如何在套接字编程中处理客户端断开连接？

Question

我使用 Sockets 编写了一个服务器端代码，它工作正常，但有一个问题我不知道如何处理这种情况：如果客户端只是关闭应用程序而不发送断开连接请求，我的服务器端程序会崩溃。我需要做些什么来避免这种情况？请指导我，我是 Socket 编程的新手。

private void OnReceive(IAsyncResult result)
{
    try
    {
        Socket clientSocket = (Socket)result.AsyncState;
        clientSocket.EndReceive(result);

        command = responseMessage = string.Empty;
        command = ByteToString(receviedData);
        receviedData = new byte[30];

        if (command=="Connect")
        {
            ClientInfo clientInfo = new ClientInfo();
            clientInfo.socket = clientSocket;
            clientInfo.IP = clientSocket.RemoteEndPoint.ToString();
            connectedClients.Add(clientInfo);
            responseMessage = "Connection established...";
        }
        else if (command=="Disconnect")
        {
            for (int i = 0; i < connectedClients.Count; i++)
            {
                if (connectedClients[i].socket == clientSocket)
                {
                    connectedClients.RemoveAt(i);
                    break;
                }
            }
            clientSocket.Close();
        }
        else
        {
            responseMessage = "Error";
        }

        byte[] responseStatus = StringToByte(responseMessage);
        for (int i = 0; i < connectedClients.Count; i++)
        {
            if (connectedClients[i].socket==clientSocket)
            {
                connectedClients[i].socket.BeginSend(responseStatus, 0, responseStatus.Length,SocketFlags.None, new AsyncCallback(OnSend), connectedClients[i].socket);
                break;
            }
        }

    }
    catch(Exception ex)
    {
      throw new Exception(ex.Message);
    }
}

Answer 1

您的应用程序崩溃了，因为您在方法的 catch 块中抛出了异常。

如果您不希望您的应用程序崩溃，您需要从 catch 块中删除 throw new Exception(ex.Message); 行。 将其替换为处理错误并将您的应用程序优雅地恢复到安全状态的代码。通过阅读您的代码，这应该通过从 connectedClients 中删除 clientSocket 来完成

其次，最好只使用throw; 而不是throw new Exception(ex.Message);。 throw; 将重新抛出原始异常对象，从而保留堆栈跟踪和其他有助于调试软件的重要信息。 使用new Exception("Message") 将使用当前堆栈跟踪创建一个全新的异常对象。

private void OnReceive(IAsyncResult result)
{
    try
    {
        Socket clientSocket = (Socket)result.AsyncState;
        clientSocket.EndReceive(result);

        command = responseMessage = string.Empty;
        command = ByteToString(receviedData);
        receviedData = new byte[30];

        if (command=="Connect")
        {
            ClientInfo clientInfo = new ClientInfo() {
               socket = clientSocket,
               IP = clientSocket.RemoteEndPoint.ToString(),
               }; 
            connectedClients.Add(clientInfo);
            responseMessage = "Connection established...";
        }
        else if (command=="Disconnect")
        {
            removeClientInfo(clientSocket);
            clientSocket.Close();
        }
        else
        {
            responseMessage = "Error";
        }

        byte[] responseStatus = StringToByte(responseMessage);
        for (int i = 0; i < connectedClients.Count; i++)
        {
            if (connectedClients[i].socket==clientSocket)
            {
                connectedClients[i].socket.BeginSend(responseStatus, 0, responseStatus.Length,SocketFlags.None, new AsyncCallback(OnSend), connectedClients[i].socket);
                break;
            }
        }

    }
    catch(Exception ex)
    {
      // add error handling and gracefully recover
      // caution: The way done here, might work, but it smells :-/
      removeClientInfo((Socket)result.AsyncState);
      ((Socket)result.AsyncState).Close();
    }
}

/// removes the client info from the connectedClients enumerable
private void removeClientInfo(Socket socket)
{
    for (int i = 0; i < connectedClients.Count; i++)
    {
        if (connectedClients[i].socket == socket)
        {
            connectedClients.RemoveAt(i);
            break;
        }
    }
}

Answer 2

您在 catch 块内抛出了一个新异常，除非您正在执行一些日志记录或类似操作，否则这没有多大意义。将 catch 块更改为：

catch(SocketException)
{ 
}

此外，您应该检查从EndReceive 返回的读取字节数。如果您收到零字节，则表示客户端已关闭连接：

int numReceived = clientSocket.EndReceive(result);
if(numReceived == 0)
{
    //client has shutdown the connection
}