百思不得其解VB编小程序的错误

给定的端口名称未以 COM/com 开头或未解析为有效的串行端口,这个是什么原因

上代码，让我们见识见识。

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Imports System.IO.Ports
Imports System.Text
Public Class Form1
  '声明一个通讯端口对象
  Dim RS232 As SerialPort
  Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
  For Each sp As String In SerialPort.GetPortNames
  cmbCOM.Items.Add(sp)
  Next
  cmbCOM.Sorted = True
  cmbCOM.SelectedIndex = 0
  End Sub

  Private Sub btnStart_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnStart.Click
  Dim mBaudRate As Integer
  Dim mParity As Parity
  Dim mDataBit As Integer
  Dim mStopbit As StopBits
  Dim mPortName As String
  mPortName = cmbCOM.SelectedIndex.ToString
  mBaudRate = 9600
  mParity = Parity.None
  mDataBit = 8
  mStopbit = StopBits.One
  '建立一个通讯端口对象
  RS232 = New SerialPort(mPortName, mBaudRate, mParity, mDataBit, mStopbit)
  RS232.Encoding = Encoding.Unicode
  If Not RS232.IsOpen Then
  RS232.Open()
  btnSend.Enabled = True
  btnReceive.Enabled = True
  Else
  MsgBox("~~通讯端口打开错误(通讯端口已被打开~~)", MsgBoxStyle.Critical Or MsgBoxStyle.OkCancel)
  End
  End If
  End Sub

  Private Sub btnSend_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSend.Click
  RS232.Write(txtSend.Text)
  End Sub

  Private Sub btnReceive_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnReceive.Click
  Dim InString As String
  InString = ""
  Try
  RS232.ReadTimeout = 1000
  InString = RS232.ReadExisting()
  If InString Is Nothing Then
  Exit Sub
  Else
  txtReceive.Text = InString
  End If
  Catch ex As Exception
  MessageBox.Show("读取错误：" + ex.ToString, "错误通知", MessageBoxButtons.OK, MessageBoxIcon.Exclamation)
  End Try
  End Sub

  Private Sub btnClose_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnClose.Click
  If RS232 Is Nothing OrElse Not RS232.IsOpen Then
  MsgBox("~~通讯端口尚未打开~~", MsgBoxStyle.Critical Or MsgBoxStyle.OkCancel)
  Else
  RS232.Close()
  btnStart.Enabled = False
  btnReceive.Enabled = False
  End If
  End Sub

  Private Sub btnEnd_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnEnd.Click
  If Not RS232 Is Nothing Then
  If RS232.IsOpen Then RS232.Close()
  End If
  End Sub

  Private Sub cmbCOM_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmbCOM.SelectedIndexChanged

  End Sub
End Class

cmbCOM.SelectedIndex.ToString
这个里面必须是com1 com2之类的