Async Event Handlers: To Use `async void` or Not?

Asynchronous Event Handlers: A Deep Dive into Using async void

While it is generally accepted to avoid the "fire and forget" async void approach, the question is: should asynchronous void event handlers be avoided as well?

Consider the following code snippet:

<code class="language-csharp">private async void Form_Load(object sender, System.EventArgs e)
{
    await Task.Delay(2000); // 执行异步操作
    // ...
}</code>

As an alternative, a more structured approach can be chosen:

<code class="language-csharp">Task onFormLoadTask = null;

private void Form_Load(object sender, System.EventArgs e)
{
    this.onFormLoadTask = OnFormLoadTaskAsync(sender, e);
}

private async Task OnFormLoadTaskAsync(object sender, System.EventArgs e)
{
    await Task.Delay(2000); // 执行异步操作
    // ...
}</code>

Apart from potential reentrancy issues, what are the hidden pitfalls of using asynchronous event handlers?

Guidelines and Potential Risks

Surprisingly, general guidelines recommend using async void in event handlers, which makes it an exception to the "don't use async void" rule. This is because:

• Event handlers are usually anonymous methods used to respond to specific events and do not participate in the traditional control flow.
• If exceptions occur in an asynchronous event handler, they are automatically propagated to the UI thread's exception handler, making it easier to handle them.

However, in some cases it may be more beneficial to break out the logic of an asynchronous event handler for the following reasons:

• Unit Testing: It simplifies separate mocking and testing of the core functionality of an event handler.
• Code readability: By separating asynchronous code from event processing logic, code readability can be improved.

Example:

<code class="language-csharp">public async Task OnFormLoadAsync(object sender, EventArgs e)
{
    await Task.Delay(2000);
    ...
}

private async void Form_Load(object sender, EventArgs e)
{
    await OnFormLoadAsync(sender, e);
}</code>

By following these guidelines, you can use asynchronous event handlers effectively while mitigating potential risks and ensuring the maintainability of your code.

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