How to get value based on dropdownlist using webservice and Jquery?

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How to get value based on dropdownlist using webservice and Jquery? I have done the code for the same. But its not working.
Code is as below:

[WebMethod]
       [ScriptMethod(ResponseFormat = ResponseFormat.Json)]
       public string[] GetUsers(string username)
       {
           List<string> customers = new List<string>();
           using (SqlConnection conn = new SqlConnection())
           {
               conn.ConnectionString = ConfigurationManager
                       .ConnectionStrings["constr"].ConnectionString;
               using (SqlCommand cmd = new SqlCommand())
               {
                   cmd.CommandText = "select UserName from Users where RoleId=1";

                   //cmd.CommandText = "select UserName from Users where " +
                   //"RoleId like @SearchText + '%'";
                   cmd.Parameters.AddWithValue("@SearchText", username);
                   cmd.Connection = conn;
                   conn.Open();
                   using (SqlDataReader sdr = cmd.ExecuteReader())
                   {
                       while (sdr.Read())
                       {
                           sdr["UserName"].ToString();
                       }
                   }
                   conn.Close();
               }
               return customers.ToArray();
           }
       }