In this article I will explain how to validate ASP.Net Checkbox control using ASP.Net Custom Validator and JavaScript
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
    <title></title>
    <script type = "text/javascript">
        function ValidateCheckBox(sender, args) {
            if (document.getElementById("<%=CheckBox1.ClientID %>").checked == true) {
                args.IsValid = true;
            } else {
                args.IsValid = false;
            }
        }
    </script>
</head>
<body>
    <form id="form1" runat="server">
    <asp:CheckBox ID="CheckBox1" runat="server" />
    <asp:CustomValidator ID="CustomValidator1" runat="server" ErrorMessage="Required" ClientValidationFunction = "ValidateCheckBox"></asp:CustomValidator><br />
    <asp:Button ID="Button1" runat="server" Text="Submit"/>
    </form>
</body>
</html>
 
Explanation
Above I have placed an ASP.Net Custom Validator besides the ASP.Net Checkbox control. For the ASP.Net Custom Validator I have specified the ClientValidationFunction property to a JavaScript function ValidateCheckBox which validates the whether the checkbox is checked or not.

Screenshot
Validate ASP.Net CheckBox using JavaScript and Custom Validator
 

The above code has been tested in the following browsers

Internet Explorer  FireFox  Chrome  Safari  Opera 

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Downloads
You can download the sample source code using the download link provided below.
ValidateASP.NetCheckBox.zip