Does Task.WhenAll wait for all the tasks in case of exceptions

Question

I have two tasks. I run both of them with Task.WhenAll. What happens if one of them throws an exception? Would the other one complete?

Answer 1

Just run this code to test it:

private static async Task TestTaskWhenAll()
{
    try
    {
        await Task.WhenAll(
            ShortOperationAsync(),
            LongOperationAsync()
        );
    }
    catch (Exception exception)
    {
        Console.WriteLine(exception.Message); // Short operation exception
        Debugger.Break();
    }
}

private static async Task ShortOperationAsync()
{
    await Task.Delay(1000);
    throw new InvalidTimeZoneException("Short operation exception");

}

private static async Task LongOperationAsync()
{
    await Task.Delay(5000);
    throw new ArgumentException("Long operation exception");
}

Debugger will stop in 5 seconds. Both exceptions are thrown, but Debugger.Break() is hit only once. What is more, the exception value is not AggregateException, but InvalidTimeZoneException. This is because of new async/await which does the unwrapping into the actual exception. You can read more here. If you want to read other Exceptions (not only the first one), you would have to read them from the Task returned from WhenAll method call.