Core: How to return result - Asynchronous Client Socket Example

[tinonetic]

Hi there,

I am using .NET Core 3.1

I have a requirement to send data asynchronously via TCP. It is a collection of serialised messages(ICollection<string>).

I searched and found a good starting example from the Microsoft (see below). The sample seems to be in under .NET Framework, but I assume it applies to .NET Core as well.

What I am doing:

1. I am repurposing the code as a non-static class
2. I would like to send a collection of messages. I know I can rewrite it to send the collection of strings in the main method.
3. I would like to receive a response for each message sent and do something with it. The current response is stored statically in private static String response = String.Empty;
4. My challenge begins from item _3._. How do I return back that response message that seems only accessible from within private static void ReceiveCallback( IAsyncResult ar )?

I do not think changing it to private static string ReceiveCallback( IAsyncResult ar ) would work. If so, how do I read it from client.BeginReceive( state.buffer, 0, StateObject.BufferSize, 0, new AsyncCallback(ReceiveCallback), state);?

An additional question is: Is it recommended practice to open a TCP connection, send the multiple messages, then close it? Or to open a TCP connection for each message being sent?

```c#
using System;
using System.Net;
using System.Net.Sockets;
using System.Threading;
using System.Text;

// State object for receiving data from remote device.
public class StateObject {
// Client socket.
public Socket workSocket = null;
// Size of receive buffer.
public const int BufferSize = 256;
// Receive buffer.
public byte[] buffer = new byte[BufferSize];
// Received data string.
public StringBuilder sb = new StringBuilder();
}

public class AsynchronousClient {
// The port number for the remote device.
private const int port = 11000;

// ManualResetEvent instances signal completion.  
private static ManualResetEvent connectDone =
    new ManualResetEvent(false);  
private static ManualResetEvent sendDone =
    new ManualResetEvent(false);  
private static ManualResetEvent receiveDone =
    new ManualResetEvent(false);  

// The response from the remote device.  <------ ### the response data that I want to access, non statically
private static String response = String.Empty;  

private static void StartClient() {  
    // Connect to a remote device.  
    try {  
        // Establish the remote endpoint for the socket.  
        // The name of the
        // remote device is "host.contoso.com".  
        IPHostEntry ipHostInfo = Dns.GetHostEntry("host.contoso.com");  
        IPAddress ipAddress = ipHostInfo.AddressList[0];  
        IPEndPoint remoteEP = new IPEndPoint(ipAddress, port);  

        // Create a TCP/IP socket.  
        Socket client = new Socket(ipAddress.AddressFamily,  
            SocketType.Stream, ProtocolType.Tcp);  

        // 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();  

        // Receive the response from the remote device.  
        Receive(client);  
        receiveDone.WaitOne();  

        // Write the response to the console.  
        Console.WriteLine("Response received : {0}", response);  

        // Release the socket.  
        client.Shutdown(SocketShutdown.Both);  
        client.Close();  

    } catch (Exception e) {  
        Console.WriteLine(e.ToString());  
    }  
}  

private static void ConnectCallback(IAsyncResult ar) {  
    try {  
        // Retrieve the socket from the state object.  
        Socket client = (Socket) ar.AsyncState;  

        // Complete the connection.  
        client.EndConnect(ar);  

        Console.WriteLine("Socket connected to {0}",  
            client.RemoteEndPoint.ToString());  

        // Signal that the connection has been made.  
        connectDone.Set();  
    } catch (Exception e) {  
        Console.WriteLine(e.ToString());  
    }  
}  

private static void Receive(Socket client) {  
    try {  
        // Create the state object.  
        StateObject state = new StateObject();  
        state.workSocket = client;  

        // Begin receiving the data from the remote device.  
        client.BeginReceive( state.buffer, 0, StateObject.BufferSize, 0,  
            new AsyncCallback(ReceiveCallback), state);  //<------ The receive callback is here, how do I return the result to the caller?
    } catch (Exception e) {  
        Console.WriteLine(e.ToString());  
    }  
}  

private static void ReceiveCallback( IAsyncResult ar ) {  
    try {  
        // Retrieve the state object and the client socket
        // from the asynchronous state object.  
        StateObject state = (StateObject) ar.AsyncState;  
        Socket client = state.workSocket;  

        // Read data from the remote device.  
        int bytesRead = client.EndReceive(ar);  

        if (bytesRead > 0) {  
            // There might be more data, so store the data received so far.  
        state.sb.Append(Encoding.ASCII.GetString(state.buffer,0,bytesRead));  

            // Get the rest of the data.  
            client.BeginReceive(state.buffer,0,StateObject.BufferSize,0,  
                new AsyncCallback(ReceiveCallback), state);  
        } else {  
            // All the data has arrived; put it in response.  
            if (state.sb.Length > 1) {  
                response = state.sb.ToString();  //<--------- ### Where it is assigned, I want it returned
            }  
            // Signal that all bytes have been received.  
            receiveDone.Set();  
        }  
    } catch (Exception e) {  
        Console.WriteLine(e.ToString());  
    }  
}  

private static void Send(Socket client, String data) {  
    // Convert the string data to byte data using ASCII encoding.  
    byte[] byteData = Encoding.ASCII.GetBytes(data);  

    // Begin sending the data to the remote device.  
    client.BeginSend(byteData, 0, byteData.Length, 0,  
        new AsyncCallback(SendCallback), client);  
}  

private static void SendCallback(IAsyncResult ar) {  
    try {  
        // Retrieve the socket from the state object.  
        Socket client = (Socket) ar.AsyncState;  

        // Complete sending the data to the remote device.  
        int bytesSent = client.EndSend(ar);  
        Console.WriteLine("Sent {0} bytes to server.", bytesSent);  

        // Signal that all bytes have been sent.  
        sendDone.Set();  
    } catch (Exception e) {  
        Console.WriteLine(e.ToString());  
    }  
}  

public static int Main(String[] args) {  
    StartClient();  
    return 0;  
}

}
```

Thank you!

[svick]

I searched and found a good starting example from the Microsoft (see below)

That's actually not a good example, because it uses the outdated Begin*/End* pattern (also known as APM). Instead, you should use using async-await.

And if you switched to that, it would make changing the code the way you want much easier, because there aren't any callbacks anymore, instead you do e.g. await client.ReceiveAsync(…) and, after processing the response, return the result.

[tinonetic]

Hi @svick,

Thank-you for for the answer!

Is there any documented example that illustrates this? Similar to the one above?

What do I need to change in the Microsoft sample?

[scalablecory]

Consider using StreamReader / StreamWriter.

```c#
ICollection strings = ...;
using Socket socket = ...;
using var stream = new NetworkStream(socket);
using var writer = new StreamWriter(stream);

foreach(string s in strings)
{
await writer.WriteLineAsync(s);
}
await writer.FlushAsync();
```

If your strings contain newlines, you will want to length-prefix your messages or escape the newlines prior to writing them.

Is it recommended practice to open a TCP connection, send the multiple messages, then close it? Or to open a TCP connection for each message being sent?

Establishing a TCP connection is generally a lot more expensive than using an existing one. But, this is ultimately scenario-dependent and you will want to do some more learning and prototyping here to see what is right for you.

[tinonetic]

Thank-you @scalablecory, @svick !