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SocketAsyncEventArgs Class


Represents an asynchronous socket operation.

public ref class SocketAsyncEventArgs : EventArgs, IDisposable
public class SocketAsyncEventArgs : EventArgs, IDisposable
type SocketAsyncEventArgs = class
    inherit EventArgs
    interface IDisposable
Public Class SocketAsyncEventArgs
Inherits EventArgs
Implements IDisposable


The following code example implements the connection logic for the socket server that uses the SocketAsyncEventArgs class. After accepting a connection, all data read from the client is sent back to the client. The read and echo back to the client pattern is continued until the client disconnects. The BufferManager class that is used by this example is displayed in the code example for the SetBuffer(Byte[], Int32, Int32) method. The SocketAsyncEventArgsPool class that is used in this example is displayed in the code example for the SocketAsyncEventArgs constructor.

// Implements the connection logic for the socket server.
// After accepting a connection, all data read from the client
// is sent back to the client. The read and echo back to the client pattern
// is continued until the client disconnects.
class Server
    private int m_numConnections;   // the maximum number of connections the sample is designed to handle simultaneously
    private int m_receiveBufferSize;// buffer size to use for each socket I/O operation
    BufferManager m_bufferManager;  // represents a large reusable set of buffers for all socket operations
    const int opsToPreAlloc = 2;    // read, write (don't alloc buffer space for accepts)
    Socket listenSocket;            // the socket used to listen for incoming connection requests
    // pool of reusable SocketAsyncEventArgs objects for write, read and accept socket operations
    SocketAsyncEventArgsPool m_readWritePool;
    int m_totalBytesRead;           // counter of the total # bytes received by the server
    int m_numConnectedSockets;      // the total number of clients connected to the server
    Semaphore m_maxNumberAcceptedClients;

    // Create an uninitialized server instance.
    // To start the server listening for connection requests
    // call the Init method followed by Start method
    // <param name="numConnections">the maximum number of connections the sample is designed to handle simultaneously</param>
    // <param name="receiveBufferSize">buffer size to use for each socket I/O operation</param>
    public Server(int numConnections, int receiveBufferSize)
        m_totalBytesRead = 0;
        m_numConnectedSockets = 0;
        m_numConnections = numConnections;
        m_receiveBufferSize = receiveBufferSize;
        // allocate buffers such that the maximum number of sockets can have one outstanding read and
        //write posted to the socket simultaneously
        m_bufferManager = new BufferManager(receiveBufferSize * numConnections * opsToPreAlloc,

        m_readWritePool = new SocketAsyncEventArgsPool(numConnections);
        m_maxNumberAcceptedClients = new Semaphore(numConnections, numConnections);

    // Initializes the server by preallocating reusable buffers and
    // context objects.  These objects do not need to be preallocated
    // or reused, but it is done this way to illustrate how the API can
    // easily be used to create reusable objects to increase server performance.
    public void Init()
        // Allocates one large byte buffer which all I/O operations use a piece of.  This gaurds
        // against memory fragmentation

        // preallocate pool of SocketAsyncEventArgs objects
        SocketAsyncEventArgs readWriteEventArg;

        for (int i = 0; i < m_numConnections; i++)
            //Pre-allocate a set of reusable SocketAsyncEventArgs
            readWriteEventArg = new SocketAsyncEventArgs();
            readWriteEventArg.Completed += new EventHandler<SocketAsyncEventArgs>(IO_Completed);

            // assign a byte buffer from the buffer pool to the SocketAsyncEventArg object

            // add SocketAsyncEventArg to the pool

    // Starts the server such that it is listening for
    // incoming connection requests.
    // <param name="localEndPoint">The endpoint which the server will listening
    // for connection requests on</param>
    public void Start(IPEndPoint localEndPoint)
        // create the socket which listens for incoming connections
        listenSocket = new Socket(localEndPoint.AddressFamily, SocketType.Stream, ProtocolType.Tcp);
        // start the server with a listen backlog of 100 connections

        // post accepts on the listening socket
        SocketAsyncEventArgs acceptEventArg = new SocketAsyncEventArgs();
        acceptEventArg.Completed += new EventHandler<SocketAsyncEventArgs>(AcceptEventArg_Completed);

        //Console.WriteLine("{0} connected sockets with one outstanding receive posted to each....press any key", m_outstandingReadCount);
        Console.WriteLine("Press any key to terminate the server process....");

    // Begins an operation to accept a connection request from the client
    // <param name="acceptEventArg">The context object to use when issuing
    // the accept operation on the server's listening socket</param>
    public void StartAccept(SocketAsyncEventArgs acceptEventArg)
        // loop while the method completes synchronously
        bool willRaiseEvent = false;
        while (!willRaiseEvent)

            // socket must be cleared since the context object is being reused
            acceptEventArg.AcceptSocket = null;            
            willRaiseEvent = listenSocket.AcceptAsync(acceptEventArg);
            if (!willRaiseEvent)

    // This method is the callback method associated with Socket.AcceptAsync
    // operations and is invoked when an accept operation is complete
    void AcceptEventArg_Completed(object sender, SocketAsyncEventArgs e)
        // Accept the next connection request

    private void ProcessAccept(SocketAsyncEventArgs e)
        Interlocked.Increment(ref m_numConnectedSockets);
        Console.WriteLine("Client connection accepted. There are {0} clients connected to the server",

        // Get the socket for the accepted client connection and put it into the
        //ReadEventArg object user token
        SocketAsyncEventArgs readEventArgs = m_readWritePool.Pop();
        readEventArgs.UserToken = e.AcceptSocket;

        // As soon as the client is connected, post a receive to the connection
        bool willRaiseEvent = e.AcceptSocket.ReceiveAsync(readEventArgs);
        if (!willRaiseEvent)

    // This method is called whenever a receive or send operation is completed on a socket
    // <param name="e">SocketAsyncEventArg associated with the completed receive operation</param>
    void IO_Completed(object sender, SocketAsyncEventArgs e)
        // determine which type of operation just completed and call the associated handler
        switch (e.LastOperation)
            case SocketAsyncOperation.Receive:
            case SocketAsyncOperation.Send:
                throw new ArgumentException("The last operation completed on the socket was not a receive or send");

    // This method is invoked when an asynchronous receive operation completes.
    // If the remote host closed the connection, then the socket is closed.
    // If data was received then the data is echoed back to the client.
    private void ProcessReceive(SocketAsyncEventArgs e)
        // check if the remote host closed the connection
        if (e.BytesTransferred > 0 && e.SocketError == SocketError.Success)
            //increment the count of the total bytes receive by the server
            Interlocked.Add(ref m_totalBytesRead, e.BytesTransferred);
            Console.WriteLine("The server has read a total of {0} bytes", m_totalBytesRead);

            //echo the data received back to the client
            e.SetBuffer(e.Offset, e.BytesTransferred);
            Socket socket = (Socket)e.UserToken;
            bool willRaiseEvent = socket.SendAsync(e);
            if (!willRaiseEvent)

    // This method is invoked when an asynchronous send operation completes.
    // The method issues another receive on the socket to read any additional
    // data sent from the client
    // <param name="e"></param>
    private void ProcessSend(SocketAsyncEventArgs e)
        if (e.SocketError == SocketError.Success)
            // done echoing data back to the client
            Socket socket = (Socket)e.UserToken;
            // read the next block of data send from the client
            bool willRaiseEvent = socket.ReceiveAsync(e);
            if (!willRaiseEvent)

    private void CloseClientSocket(SocketAsyncEventArgs e)
        Socket socket = (Socket)e.UserToken;

        // close the socket associated with the client
        // throws if client process has already closed
        catch (Exception) { }

        // decrement the counter keeping track of the total number of clients connected to the server
        Interlocked.Decrement(ref m_numConnectedSockets);

        // Free the SocketAsyncEventArg so they can be reused by another client

        Console.WriteLine("A client has been disconnected from the server. There are {0} clients connected to the server", m_numConnectedSockets);


The SocketAsyncEventArgs class is part of a set of enhancements to the System.Net.Sockets.Socket class that provide an alternative asynchronous pattern that can be used by specialized high-performance socket applications. This class was specifically designed for network server applications that require high performance. An application can use the enhanced asynchronous pattern exclusively or only in targeted hot areas (for example, when receiving large amounts of data).

The main feature of these enhancements is the avoidance of the repeated allocation and synchronization of objects during high-volume asynchronous socket I/O. The Begin/End design pattern currently implemented by the System.Net.Sockets.Socket class requires a System.IAsyncResult object be allocated for each asynchronous socket operation.

In the new System.Net.Sockets.Socket class enhancements, asynchronous socket operations are described by reusable SocketAsyncEventArgs objects allocated and maintained by the application. High-performance socket applications know best the amount of overlapped socket operations that must be sustained. The application can create as many of the SocketAsyncEventArgs objects that it needs. For example, if a server application needs to have 15 socket accept operations outstanding at all times to support incoming client connection rates, it can allocate 15 reusable SocketAsyncEventArgs objects for that purpose.

The pattern for performing an asynchronous socket operation with this class consists of the following steps:

  1. Allocate a new SocketAsyncEventArgs context object, or get a free one from an application pool.

  2. Set properties on the context object to the operation about to be performed (the completion callback method, the data buffer, the offset into the buffer, and the maximum amount of data to transfer, for example).

  3. Call the appropriate socket method (xxxAsync) to initiate the asynchronous operation.

  4. If the asynchronous socket method (xxxAsync) returns true, in the callback, query the context properties for completion status.

  5. If the asynchronous socket method (xxxAsync) returns false, the operation completed synchronously. The context properties may be queried for the operation result.

  6. Reuse the context for another operation, put it back in the pool, or discard it.

The lifetime of the new asynchronous socket operation context object is determined by references by the application code and asynchronous I/O references. It is not necessary for the application to retain a reference to an asynchronous socket operation context object after it is submitted as a parameter to one of the asynchronous socket operation methods. It will remain referenced until the completion callback returns. However it is advantageous for the application to retain the reference to the context so that it can be reused for a future asynchronous socket operation.



Creates an empty SocketAsyncEventArgs instance.


Initializes the SocketAsyncEventArgs.



Gets or sets the socket to use or the socket created for accepting a connection with an asynchronous socket method.


Gets the data buffer to use with an asynchronous socket method.


Gets or sets an array of data buffers to use with an asynchronous socket method.


Gets the number of bytes transferred in the socket operation.


Gets the exception in the case of a connection failure when a DnsEndPoint was used.


The created and connected Socket object after successful completion of the ConnectAsync method.


Gets the maximum amount of data, in bytes, to send or receive in an asynchronous operation.


Gets or sets a value that specifies if socket can be reused after a disconnect operation.


Gets the type of socket operation most recently performed with this context object.


Gets the region of memory to use as a buffer with an asynchronous socket method.


Gets the offset, in bytes, into the data buffer referenced by the Buffer property.


Gets the IP address and interface of a received packet.


Gets or sets the remote IP endpoint for an asynchronous operation.


Gets or sets an array of buffers to be sent for an asynchronous operation used by the SendPacketsAsync(SocketAsyncEventArgs) method.


Gets or sets a bitwise combination of TransmitFileOptions values for an asynchronous operation used by the SendPacketsAsync(SocketAsyncEventArgs) method.


Gets or sets the size, in bytes, of the data block used in the send operation.


Gets or sets the protocol to use to download the socket client access policy file.


Gets or sets the result of the asynchronous socket operation.


Gets the results of an asynchronous socket operation or sets the behavior of an asynchronous operation.


Gets or sets a user or application object associated with this asynchronous socket operation.



Releases the unmanaged resources used by the SocketAsyncEventArgs instance and optionally disposes of the managed resources.


Determines whether the specified object is equal to the current object.

(Inherited from Object)

Frees resources used by the SocketAsyncEventArgs class.


Serves as the default hash function.

(Inherited from Object)

Gets the Type of the current instance.

(Inherited from Object)

Creates a shallow copy of the current Object.

(Inherited from Object)

Represents a method that is called when an asynchronous operation completes.

SetBuffer(Byte[], Int32, Int32)

Sets the data buffer to use with an asynchronous socket method.

SetBuffer(Int32, Int32)

Sets the data buffer to use with an asynchronous socket method.


Sets the region of memory to use as a buffer with an asynchronous socket method.


Returns a string that represents the current object.

(Inherited from Object)



The event used to complete an asynchronous operation.

Applies to

See also