Question

Is it bad to have expensive code at the start of an async method, before the first await is called? Should this code be wrapped with a TaskEx.Run instead?

public async Task Foo()
{
    // Do some initial expensive stuff.
    // ...

    // First call to an async method with await.
    await DoSomethingAsync;
}

Answer 1

As Reed says, it really depends on the context. The code has to run at some point – but depending on the context, it may end up being run on a thread pool thread instead of some critical one.

Rather than using Task.Run, I’d use TaskEx.Yield:

public async Task Foo()
{
    await TaskEx.Yield();
    // Do expensive stuff
}

As far as I’m aware, that’s basically a way of immediately returning to the caller, but allowing the rest of the async method to be scheduled straightaway. If you’re in something like a Windows Forms UI thread, there’s no point in doing this as you’ll be back to the UI thread (and running expensive code there) immediately – but it would make sense if you’re in a context where the current thread shouldn’t be blocked, but continuations are run on another thread.