How do I make a checkbox required on an ASP.NET form?

Question

I've done some searching on this, and I've found several partial answers, however nothing that gives me that warm fuzzy "this is the right way to do this". To answer the most frequently occurring complaint against this question: "checkboxes can have two legitimate states - checked and unchecked", this is an "I accept the terms and conditions..." checkbox which must be checked in order to complete a registration, hence checking the box is required from a business logic standpoint.

Please provide complete cut-n-paste ready code fragments with your response! I know there are several pieces to this -- the CustomValidator (presumably), the code-behind, some javascript and possibly a check for IsValid, and the frustrating part for me is that in each example I've seen, one of these critical pieces is missing!

Answer 1

javascript function for client side validation (using jQuery)...

function CheckBoxRequired_ClientValidate(sender, e)
{
    e.IsValid = jQuery(".AcceptedAgreement input:checkbox").is(':checked');
}

code-behind for server side validation...

protected void CheckBoxRequired_ServerValidate(object sender, ServerValidateEventArgs e)
{
    e.IsValid = MyCheckBox.Checked;
}

ASP.Net code for the checkbox & validator...

<asp:CheckBox runat="server" ID="MyCheckBox" CssClass="AcceptedAgreement" />
<asp:CustomValidator runat="server" ID="CheckBoxRequired" EnableClientScript="true"
    OnServerValidate="CheckBoxRequired_ServerValidate"
    ClientValidationFunction="CheckBoxRequired_ClientValidate">You must select this box to proceed.</asp:CustomValidator>

and finally, in your postback - whether from a button or whatever...

if (Page.IsValid)
{
    // your code here...
}