public static bool IsUserAvailable(string username)

{'username':'value'}

      public static string ToJson(this object data)
      {
          return new JavaScriptSerializer().Serialize(data);
      }

        public static string GetJsonTest0()
      {
          var data = "a1";
          return data.ToJson();
      }

      public static string GetJsonTest1()
      {
          var data = new List<string> { "a1", "a2", "a3" };
          return data.ToJson();
      }

      public static string GetJsonTest2()
      {

          var lst =
             new List<KeyValuePair<string, object>>
                   {
                       new KeyValuePair<string, object>("Name", "علی"),
                       new KeyValuePair<string, object>("Number", 10),
                       new KeyValuePair<string, object>("Desc", "منابع مورد نیاز")
                   };

          return lst.ToJson();

      }

"a1"
["a1","a2","a3"]
[{"Key":"Name","Value":"علی"},{"Key":"Number","Value":10},{"Key":"Desc","Value":"منابع مورد نیاز"}]