How to make checkboxlist required on clientside using validator in ASP.Net?

You can do that by using CustomValidator.

Here is the simple code:

<script> 

function ValidateChkBxLst(source, args) {

var chkListModules = 
document.getElementById('<%= chkBxLst.ClientID%>');
var chkListinputs = chkListModules.getElementsByTagName("input");
for (var i = 0; i < chkListinputs.length; i++) { 
if (chkListinputs[i].checked) {
args.IsValid = true;
return; 
} 
} 
args.IsValid = false; 
}
</script> 

<asp:checkboxlist id="chkBxLst" repeatcolumns="3" 
repeatdirection="Vertical" runat="server"/>

<asp:customvalidator clientvalidationfunction="ValidateChkBxLst" 
errormessage="CheckBox List" id="cvList" runat="server" 
text="Please select at least one checkbox." />

Here I have created "CustomValidator" which is pointing to "ValidateChkBxLst" Javascript function to validate it clientside without postback. Isn't it simple?

Thanks