显示 Telnet 客户端收到的消息时的延迟

Question

我正在尝试在 VB.NET 中实现 Telnet 客户端。我以this 代码为例：

我正在实施的程序如下：

1. 我单击“打开 Telnet”按钮打开 Telnet 会话。
2. 我在左侧的文本框中写了一条指令（字符串），然后单击Button1 将消息发送到 Telnet 服务器（带有嵌入式以太网端口的电子板）。
3. Telnet 服务器发送的应答显示在左侧的文本框中。

我在示例和正在执行的实现中遇到的问题是消息显示延迟。例如，如果我发送字符串1FFFFFF + vbCrLf，我应该会收到来自服务器的消息Unknown HMI code!。我已经与 Wireshark 确认该消息是在我使用 VB.NET 程序发送指令后由 Telnet 服务器发送的，但只有当我再次单击 Button1 时它才会显示在右侧的文本框中（无论左边的文本框中写的是什么）。

您能否告诉我代码中是否缺少某些内容？

下面是我的代码：

Imports System
Imports System.IO
Imports System.Net.Sockets
Imports System.Security.Cryptography.X509Certificates
Imports System.Text
Imports System.Threading
Imports System.Net.Http
Imports System.Net.Security
Imports System.Net.IPAddress
Imports System.Net

Public Class Form1
    ' Create a TcpClient.
    Dim client As New TcpClient
    Dim stream As NetworkStream

    ' Function to write/read a TCP stream. 
    Shared Sub Connect(server As [String], message As [String])
        Try
            ' Translate the passed message into ASCII and store it as a Byte array.
            Dim data As [Byte]() = System.Text.Encoding.ASCII.GetBytes(message)

            ' Send the message to the connected TcpServer. 
            Form1.stream.Write(data, 0, data.Length)

            Console.WriteLine("Sent: {0}", message)

            ' Buffer to store the response bytes.
            data = New [Byte](256) {}

            ' String to store the response ASCII representation.
            Dim responseData As [String] = [String].Empty

            ' Read the first batch of the TcpServer response bytes.
            Dim bytes As Int32 = Form1.stream.Read(data, 0, data.Length)
            responseData = System.Text.Encoding.ASCII.GetString(data, 0, bytes)
            Console.WriteLine("Received: {0}", responseData)
            Form1.TelnetRx.Text += responseData + vbCrLf
            Form1.TelnetRx.Refresh()

        Catch e As ArgumentNullException
            Console.WriteLine("ArgumentNullException: {0}", e)
        Catch e As SocketException
            Console.WriteLine("SocketException: {0}", e)
        End Try

        Console.WriteLine(ControlChars.Cr + " Press Enter to continue...")
        Console.Read()
    End Sub

    ' Function to open a Telnet session.
    Public Function OpenTelnetSession(server As String, Port As Int32) As Boolean
        Dim ipAddress As IPAddress = Parse(server)
        client.Connect(ipAddress, Port)
        stream = Me.client.GetStream()
        Return True
    End Function

    ' Button to send a message.
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Connect("192.168.8.110", TCP_Order.Text + vbCrLf)
    End Sub

    ' Button to open the Telnet session.
    Private Sub Button2_Click(sender As Object, e As EventArgs) Handles OpenTelnet.Click
        OpenTelnetSession("192.168.8.110", 10001)
    End Sub

    ' Button to close the Telnet session.
    Private Sub CloseTelnet_Click(sender As Object, e As EventArgs) Handles CloseTelnet.Click

    End Sub
End Class

Answer 1

因为您没有读取响应的整个缓冲区，所以您只是从中获取 256 个字节：

data = New [Byte](256) {} ' <-

您还必须在收到响应后通过关闭流和TcpClient 来释放资源。始终通过Using 语句创建一次性对象以保证这一点。

同步示例

---

下例以同步阻塞模式连接到一个端点，调用者线程被阻塞，直到端点返回响应或抛出异常（例如连接超时）。

Private Function Connect(server As String, port As Integer, Msg As String) As String
    Using client As New TcpClient(server, port),
        netStream = client.GetStream,
        sr = New StreamReader(netStream, Encoding.UTF8)

        Dim msgBytes = Encoding.UTF8.GetBytes(Msg)

        netStream.Write(msgBytes, 0, msgBytes.Length)

        Return sr.ReadToEnd
    End Using
End Function

和调用者：

Private Sub TheCaller()
    Dim resp As String = Nothing

    Try
        Dim server = "192.168.8.110"
        Dim port = 10001
        Dim msg = $"1FFFFFF{ControlChars.CrLf}{ControlChars.CrLf}"
        
        resp = Connect(server, port, msg)
    Catch ex As ArgumentNullException
        resp = ex.Message
    Catch ex As SocketException
        resp = ex.SocketErrorCode.ToString
    Catch ex As Exception
        resp = ex.Message
    Finally
        If resp IsNot Nothing Then
            UpdateStatus(resp)
        End If
    End Try
End Sub

异步示例

---

您可能希望使用异步操作，因为您正在开发 WinForms 应用程序，并且我认为您不想阻塞 UI 线程。这里需要调用TcpClient的Async方法和读/写流：

Private Async Function ConnectAsync(server As String,
                                    port As Integer, msg As String) As Task(Of String)
    Using client As New TcpClient
        Await client.ConnectAsync(server, port)

        Using netStream = client.GetStream,
            sw = New StreamWriter(netStream, Encoding.UTF8) With {.AutoFlush = True },
            sr = New StreamReader(netStream, Encoding.UTF8)

            Await sw.WriteLineAsync(msg)

            Return Await sr.ReadToEndAsync()
        End Using
    End Using
End Function

还有一个Async 来电者：

Private Async Sub TheCaller()
    Dim resp As String = Nothing

    Try
        Dim server = "192.168.8.110"
        Dim port = 10001
        Dim msg = $"1FFFFFF{ControlChars.CrLf}{ControlChars.CrLf}"
        
        resp = Await ConnectAsync(server, port, msg)
    Catch ex As ArgumentNullException
        resp = ex.Message
    Catch ex As SocketException
        resp = ex.SocketErrorCode.ToString
    Catch ex As Exception
        resp = ex.Message
    Finally
        If resp IsNot Nothing Then
            UpdateStatus(resp)
        End If
    End Try
End Sub

代码 sn-ps 中的UpdateStatus 只是一种将响应附加到文本框的方法..

Private Sub UpdateStatus(txt As String)
    StatusTextBox.AppendText(txt)
    StatusTextBox.AppendText(ControlChars.NewLine)
End Sub