custom validator

// type this part between form tag

Custom text:<br />
<asp:TextBox runat="server" id="txtCustom" />
<asp:CustomValidator runat="server" id="cusCustom" controltovalidate="txtCustom" onservervalidate="cusCustom_ServerValidate" errormessage="The text must be exactly 8 characters long!" />
<br /><br />
// type this part in code behind

protected void cusCustom_ServerValidate(object sender, ServerValidateEventArgs e)
{
    if(e.Value.Length == 8)
        e.IsValid = true;
    else
        e.IsValid = false;
}