As you know, ASP.Net has bunch of useful validation controls and they realy make our project so easy in terms of validations. But ASP.Net doesn't have any control to validate a check box !
There are lots of way to validate a check box and here you will find out some of them.
<script runat="server" language="vbscript">
Sub CustomValidator1_ServerValidate(ByVal source As Object, ByVal args As ServerValidateEventArgs)
args.IsValid = (Privacy1.Checked = True)
If CheckBox1.Checked = False Then
Label1.Visible = True
Label1.Visible = False
'here is your function that you want to run after the checkbox is checked !
That's it ! Now you have two kinds of CheckBox RequiredFieldValidator. When you run the page and click one of the buttons, you will see that the checkbox will have a Required Field Validator. We can do that via Custon Validator Control on ASP.Net 3.5 Toolbar on the first examples and on the second one we are able to that manually !
I hope It helps :)