Async code is a great way to keep your app’s UI responsive. You can start an async operation from the UI thread, await it without blocking the UI thread, and naturally resume on the UI thread when it’s done. This is a very powerful feature, and most of the time you don’t even need to think about it; it “just works”. However, this works only if the async operation is started from a thread that has a synchronization context (such as the UI thread in Windows Forms, WPF or WinRT).
Asynchrony in C# 5 is awesome, and I’ve been using it a lot since it was introduced. But there are few annoying limitations; for instance, you cannot pass parameters by reference (ref or out) to an asynchronous method. There are good reasons for that; the most obvious is that if you pass a local variable by reference, it is stored on the stack, but the current stack won’t remain available during the whole execution of the async method (only until the first await), so the location of the variable won’t exist anymore.