Exception not thrown in an async Task configuration

Question

I have the following method

    public async Task<bool> Connect()
    {
        lock (_connectingLock)
        {
            if (_connecting)
                throw new IOException("Already connecting");
            _connecting = true;
        }

        try {
             await tcpClient.ConnectAsync(...); 
        }           
        catch (SocketException e)
        {
            return false;
        }
        finally
        {
            lock (_connectingLock)
            {
                _connecting = false;
            }
        }
    }

Now, I would expect consecutive calls to Connect() to throw an IOException, but it doesn't happen!

What could be the cause?

Answer 1

Calls to Connect() can't throw exceptions directly. Async methods do not throw exceptions. Instead, they will return tasks which, when awaited, will throw IOException . (ie the tasks are faulted.)

If that's not what you want, you should separate the calls:

public Task<bool> Connect()
{
    // Eager validation of state...
    lock (_connectingLock)
    {
        if (_connecting)
            throw new IOException("Already connecting");
        _connecting = true;
    }
    return ConnectImpl();
}

private async Task<bool> ConnectImpl()
{
    try {
         await tcpClient.ConnectAsync(...); 
    }           
    catch (SocketException e)
    {
        return false;
    }
    finally
    {
        lock (_connectingLock)
        {
            _connecting = false;
        }
    }
}

It's not clear whether that's appropriate in this case though. It's generally fine to throw things like ArgumentException eagerly, but if the error doesn't represent a bug in the calling code itself, I think returning a faulted task instead is fine.