public class Tools_
{
public Tools_(Label laberror)
{
this.laberror=laberror;
}
private static ArrayList al = new ArrayList();
private  Label laberror;
public  void checkvalidaty(RequiredFieldValidator val,Label lab,string error)
{

if (!val.IsValid)
{
lab.CssClass="error";
al.Add(error);  
laberror.Text=error;
return;

}
else
{
lab.CssClass="td";
}


}
}

Tools_ tls = new Tools_(Laberror);
tls.checkvalidaty(valuserselected,Labuserselected,"Please choose value from the list!");