Sub FunctionName(sender as Object, args as ServerValidateEventArgs)
  ...
End Sub
其中注意两个参数
 value:指示当前的CustomValidator验证的是哪一个控件
 IsValid：为真时，表示页面控件已经通过验证。
举一个验证页面中文本框的输入是否为素数的例子：
<script language="vb" runat="server">
  Sub btnSubmit_Click(sender as Object, e as EventArgs)
    If Page.IsValid then
      Response.Write("<font color=""red""><i>" & txtPrimeNumber.Text & _
                     " is, indeed, a good prime number.</i></font>")
    Else
      Response.Write("<font color=""red""><i>" & txtPrimeNumber.Text & _
                     " is <b>not</b> a prime number.</i></font>")
    End If
  End Sub


  Sub PrimeNumberCheck(sender as Object, args as ServerValidateEventArgs)
    Dim iPrime as Integer = Cint(args.Value), iLoop as Integer, _
        iSqrt as Integer = CInt(Math.Sqrt(iPrime))

    For iLoop = 2 to iSqrt
      If iPrime mod iLoop = 0 then
        args.IsValid = False
        Exit Sub
      End If
    Next

    args.IsValid = True
  End Sub
</script>

<form method="post" runat="server">
  Enter your favorite prime number:
  <asp:textbox id="txtPrimeNumber" runat="server" />

  <%-- Create the CustomValidator control --%>
  <asp:CustomValidator runat="server" id="custPrimeCheck"
        ControlToValidate="txtPrimeNumber"
        OnServerValidate="PrimeNumberCheck"
        ErrorMessage="Invalid Prime Number" />

  <%-- Create two CompareValidator controls: the first ensures that 
      the number entered by the user is an Integer; the second 
      makes sure it is positive. --%>
  <asp:CompareValidator runat="server" id="compPrimeNumber"
        Operator="DataTypeCheck" Type="Integer"
        Display="Dynamic" ControlToValidate="txtPrimeNumber"
        ErrorMessage = "You must enter an integer value." />
  <asp:CompareValidator runat="server" id="compPrimeNumberPositive"
        Operator="GreaterThan" Type="Integer"
        Display="Dynamic" ValueToCompare="0"
        ControlToValidate="txtPrimeNumber"
        ErrorMessage = "You must enter a value greater than zero." />

  <p><asp:button id="btnSubmit" runat="server"
          OnClick="btnSubmit_Click" Text="Submit" />
</form>
可以看到，在验证控件中，   OnServerValidate="PrimeNumberCheck"中，要定义具体的onservervalidate事件
，之后，在具体的事件处理过程中，一定要返回args.isvalid的值，以价讲明是否验证成功；当然，最后要用page.isvalid属性
来进行全面验证