Asp.NET DropDownList在回发后重置SelectedIndex

!(Page.IsPostBack)

AutoPostBack

SelectedIndex = 0

<form id=\"AddAssignmentForm\" runat=\"server\">
<asp:ScriptManager ID=\"ScriptManager1\" EnablePartialRendering=\"true\" runat=\"server\" />

<asp:UpdatePanel ID=\"CommentUpdate\" runat=\"server\">
<ContentTemplate>

Add Comment
<asp:DropDownList ID=\"ddlCategory\" runat=\"server\" Width=\"206\" OnSelectedIndexChanged=\"ddlCategory_SelectedIndexChanged\" AutoPostBack=\"true\" />
<asp:TextBox ID=\"txtName\" runat=\"server\" Width=\"200\" />
<asp:TextBox ID=\"txtAbbrv\" runat=\"server\" Width=\"200\" />
<asp:TextBox ID=\"txtDescription\" runat=\"server\" Width=\"200\" Height=\"90\" TextMode=\"MultiLine\" />

</ContentTemplate>
</asp:UpdatePanel>
</form>

private void Page_Load(object sender, System.EventArgs e)
{
    if (!Page.IsPostBack)
    {
        GetCategories();
    }
}

public void GetCategories()
{
    String strSql = @\"SELECT Name, Total
                        FROM MyTable\";

    if (con.State == ConnectionState.Closed)
        con.Open();

    OleDbCommand cmdsql = new OleDbCommand(strSql, con);
    OleDbDataReader cmdReader = cmdsql.ExecuteReader();

    if (cmdReader.HasRows)
    {
        while (cmdReader.Read())
        {
            ddlCategory.Items.Add(new ListItem(cmdReader[\"Category_Name\"].ToString(), cmdReader[\"Total\"].ToString()));

        }
        ddlCategory.SelectedIndex = -1;
    }


    cmdReader.Close();
    con.Close();
}

public void FillForm(int index)
{
    ListItem item = ddlCategory.Items[index];
    txtName.Text = item.Text + \" \" + (Convert.ToInt32(item.Value) + 1).ToString();
    txtAbbrv.Text = item.Text.Substring(0, 1) + (Convert.ToInt32(item.Value) + 1).ToString();
}

public void ddlCategory_SelectedIndexChanged(Object sender, EventArgs e)
{
    //When I break here SelectedIndex always = 1.
    FillForm(ddlCategory.SelectedIndex);
}