连接中的客户端，如何判断服务器关闭，用vb.net编写

在 VB.NET 中判断服务器是否关闭可以通过捕获异常来实现。当客户端尝试与服务器建立连接时，如果连接失败或者在数据接收的过程中发生异常，可以认为服务器已关闭。以下是一个示例代码：

Imports System
Imports System.Net
Imports System.Net.Sockets
Imports System.Threading

Public Class Client
    Private clientSocket As TcpClient
    Private serverIP As String
    Private serverPort As Integer
    Private isConnected As Boolean

    Public Sub New(ByVal ip As String, ByVal port As Integer)
        serverIP = ip
        serverPort = port
        isConnected = False
    End Sub

    Public Sub Connect()
        Try
            clientSocket = New TcpClient()
            clientSocket.Connect(serverIP, serverPort)
            isConnected = True
            StartListening()
        Catch ex As SocketException
            Console.WriteLine("Failed to connect to the server: " & ex.Message)
        End Try
    End Sub

    Public Sub Disconnect()
        If isConnected Then
            isConnected = False
            clientSocket.Close()
        End If
    End Sub

    Private Sub StartListening()
        Dim receiveThread As New Thread(AddressOf ReceiveData)
        receiveThread.Start()
    End Sub

    Private Sub ReceiveData()
        While isConnected
            Try
                Dim stream As NetworkStream = clientSocket.GetStream()
                Dim buffer(clientSocket.ReceiveBufferSize) As Byte
                Dim bytesRead As Integer = stream.Read(buffer, 0, clientSocket.ReceiveBufferSize)
                Dim receivedData As String = System.Text.Encoding.ASCII.GetString(buffer, 0, bytesRead)
                Console.WriteLine("Received data from server: " & receivedData)

                ' 处理接收到的数据
                ' ...

            Catch ex As SocketException
                Console.WriteLine("Lost connection to the server: " & ex.Message)
                Exit While ' 退出循环，结束连接
            End Try
        End While
    End Sub
End Class

Public Class MainProgram
    Public Shared Sub Main()
        Dim serverIP As String = "127.0.0.1" ' 服务器的 IP 地址
        Dim serverPort As Integer = 1234 ' 服务器的端口号

        Dim client As New Client(serverIP, serverPort)
        client.Connect()

        ' 执行其他操作
        ' ...

        client.Disconnect()
    End Sub
End Class

在上述代码中，当在接收数据的过程中发生 `SocketException` 异常时，会打印出错误信息并退出循环，结束连接。你可以根据实际需求修改代码，比如添加重连逻辑或其他处理方式。请注意，这只是一种简单的判断方式，具体情况可能因网络环境等因素而有所不同。