Search Data from Database using Auto Complete Functionality

[WebMethod]
   public static List<string> GetEmployeeName(string userName)
   {
       List<string> srchResult = new List<string>();
       using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["constring"].ConnectionString))
       {
           using (SqlCommand cmd = new SqlCommand())
           {
               cmd.CommandText = "select userName from Credentials where userName LIKE ''+@SearchusrName+'%'";
               cmd.Connection = con;
               con.Open();
               cmd.Parameters.AddWithValue("@SearchusrName", userName);
               SqlDataReader dr = cmd.ExecuteReader();
               while (dr.Read())
               {
                   empResult.Add(dr["userName"].ToString());
               }
               con.Close();
               return srchResult;

           }
       }
   }

$(document).ready(function () {
            SearchUserName();
        });
        function SearchUserName() {
            $("#txtEmpName").autocomplete({
                source: function (request, response) {
                    $.ajax({
                        type: "POST",
                        contentType: "application/json; charset=utf-8",
                        url: "Default.aspx/GetEmployeeName",
                        data: "{'userName':'" + document.getElementById('txtEmpName').value + "'}",
                        dataType: "json",
                        success: function (data) {
                            response(data.d);
                        },
                        error: function (result) {
                            alert("No Match found !");
                        }
                    });
                }
            });
        }