Filter GridView by DropDownList

Filter GridView by DropDownList

using System;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Data;
using Microsoft.SharePoint;

namespace SampleProject.DisplayGridView
{
    public partial class DisplayGridViewUserControl : UserControl
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                Load_Items("");
            }
        } //end Page_Load

        protected void Filter_Items(Object sender, EventArgs e)
        {
            Load_Items(DropDownList1.SelectedValue);

        } //end Filter_Items

        protected void Load_Items(string filter)
        {
            //Select the current site
            SPWeb thisWeb = SPContext.Current.Web;
            SPQuery query = new SPQuery();
            if (filter != "")
            {
                query.Query = string.Concat(
                    "<Where>",
                        "<Eq>",
                            "<FieldRef Name='Class' />",
                            "<Value Type='Text'>", filter, "</Value>",
                        "</Eq>",
                    "</Where>");
            }
            query.ViewFields = string.Concat(
                "<FieldRef Name='ID' />",
                "<FieldRef Name='Title' />",
                "<FieldRef Name='Class' />");
            query.ViewFieldsOnly = true;

            //The list name is Sample List.
            SPList thisList = thisWeb.Lists["Sample List"];
            DataTable dt = new DataTable();
            dt = thisList.GetItems(query).GetDataTable();

            GridView1.DataSource = dt;
            GridView1.AutoGenerateColumns = true;
            GridView1.DataBind();

        } //end Load_Items
    }
}