Why do I get a winrt::hresult_no_interface exception when I try to register an auto-revoke event handler?

If you register an event handler in C++/WinRT, the registration function returns a winrt::event_token, and you can pass that event token back to the event in order to unregister the handler.

An alternative that is popular in some circles is to register with winrt::auto_revoke as a marker first parameter ahead of the usual event handler parameters. If you do this, then the registration function returns a winrt::event_revoker instead of a winrt::event_token. The event_revoker automatically unregisters the event at destruction.

What does it mean when attempting to register an auto-revoke event handler throws the hresult_no_interface exception?

Internally, the event_revoker is a class with three members:

• A weak reference to the event source.
• A method to call to unregister the event.
• The event_token to unregister with.

The hresult_no_interface exception can come out of the first part: Creating the weak reference. You get the exception if the event source doesn’t support weak references. (This is common in the Windows.UI.Composition namespace.) In that case, you cannot use the auto-revoke feature will have to fall back to revoking your event handlers manually.