更新DropDownList时，未正确选择所选值;而是返回随机值

protected void wbsDropDown_Change(object sender, EventArgs e)
{
    //txtWBSDesc.Text = wbsDropDown.SelectedItem.Value.ToString();
    //txtWBSDesc.Text = wbsDropDown.SelectedValue.ToString().Trim();   
    txtWBSDesc.Text = this.wbsDropDown.SelectedValue.ToString().Trim();
}
protected void wbsDropDown_Change(object sender