Can we use "async void" event handlers in web user control? I tried and failed with "An asynchronous operation cannot be started at this time. Asynchronous operations may   only be started within an asynchronous  handler or module or during certain events in the Page lifecycle. If  this exception occurred while executing a Page, ensure that the Page  is marked <%@ Page Async="true" %>.". And I'm pretty sure that I have enabled async for the container page.