessage>";
            }
        }

i m doing this. i know wcf has power to do this but how it does i dont' know plz help me to achieve this. thanx in advance.