Why Required Field not working for dropdownlist?

then bind dropdown like as

 if (objDs.Tables[0].Rows.Count > 0)
        {
            ddlClass.DataSource = objDs.Tables[0];
            ddlClass.DataTextField = "class_name";
            ddlClass.DataValueField = "id";
            ddlClass.DataBind();
            ddlClass.Items.Insert(0, new ListItem("----------- Select Class -----------", "0"));
}

and apply validation as above you add initial value="0" in requiredfield validater

oops..Thanx


but if dropdown list dynamically bind then how to validate?