Option Explicit
Private ErrorString As String
Private r As Long

Private Sub chkNotify_Click()
  picNotify.BackColor = Me.BackColor
  If chkNotify.Value = 1 Then
    SubClass picNotify.hWnd
  Else
    UnSubClass picNotify.hWnd
  End If
End Sub

Private Sub cmdSendString_Click()
  ' for Notify
  picNotify.BackColor = RGB(255, 0, 0)
  r = mSendString(txtSendString.Text, ErrorString, picNotify.hWnd)
  If r <> 0 Then
    MsgBox ErrorString & " (" & CStr(r) & ")", vbCritical, "Error"
  Else
    txtGetString.Text = ErrorString
  End If
End Sub

Private Sub Form_Unload(Cancel As Integer)
  If chkNotify.Value = 1 Then
    UnSubClass picNotify.hWnd
  End If
End Sub