async / await vs BeginRead, EndRead

问题

I don't quite 'get' async and await yet, and I'm looking for some clarification around a particular problem I'm about to solve. Basically, I need to write some code that'll handle a TCP connection. It'll essentially just receive data and process it until the connection is closed.

I'd normally write this code using the NetworkStream BeginRead and EndRead pattern, but since the async / await pattern is much cleaner, I'm tempted to use that instead. However, since I admittedly don't fully understand exactly what is involved in these, I'm a little wary of the consequences. Will one use more resources than the other; will one use a thread where another would use IOCP, etc.

Convoluted example time. These two do the same thing - count the bytes in a stream:

class StreamCount
{
    private Stream str;
    private int total = 0;
    private byte[] buffer = new byte[1000];

    public Task<int> CountBytes(Stream str)
    {
        this.str = str;

        var tcs = new TaskCompletionSource<int>();
        Action onComplete = () => tcs.SetResult(total);
        str.BeginRead(this.buffer, 0, 1000, this.BeginReadCallback, onComplete);

        return tcs.Task;
    }

    private void BeginReadCallback(IAsyncResult ar)
    {
        var bytesRead = str.EndRead(ar);
        if (bytesRead == 0)
        {
            ((Action)ar.AsyncState)();
        }
        else
        {
            total += bytesRead;
            str.BeginRead(this.buffer, 0, 1000, this.BeginReadCallback, ar.AsyncState);
        }
    }
}

... And...

    public static async Task<int> CountBytes(Stream str)
    {
        var buffer = new byte[1000];
        var total = 0;
        while (true)
        {
            int bytesRead = await str.ReadAsync(buffer, 0, 1000);
            if (bytesRead == 0)
            {
                break;
            }
            total += bytesRead;
        }
        return total;
    }

To my eyes, the async way looks cleaner, but there is that 'while (true)' loop that my uneducated brain tells me is going to use an extra thread, more resources, and therefore won't scale as well as the other one. But I'm fairly sure that is wrong. Are these doing the same thing in the same way?

回答1:

To my eyes, the async way looks cleaner, but there is that 'while (true)' loop that my uneducated brain tells me is going to use an extra thread, more resources, and therefore won't scale as well as the other one.

Nope, it won't. The loop will only use a thread when it's actually running code... just as it would in your BeginRead callback. The await expression will return control to whatever the calling code is, having registered a continuation which jumps back to the right place in the method (in an appropriate thread, based on the synchronization context) and then continues running until it either gets to the end of the method or hits another await expression. It's exactly what you want :)

It's worth learning more about how async/await works behind the scenes - you might want to start with the MSDN page on it, as a jumping off point.

来源：https://stackoverflow.com/questions/26652328/async-await-vs-beginread-endread

标签

asynchronous

async-await

networkstream