FindControl
is only capable of finding server-side controls, not plain HTML tags. Therefore in this particular Repeater it is not likely to find anything. However you can always turn HTML tag into server-side control with runat
:
<input runat="server" type="checkbox" name="checkboxSelect" id="checkboxSelect" class="custom" />
Now every checkboxSelect
will be added into collection of controls, and FindControl
will be able to find it.