Dev Blog

Summary ​

Conclusions ​

The Windows Forms UI thread processes all messages one at a time, so a long synchronous operation in a handler "freezes" the window. Task and Task<T> represent operations that will complete later, and the await operator releases the thread while waiting and, thanks to the synchronization context, returns the continuation to the UI thread. I/O operations are called through their …Async methods, and computations are moved to the thread pool with Task.Run. IProgress<T> safely passes progress to the form, CancellationTokenSource and CancellationToken implement cooperative cancellation and timeouts, IAsyncEnumerable<T> yields results gradually, and Task.WhenAll, WhenAny, WhenEach, and SemaphoreSlim manage several tasks. A task's result is obtained only through await, async void is used only for event handlers, and in .NET 10 the asynchronous InvokeAsync, ShowDialogAsync, and TaskDialog.ShowDialogAsync methods are available without any configuration.

Self-check questions ​

  1. Why does a long-running event handler make a Windows Forms window "Not Responding"?
  2. How do I/O operations differ from computations? How do you run each of them in a GUI application?
  3. What is a Task? What states can it have?
  4. Why are task.Result and task.Wait() not used on the UI thread?
  5. What happens when the await operator is applied to an incomplete task?
  6. Which types can an async method return? When is async void acceptable?
  7. What does CS4014 warn about?
  8. What is a synchronization context? How does the behavior of await differ in a console program and in a Windows Forms application?
  9. When does the "Cross-thread operation not valid" exception occur, and how do you avoid it?
  10. How does a deadlock occur because of Result? What is ConfigureAwait(false) for?
  11. How do IProgress<T> and Progress<T> work? On which thread does the Progress<T> handler run?
  12. How do you implement canceling an operation with a button and a timeout?
  13. What are the asynchronous streams IAsyncEnumerable<T> and the await foreach loop for?
  14. How do Task.WhenAll, Task.WhenAny, and Task.WhenEach differ? How do you limit the number of concurrent operations?
  15. How are exceptions from await and Task.WhenAll caught? What happens to an exception in async void?
  16. What are Control.InvokeAsync and Form.ShowDialogAsync for? Is suppressing WFO5002 needed in .NET 10?