Exception details not shown for async commands

[gabrielmaldi]

When using CommandHandler.Create(MethodInfo) with an async method, if an exception is thrown, the details are not printed to the console.

class Program
{
    public void TestCommand()
    {
        throw new InvalidOperationException();
    }

    public async void TestCommandAsync()
    {
        throw new InvalidOperationException();
    }

    static int Main(string[] args)
    {
        return new CommandLineBuilder(new RootCommand()
        {
            new Command("test")
            {
                Handler = CommandHandler.Create(typeof(Program).GetMethod("TestCommand"))
            },
            new Command("test-async")
            {
                Handler = CommandHandler.Create(typeof(Program).GetMethod("TestCommandAsync"))
            }
        })
            .UseDefaults()
            .Build()
            .Invoke(args);
    }
}

This is the output generated by running both commands:

For the async method, only Unhandled exception is printed.

The behavior is the same if we use:

static async Task<int> Main(string[] args)
{
    return await new CommandLineBuilder(new RootCommand()
    ...
        .InvokeAsync(args);

Is there a way to get the full exception details printed to the console when using async commands?

Thanks!

[elgonzo]

That's a problem with your code, not a problem with System.CommandLine.

Your async method definition is using async in conjunction with void return type, hence no exception being caught. As per async documentation (https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/async#return-types):

You use the void return type primarily to define event handlers, which require that return type.
The caller of a void-returning async method can't await it and can't catch exceptions that the method throws.

and elsewhere in the async programming guide, for example https://docs.microsoft.com/en-us/dotnet/csharp/async#important-info-and-advice:

async void should only be used for event handlers.

async void is the only way to allow asynchronous event handlers to work because events do not have return types (thus cannot make use of Task and Task<T>). Any other use of async void does not follow the TAP model and can be challenging to use, such as:

• Exceptions thrown in an async void method can't be caught outside of that method.
• async void methods are difficult to test.
• async void methods can cause bad side effects if the caller isn't expecting them to be async.

(Emphasis mine)

Try using Task as return type for your async method...

[gabrielmaldi]

@elgonzo, you're completely right. Thanks!