How Can I Catch Exceptions from Async Void Methods in C#?

Handling exceptions in asynchronous void methods

In Microsoft's .NET Async CTP, asynchronous methods behave differently when throwing exceptions. In an asynchronous Task or an asynchronous Task method, exceptions are caught and attached to the Task object. However, for asynchronous void methods, there is no Task object to handle exceptions.

Catch exception

To catch an exception thrown by an asynchronous void method, you must explicitly wait for the method call or use the Wait() method:

1. Waiting for method call:

    public async Task Foo()
    {
        var x = await DoSomethingAsync();
    }
   
    public async void DoFoo()
    {
        try
        {
            await Foo();
        }
        catch (ProtocolException ex)
        {
            // 由于在异步方法中等待了调用，因此将捕获异常。
        }
    }

2. Use Wait() method:

    public void DoFoo()
    {
        try
        {
            Foo().Wait();
        }
        catch (ProtocolException ex)
        {
            // 由于等待了调用的完成，因此将捕获异常。
        }
    }

Error handling semantics

As Stephen Cleary pointed out, asynchronous void methods have different error handling semantics:

"When an async void method throws an exception, the exception is raised directly on the SynchronizationContext that was active when the asynchronous void method was started."

Other instructions

• If .NET executes the method synchronously, using Wait() may cause the application to block.
• See the explanation below for details on how the compiler handles this behavior: https://www.php.cn/link/c4d856156b2c629314b9edd13523fe8e

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