Problem in DropDownList's SelectedIndexChanged Event

hi guys,
i am working on a library automation system in ASP.NET with C#. I think this part of the forum is suitable.
When i come to my problem
in admin panel i designed a "Book Addition" page . In this part admin choose category and subcategory to add the book. The category names will be listed on Page_Load, admin chooses a category name and the subcategories which is related to the choosen category will listed on ddlSubcategory.

here is my code

Code:

int i;

protected void ddlCat_SelectedIndexChanged(object sender, EventArgs e)
    {
        ddlScat.Items.Clear();

        SqlConnection connect = dbConnection.GetConnection();
        SqlCommand cmd2 = new SqlCommand("select c_id from categories where c_name=@catname", connect);
        cmd2.Parameters.AddWithValue("@catname", ddlCat.SelectedValue.ToString());
        //cmd2.Parameters.AddWithValue("@catname", ddlCategory.SelectedItem.Value);
        connect.Open();
        SqlDataReader rdr2 = cmd2.ExecuteReader();

        while (rdr2.Read())
        {
            i = Int32.Parse(rdr2.GetValue(0).ToString());
        }
        connect.Close();
        connect.Open();
        SqlCommand command2 = new SqlCommand("select s_name,s_id from subcategories where c_id=@i", connect);
        command2.Parameters.AddWithValue("@i", i);

        SqlDataAdapter da = new SqlDataAdapter(command2);
        DataTable dt = new DataTable();

        da.Fill(dt);

        ddlScat.DataSource = dt;
        ddlScat.DataTextField = "s_name";
        ddlScat.DataValueField = "s_id";
        ddlScat.DataBind();

        connect.Close();

    }

there is no warning or error. And Page_load part is working correctly but when i clicked ddlCat ( drop down list of Categories ) the ddlScat ' s( drop down list of subcategories ) items do not change.

i am waiting your answers..

Have you set the "AutoPostback" property on the dropdownlist to True?

yeah that's it thanks a lot Magos