dropdown list not redirecting to update gridview

See more:

C#

ASP.NET

i have a dropdown list and data grid..

dropdownlist is seleting categoryId value from the categories database table.

i want that as soon as the value is seleted from the dropdown list..
the griw view show the products beleng to that catagory.

but my problem is that when i select catagoryname in the dropdownlist it is seleted . but not updating the gridview on the bases of category name..

i think dropdown list is just selecting. not redircting to update grid view. here is the code. what changes i have to do to work it correctly.

public partial class Admin_procat : System.Web.UI.Page
{
    dbcon obj = new dbcon();
    protected void Page_Load(object sender, EventArgs e)
    {
        //dbcon obj = new dbcon();
        if (!IsPostBack)
        {
            string querydr = "select * from ESK_Categories";
            SqlDataReader dr = obj.fillcomb(querydr);
            DropDownList1.DataSource = dr;
            DropDownList1.DataValueField = "CategoryName";
            DropDownList1.DataBind();

           // Response.Redirect("procat.aspx");
           
        }
        if (!IsPostBack)
        {
            bind();

        }

       
    }
    public void bind()
    {
        //string query = "select * from ESK_Products";
        string query = "SELECT * FROM ESK_Products LEFT JOIN ESK_Categories ON ESK_Categories.CategoryID=ESK_Products.CategoryID Where ESK_Categories.CategoryName='" + DropDownList1.Text + "'";  
        DataSet ds = obj.fillgrid(query);
        GridView1.DataSource = ds.Tables[0];
        GridView1.DataBind();
    }
    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        //Response.Redirect("procat.aspx");
    }
}