c# sockets handling multiple clients

Question

I have the following code which I want to implement as my server. As I understand it is async. and should allow connections from multiple clients...

public void Start()
{          
    TcpListener listener = new TcpListener(IPAddress.Any, 10250);
    listener.Start();
    Console.WriteLine("Listening...");

    while (true)
    {
        IAsyncResult res = listener.BeginAcceptTcpClient(HandleAsyncConnection, listener);
        connectionWaitHandle.WaitOne();
    }
}

private void HandleAsyncConnection(IAsyncResult res)
{
    TcpListener listener = (TcpListener)res.AsyncState;
    TcpClient client = listener.EndAcceptTcpClient(res);
    connectionWaitHandle.Set();

    StringBuilder sb = new StringBuilder();
    var data = new byte[client.ReceiveBufferSize];

    using (NetworkStream ns = client.GetStream())
    {             
        // Test reply
        Byte[] replyData = System.Text.Encoding.ASCII.GetBytes(DateTime.Now.ToString());
        ns.Write(replyData, 0, replyData.Length);
        ns.Flush();
        ns.Close();
    }

    client.Close();
}

I have a test app which simply fires requests to my server. As you can see in the code the server just replies with its date/time. The test app sends say 20 requests which are simply test strings. For each of these requests it opens a socket, sends the data to my server and then closes the socket again.

This works fine with one test app running. However, if I open two test apps the second one cannot connect to the server. I thought because I am handling the request async. and because my test app opens then closes the socket before each call I could handle requests from multiple clients?

I have some sample code that does exactly what you want but it's on my laptop. I'll post tonight if I remember and if someone hasn't already answered for you. Sorry about the delay. — BenCr
– BenCr, Commented Apr 21, 2011 at 10:13

Community · Accepted Answer · 2017-05-23 11:53:59Z

20

Edit

If using >=.Net4.5, it's better to use the new network methods that then permit the adoption of async and await. As such, it might be better to follow the example I provided in this post as a starting point.

Original Post

The following code demonstrates how to accept multiple clients asynchronously without spinning off a new thread per connection.

private TcpListener listener;
public void Start()
{          
    listener = new TcpListener(IPAddress.Any, 10250);
    listener.Start();
    Console.WriteLine("Listening...");
    StartAccept();

}
private void StartAccept()
{
    listener.BeginAcceptTcpClient(HandleAsyncConnection, listener);
}
private void HandleAsyncConnection(IAsyncResult res)
{
    StartAccept(); //listen for new connections again
    TcpClient client = listener.EndAcceptTcpClient(res);
    //proceed

}

You can use this pattern for most async operations.

edited May 23, 2017 at 11:53

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answered Apr 21, 2011 at 10:33

spender

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8 Comments

Remotec Over a year ago

Does there need to be a loop inside StartAccept() to keep the app. active until a connection arrives?

spender Over a year ago

I assume this is a Console app then. I'd suggest calling Start from the application's Main method, then preventing the program from closing with a Console.ReadKey or similar.

Remotec Over a year ago

This seems to be the code thats giving me the best results so far.

Oghenero E. Over a year ago

@spender i like this code exaple for its simplicity. but after the //proceed can you help with code sample to manage multiple clients. ie sending message to each one uniquely.

spender Over a year ago

@Cesar Yes, otherwise the server will accept only a single client. I would advise against such code with >.Net 4.5 and encourage you to adopt the new network methods that allow you to use async/await. See my answer here: stackoverflow.com/questions/12630827/…

|

jgauffin · Accepted Answer · 2011-04-21 10:33:16Z

The way you did it, there is no benefit compared to using AcceptTcpClient. Simply loop and create a new thread for each accepted connection:

public void Start()
{          
    TcpListener listener = new TcpListener(IPAddress.Any, 10250);
    listener.Start();
    Console.WriteLine("Listening...");
    while (canRun)
    {
       var client = listener.AcceptTcpClient();
       new Thread(ClientThread).Start(client);
    }
}

private void ClientThread(IAsyncResult res)
{
    TcpClient client = (TcpClient)res.AsyncState;

    StringBuilder sb = new StringBuilder();
    var data = new byte[client.ReceiveBufferSize];

    using (NetworkStream ns = client.GetStream())
    {             
        // Test reply
        Byte[] replyData = System.Text.Encoding.ASCII.GetBytes(DateTime.Now.ToString());
        ns.Write(replyData, 0, replyData.Length);
        ns.Flush();
        ns.Close();
    }

    client.Close();
}

A golden rule when using asynchronous methods is to NOT block the operation. Blocking would defeat the purpose of using async ops.

public void Start()
{          
    TcpListener listener = new TcpListener(IPAddress.Any, 10250);
    listener.Start();
    Console.WriteLine("Listening...");
    listener.BeginAcceptTcpClient(OnAccept, listener);
}

private void OnAccept(IAsyncResult res)
{
    TcpListener listener = (TcpListener)res.AsyncState;
    TcpClient client = listener.EndAcceptTcpClient(res);

    StringBuilder sb = new StringBuilder();
    var data = new byte[client.ReceiveBufferSize];

    using (NetworkStream ns = client.GetStream())
    {             
        // Test reply
        Byte[] replyData = System.Text.Encoding.ASCII.GetBytes(DateTime.Now.ToString());
        ns.Write(replyData, 0, replyData.Length);
        ns.Flush();
        ns.Close();
    }

    client.Close();
}

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