Asp.net web service return json

Question

I am using below code for my web service. I want return value in json. but this code return value in XML format only.

 [WebMethod]
 [ScriptMethod(UseHttpGet = true, ResponseFormat = ResponseFormat.Json)]
 public bool login(string uname, string pwd)
 {  
     SqlConnection con = new SqlConnection();
     SqlCommand cmd = new SqlCommand();
     con.ConnectionString = ConfigurationManager.ConnectionStrings["cn"].ConnectionString;
     if (con.State == ConnectionState.Closed)
     {
         con.Open();
     }
     cmd.CommandText = "select * from UserList where UserName=@username and Password=@pwd";
     cmd.Connection = con;
     cmd.Parameters.Add("@username", SqlDbType.VarChar, 50).Value = uname;
     cmd.Parameters.Add("@pwd", SqlDbType.VarChar, 50).Value = pwd;

     SqlDataReader dr = cmd.ExecuteReader();
     if (dr.Read() == true)
     {
         return true;
     }
     else
     {
         return false;
     }
     con.Close();
 }