Can Exceptions Thrown in Async Void Methods Be Caught by Calling Methods?

Exception handling of asynchronous void methods

When using Microsoft's Async CTP for asynchronous programming, it is crucial to understand the exception handling behavior of asynchronous void methods. Consider the following scenario:

public async void Foo()
{
    var x = await DoSomethingAsync();
}

public void DoFoo()
{
    try
    {
        Foo();
    }
    catch (ProtocolException ex)
    {
        // 异常将永远不会被捕获。
    }
}

The question is: Can the exception thrown in the asynchronous method DoFoo be caught in the calling method Foo?

Answer: Yes (but there are prerequisites)

Exceptions do bubble up to the calling code, but only if you require a call to await or Wait() Foo. Modify the code as follows:

public async Task Foo()
{
    var x = await DoSomethingAsync();
}

public async void DoFoo()
{
    try
    {
        await Foo();
    }
    catch (ProtocolException ex)
    {
        // 异常将被捕获，因为它是一个异步方法。
    }
}

public void DoFoo()
{
    try
    {
        Foo().Wait();
    }
    catch (ProtocolException ex)
    {
        // 异常将被捕获，因为调用被等待。
    }
}

This allows exception handling in the calling code.

However, it is important to note that Stephen Cleary, an authority on asynchronous programming, warns:

"Async void methods have different error handling semantics. When an exception is thrown from an async Task or async Task<T> method, the exception is caught and placed on the Task object. Right For async void methods, there is no Task object, so any exception thrown from an async void method will be raised directly on the void that was active when the async SynchronizationContext method was initiated.

Additionally, using

to wait for a task to complete may block the application if .NET decides to execute the method synchronously. Wait()

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