C# 异步客户端套接字示例

Question

I'm looking at an example of the client socket on Microsofts website and I dont understand how this code is async.

// Connect to the remote endpoint.  
client.BeginConnect(remoteEP,

new AsyncCallback(ConnectCallback), client);

connectDone.WaitOne();

//Send test data to the remote device.
Send(client, "This is a test<EOF>");

sendDone.WaitOne();  

The waitone blocks the current thread until the current waithandle receives a signal.

https://docs.microsoft.com/en-us/dotnet/framework/network-programming/asynchronous-client-socket-example

Also is it possible to write this code using task and wait?

---

When I try to make the SocketAsync with ReceiveAsync I dont seem to be able to await the RecieveAsync .

    private async Task<bool> handleReceiveAsync()
    {
        // Get size of frame
        byte[] frameLenData = new byte[4];
        uint frameLen = 0;
        
            bool bOperationFailed = false;
            SocketAsyncEventArgs socketEventArg = new SocketAsyncEventArgs();
            socketEventArg.SetBuffer(frameLenData, 0, 4);
            socketEventArg.Completed += new EventHandler<SocketAsyncEventArgs>(delegate(object s, SocketAsyncEventArgs e)
            {
            });
          
            return await _socket.ReceiveAsync(socketEventArg);
        
    }

Answer 1

The socket examples on Microsoft's website are known to be extremely bad. The way async is done here is pointless. Async is supposed to not consume a thread (for resource management reasons). This does consume a thread. This is purely worse than synchronous IO.

Modern async IO is done using async/await. It can be this simple:

await socket.ReceiveAsync(...);

This is using true async IO based on tasks. There are more details to this but I hope this gets you oriented in the right direction.

---

One key point is to "find" the modern Task-based socket APIs. Socket has legacy APIs for compatibility reasons. In the code snipped posted in the question, this is an EAP-style API which, while not entirely obsolete, is something that almost never should be used.

So you have to pick the correct overloads. Look at intellisense or documentation to see what is there. You want something that returns a task. Here is a self-contained example:

        using var socket = new Socket(SocketType.Stream, ProtocolType.Tcp);

        await socket.ConnectAsync("example.org", 80);

        await socket.SendAsync(Encoding.ASCII.GetBytes("GET / HTTP/1.0\r\n\r\n"), SocketFlags.None);

        await using var result = new MemoryStream();
        await using var networkStream = new NetworkStream(socket);
        await networkStream.CopyToAsync(result);

        var responseString = Encoding.UTF8.GetString(result.ToArray());

        Console.WriteLine(responseString);

Using sockets correctly is still a bit more involved than this. Unfortunately, it's more than this margin will contain. So I encourage you to research sockets a bit more. Hope this helps you get started.

Try to use a modern style with modern APIs and modern patterns. They are much better.