Socket.BeginAccept Method (Int32, AsyncCallback, Object)


Begins an asynchronous operation to accept an incoming connection attempt and receives the first block of data sent by the client application.

Namespace:   System.Net.Sockets
Assembly:  System (in System.dll)

[HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)]
public IAsyncResult BeginAccept(
	int receiveSize,
	AsyncCallback callback,
	object state


Type: System.Int32

The number of bytes to accept from the sender.

Type: System.AsyncCallback

The AsyncCallback delegate.

Type: System.Object

An object that contains state information for this request.

Return Value

Type: System.IAsyncResult

An IAsyncResult that references the asynchronous Socket creation.

Exception Condition

The Socket object has been closed.


Windows NT is required for this method.


The accepting socket is not listening for connections. You must call Bind and Listen before calling BeginAccept.


The accepted socket is bound.


receiveSize is less than 0.


An error occurred when attempting to access the socket. See the Remarks section for more information.

Connection-oriented protocols can use the BeginAccept method to asynchronously process incoming connection attempts. Accepting connections asynchronously enables you to send and receive data within a separate execution thread. This overload allows you to specify the number of bytes to accept in the initial transfer in the receiveSize parameter.

Before calling the BeginAccept method, you must call the Listen 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 BeginAccept method. To do this, at the very minimum, you must pass the listening Socket object to BeginAccept through the state parameter. If your callback needs more information, you can create a small class to hold the Socket and the other required information. Pass an instance of this class to the BeginAccept method through the state parameter.

Your callback method should invoke the EndAccept method. When your application calls BeginAccept, the system usually uses a separate thread to execute the specified callback method and blocks on EndAccept until a pending connection is retrieved.

EndAccept returns a new Socket that you can use to send and receive data with the remote host. You cannot use this returned Socket to accept any additional connections from the connection queue. If you want the original thread to block after you call the BeginAccept method, use WaitHandle.WaitOne. Call the Set method on a 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 CompletedSynchronously property on the returned IAsyncResult will be set to indicate that the BeginAcceptmethod completed synchronously.

For additional information on writing callback methods see Marshaling a Delegate as a Callback Method.

To cancel a pending call to the BeginAccept method, close the Socket. When the Closemethod is called while an asynchronous operation is in progress, the callback provided to the BeginAccept method is called. A subsequent call to the EndAcceptmethod will throw an ObjectDisposedException to indicate that the operation has been cancelled.


You can call use the RemoteEndPoint property of the returned Socket object to identify the remote host's network address and port number.


If you receive a SocketException, use the SocketException.ErrorCode property to obtain the specific error code. After you have obtained this code, refer to the Windows Sockets version 2 API error code documentation in the MSDN library for a detailed description of the error.


This member outputs trace information when you enable network tracing in your application. For more information, see Network Tracing in the .NET Framework.


The execution context (the security context, the impersonated user, and the calling context) is cached for the asynchronous Socket methods. After the first use of a particular context (a specific asynchronous Socket method, a specific Socket instance, and a specific callback), subsequent uses of that context will see a performance improvement.

Legacy Code Example

The following code example opens a socket and accepts an asynchronous connection. In this example, the socket accepts the initial 10 bytes of data. The number of bytes received and the data are displayed on the console by the callback delegate. See BeginReceive for a description of how the remaining data is received.

    // This server waits for a connection and then uses asynchronous operations to
    // accept the connection with initial data sent from the client.

    // Establish the local endpoint for the socket.

    IPHostEntry ipHostInfo = Dns.GetHostEntry(Dns.GetHostName());
    IPAddress ipAddress = ipHostInfo.AddressList[0];
    IPEndPoint localEndPoint = new IPEndPoint(ipAddress, 11000);

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

    // Bind the socket to the local endpoint, and listen for incoming connections.

    while (true) 
        // Set the event to nonsignaled state.

        // Start an asynchronous socket to listen for connections and receive data from the client.
        Console.WriteLine("Waiting for a connection...");

        // Accept the connection and receive the first 10 bytes of data.
        int receivedDataSize = 10;
        listener.BeginAccept(receivedDataSize, new AsyncCallback(AcceptReceiveCallback), listener);

        // Wait until a connection is made and processed before continuing.


public static void AcceptReceiveCallback(IAsyncResult ar) 
    // Get the socket that handles the client request.
    Socket listener = (Socket) ar.AsyncState;

    // End the operation and display the received data on the console.
    byte[] Buffer;
    int bytesTransferred;
    Socket handler = listener.EndAccept(out Buffer, out bytesTransferred, ar);
    string stringTransferred = Encoding.ASCII.GetString(Buffer, 0, bytesTransferred);

    Console.WriteLine("Size of data transferred is {0}", bytesTransferred);

    // Create the state object for the asynchronous receive.
    StateObject state = new StateObject();
    state.workSocket = handler;
    handler.BeginReceive( state.buffer, 0, StateObject.BufferSize, 0,
    new AsyncCallback(ReadCallback), state);

.NET Framework
Available since 2.0
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