Для One Control вы можете сделать это ..
<!-- In Designer Page -->
<asp:CustomValidator runat="server" id="cusCustom" controltovalidate="txtCustom"
onservervalidate="cusCustom_ServerValidate"
errormessage="The text must be exactly 8 characters long!" />
<br /><br />
/* In Code Behind*/
protected void cusCustom_ServerValidate(object sender, ServerValidateEventArgs e)
{
if(e.Value.Length == 8)
e.IsValid = true;
else
e.IsValid = false;
}