How can you `co_await` in a C++/WinRT lambda?

Question

Can you co_await in a C++/WinRT TimerElapsedHandler (or any other lambda in C++/WinRT)?

When I try to compile code like this:

auto pointerExitedTimerHandler = winrt::TimerElapsedHandler([](const winrt::ThreadPoolTimer&)
{
    co_await 5s;

    // Other stuff...
});

I get an error:

error C7588: A definition of a class template std::experimental::task must be provided for the return type of this coroutine to be deduced

How can I write an async handler?

Disclaimer: I work for Microsoft.

Answer 1

You simply need to provide an async return type. For example, -> winrt::fire_and_forget:

auto pointerExitedTimerHandler = winrt::TimerElapsedHandler([](const winrt::ThreadPoolTimer&) -> winrt::fire_and_forget
{
    co_await 5s;

    // Other stuff...
});

Fire and forget is a simple WinRT wrapper for async functions that ignores the result—if you need to await the result, you'll need to use a different return type.