System.Net.Sockets.Socket 類有一組增強功能，提供可供專用的高性能套接字應用程序使用的可選異步模式，SocketAsyncEventArgs 類就是這一組增強功能的一部分。 該類專爲需要高性能的網絡服務器應用程序而設計。 應用程序可以完全使用增強的異步模式，也可以僅僅在目標熱點區域（例如，在接收大量數據時）使用此模式。
這些增強功能的主要特點是可以避免在異步套接字 I/O 量非常大時發生重複的對象分配和同步。 當前由 System.Net.Sockets.Socket 類實現的開始/結束設計模式要求爲每個異步套接字操作分配一個System.IAsyncResult 對象。
在新的 System.Net.Sockets.Socket 類增強功能中，異步套接字操作由分配的可重用 SocketAsyncEventArgs 對象描述並由應用程序維護。 高性能套接字應用程序非常清楚地知道必須保持的重疊的套接字操作的量。 應用程序可以根據自身需要創建任意多的 SocketAsyncEventArgs 對象。 例如，如果服務器應用程序需要總是有 15 個未完成的套接字接收操作來支持傳入客戶端連接率，那麼可以爲此分配 15 個可重用的SocketAsyncEventArgs 對象。
使用此類執行異步套接字操作的模式包含以下步驟：
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分配一個新的 SocketAsyncEventArgs 上下文對象，或者從應用程序池中獲取一個空閒的此類對象。
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將該上下文對象的屬性設置爲要執行的操作（例如，完成回調方法、數據緩衝區、緩衝區偏移量以及要傳輸的最大數據量）。
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調用適當的套接字方法 (xxxAsync) 以啓動異步操作。
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如果異步套接字方法 (xxxAsync) 返回 true，則在回調中查詢上下文屬性來獲取完成狀態。
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如果異步套接字方法 (xxxAsync) 返回 false，則說明操作是同步完成的。 可以查詢上下文屬性來獲取操作結果。
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將該上下文重用於另一個操作，將它放回到應用程序池中，或者將它丟棄。
新的異步套接字操作上下文對象的生命週期由應用程序代碼引用和異步 I/O 引用決定。 在對異步套接字操作上下文對象的引用作爲一個參數提交給某個異步套接字操作方法之後，應用程序不必保留該引用。 在完成回調返回之前將一直引用該對象。 但是，應用程序保留對上下文的引用是有好處的，這樣該引用就可以重用於將來的異步套接字操作。
下面的代碼示例實現使用 SocketAsyncEventArgs 類的套接字服務器的連接邏輯。 接受連接之後，從客戶端讀取的所有數據都將發回客戶端。 客戶端模式的讀取和回送會一直繼續到客戶端斷開連接。 此示例使用的 BufferManager 類顯示在 SetBuffer(Byte[], Int32, Int32) 方法的代碼示例中。 此示例使用的 SocketAsyncEventArgsPool 類顯示在 SocketAsyncEventArgs 構造函數的代碼示例中。// 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,
receiveBufferSize);
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
m_bufferManager.InitBuffer();
// 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);
readWriteEventArg.UserToken = new AsyncUserToken();
// assign a byte buffer from the buffer pool to the SocketAsyncEventArg object
m_bufferManager.SetBuffer(readWriteEventArg);
// add SocketAsyncEventArg to the pool
m_readWritePool.Push(readWriteEventArg);
}
}
// 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);
listenSocket.Bind(localEndPoint);
// start the server with a listen backlog of 100 connections
listenSocket.Listen(100);
// post accepts on the listening socket
StartAccept(null);
//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....");
Console.ReadKey();
}
// 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)
{
if (acceptEventArg == null)
{
acceptEventArg = new SocketAsyncEventArgs();
acceptEventArg.Completed += new EventHandler<SocketAsyncEventArgs>(AcceptEventArg_Completed);
}
else
{
// socket must be cleared since the context object is being reused
acceptEventArg.AcceptSocket = null;
}
m_maxNumberAcceptedClients.WaitOne();
bool willRaiseEvent = listenSocket.AcceptAsync(acceptEventArg);
if (!willRaiseEvent)
{
ProcessAccept(acceptEventArg);
}
}
// 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)
{
ProcessAccept(e);
}
private void ProcessAccept(SocketAsyncEventArgs e)
{
Interlocked.Increment(ref m_numConnectedSockets);
Console.WriteLine("Client connection accepted. There are {0} clients connected to the server",
m_numConnectedSockets);
// Get the socket for the accepted client connection and put it into the
//ReadEventArg object user token
SocketAsyncEventArgs readEventArgs = m_readWritePool.Pop();
((AsyncUserToken)readEventArgs.UserToken).Socket = e.AcceptSocket;
// As soon as the client is connected, post a receive to the connection
bool willRaiseEvent = e.AcceptSocket.ReceiveAsync(readEventArgs);
if(!willRaiseEvent){
ProcessReceive(readEventArgs);
}
// Accept the next connection request
StartAccept(e);
}
// 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:
ProcessReceive(e);
break;
case SocketAsyncOperation.Send:
ProcessSend(e);
break;
default:
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
AsyncUserToken token = (AsyncUserToken)e.UserToken;
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);
bool willRaiseEvent = token.Socket.SendAsync(e);
if (!willRaiseEvent)
{
ProcessSend(e);
}
}
else
{
CloseClientSocket(e);
}
}
// 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
AsyncUserToken token = (AsyncUserToken)e.UserToken;
// read the next block of data send from the client
bool willRaiseEvent = token.Socket.ReceiveAsync(e);
if (!willRaiseEvent)
{
ProcessReceive(e);
}
}
else
{
CloseClientSocket(e);
}
}
private void CloseClientSocket(SocketAsyncEventArgs e)
{
AsyncUserToken token = e.UserToken as AsyncUserToken;
// close the socket associated with the client
try
{
token.Socket.Shutdown(SocketShutdown.Send);
}
// throws if client process has already closed
catch (Exception) { }
token.Socket.Close();
// decrement the counter keeping track of the total number of clients connected to the server
Interlocked.Decrement(ref m_numConnectedSockets);
m_maxNumberAcceptedClients.Release();
Console.WriteLine("A client has been disconnected from the server. There are {0} clients connected to the server", m_numConnectedSockets);
// Free the SocketAsyncEventArg so they can be reused by another client
m_readWritePool.Push(e);
}
}