c# - Why exactly is void async bad?

Question

So I understand why returning void from async would normally make no sense, but I've ran into a situation where I think it would be perfectly valid. Consider the following contrived example:

protected override void OnLoad(EventArgs e)
{
    if (CustomTask == null)
        // Do not await anything, let OnLoad return.
        PrimeCustomTask();
}
private TaskCompletionSource<int> CustomTask;

// I DO NOT care about the return value from this. So why is void bad?
private async void PrimeCustomTask()
{
    CustomTask = new TaskCompletionSource<int>();
    int result = 0;
    try
    {
        // Wait for button click to set the value, but do not block the UI.
        result = await CustomTask.Task;
    }
    catch
    {
        // Handle exceptions
    }
    CustomTask = null;

    // Show the value
    MessageBox.Show(result.ToString());
}

private void button1_Click(object sender, EventArgs e)
{
    if (CustomTask != null)
        CustomTask.SetResult(500);
}

I realize this is an unusual example, but I tried to make it simple and more generalized. Could someone explain to me why this is horrible code, and also how I could modify it to follow conventions correctly?

Thanks for any help.

Answer 1

Well, walking through the reasons in the "avoid async void" article:

• Async void methods have different error-handling semantics. Exceptions escaping from PrimeCustomTask will be very awkward to handle.
• Async void methods have different composing semantics. This is an argument centered around code maintainability and reuse. Essentially, the logic in PrimeCustomTask is there and that's it - it can't be composed into a higher-level async method.
• Async void methods are difficult to test. Following naturally from the first two points, it's very difficult to write a unit test covering PrimeCustomTask (or anything that calls it).

It's also important to note that async Task is the natural approach. Of the several languages that have adopted async/await, C#/VB are the only ones AFAIK that support async void at all. F# doesn't, Python doesn't, JavaScript and TypeScript don't. async void is unnatural from a language design perspective.

The reason async void was added to C#/VB was to enable asynchronous event handlers. If you change your code to use async void event handlers:

protected override async void OnLoad(EventArgs e)
{
  if (CustomTask == null)
    await PrimeCustomTask();
}

private async Task PrimeCustomTask()

Then the async void disadvantages are restricted to your event handler. In particular, exceptions from PrimeCustomTask are propagated naturally to its (asynchronous) callers (OnLoad), PrimeCustomTask can be composed (called naturally from other asynchronous methods), and PrimeCustomTask is much easier to include in a unit test.