Connection-oriented protocols can use the Socket.BeginAccept(AsyncCallback, object) method to asynchronously process incoming connection attempts. Accepting connections asynchronously gives you the ability to send and receive data within a separate execution thread. Before calling the Socket.BeginAccept(AsyncCallback, object) method, you must call the Socket.Listen(int) method to listen for and queue incoming connection requests.

You must create a callback method that implements the AsyncCallback delegate and pass its name to the Socket.BeginAccept(AsyncCallback, object) method. To do this, at the very minimum, you must pass the listening System.Net.Sockets.Socket object to Socket.BeginAccept(AsyncCallback, object) through the state parameter. If your callback needs more information, you can create a small class to hold the System.Net.Sockets.Socket and the other required information. Pass an instance of this class to the Socket.BeginAccept(AsyncCallback, object) method through the state parameter.

Your callback method should invoke the Socket.EndAccept(IAsyncResult) method. When your application calls Socket.BeginAccept(AsyncCallback, object), the system usually uses a separate thread to execute the specified callback method and blocks on Socket.EndAccept(IAsyncResult) until a pending connection is retrieved. Socket.EndAccept(IAsyncResult) will return a new System.Net.Sockets.Socket object that you can use to send and receive data with the remote host. You cannot use this returned System.Net.Sockets.Socket to accept any additional connections from the connection queue. If you want the original thread to block after you call the Socket.BeginAccept(AsyncCallback, object) method, use System.Threading.WaitHandle.WaitOne(int, bool). Call the Set method on a System.Threading.ManualResetEvent in the callback method when you want the original thread to continue executing.

The system may also use the calling thread to invoke the callback method. In this case, the IAsyncResult.CompletedSynchronously property on the returned IAsyncResult will be set to indicate that the Socket.BeginAccept(AsyncCallback, object) method completed synchronously.

For additional information on writing callback methods see [<topic://cpconcallbacksample>].

To cancel a pending call to the erload:System.Net.Sockets.Socket.BeginAccept method, close the System.Net.Sockets.Socket. When the Socket.Close method is called while an asynchronous operation is in progress, the callback provided to the erload:System.Net.Sockets.Socket.BeginAccept method is called. A subsequent call to the erload:System.Net.Sockets.Socket.EndAccept method will throw an ObjectDisposedException to indicate that the operation has been cancelled.

The following excerpt from the System.Net.Sockets.Socket class overview example outlines an asynchronous accept operation.

public class Server
{
  static void Main()
  {
    .
    .
    .
    listenSocket.BeginAccept(
      new AsyncCallback(Server.acceptCallback),
      listenSocket);
    .
    .
    .
    // EndAccept can be called here
    .
    .
    .
  }

  public static void
    acceptCallback(IAsyncResult asyncAccept)
  {
    Socket listenSocket =
      (Socket)asyncAccept.AsyncState;

    Socket serverSocket =
      listenSocket.EndAccept(asyncAccept);

    serverSocket.BeginReceive(...);
    .
    .
    .
  }
}