vb.net-将输出重定向到表单文本框

Question

I am trying to read/use the output from a python program in my vb.net project so far I'm not getting any results. What I'd like to see is the python program run (just by itself first) and all of the output get redirected into a textbox.

I've looked at some other posts about this, but I'm either missing something or not understanding something, as all I'm getting is blank output. Public Class Form1

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    Dim PythonPath = "C:\Python27\"
    Dim strPath As String = Application.StartupPath
    MessageBox.Show(PythonPath & "python.exe  """ & strPath & "\Resources\import_logs.py"" ")

    Dim start_info As New ProcessStartInfo(TextBox1.Text)

    ' Make the process and set its start information.
    Dim process As New Process()
    process.StartInfo.WindowStyle = ProcessWindowStyle.Hidden
    process.StartInfo.FileName = PythonPath & "\python.exe"
    process.StartInfo.Arguments = """" & strPath & "\resources\import_logs.py"""""
    process.StartInfo.UseShellExecute = False
    process.StartInfo.CreateNoWindow = True
    process.StartInfo.RedirectStandardOutput = True
    'process.StartInfo.RedirectStandardError = True

    AddHandler process.OutputDataReceived, AddressOf proccess_OutputDataReceived

    process.Start()
    process.BeginOutputReadLine()
End Sub

Public Sub proccess_OutputDataReceived(ByVal sender As Object, ByVal e As DataReceivedEventArgs)
    On Error Resume Next
    ' output will be in string e.Data
    ' modify TextBox.Text here
    'Server_Logs.Text = e.Data  ` Does not display anything in textbox
    MsgBox(e.Data) 'It works but I want output in text box field
End Sub
End Class

Eventually I'm going to pass arguments to the python script and I'd like to get feedback that I can then use (insert error into a database, email when it's done, etc), so I'd like it to capture the process while running and not just a data dump at the end.

Any help would be much appreciated.

Answer 1

First things first—it's no wonder you aren't sure what's wrong with your code, you're silencing all errors that could possibly help you to diagnose it. That's the only purpose of On Error Resume Next in VB.NET. That unstructured error handling was included only for backwards compatibility with the pre-.NET versions of VB and it's time to forget that it ever existed. You certainly don't want to use it in code. (I would say "in code that you're debugging", but all code is a potential candidate for debugging and ignoring errors is just dumb.)

Anyway, on to the specific problem. We know that the call to MsgBox works, but it doesn't work right when you start interacting with controls on your form. So something is falling apart there.

It turns out that the OutputDataReceived event is raised on an entirely different thread, a different one than was used to create the process and a different one than is running your application's UI. It actually just retrieves a thread from the system thread pool.

And that's where the problem lies: you cannot manipulate UI objects on a thread other than the one that created those objects (at least not without jumping through some hoops), which is precisely what your code tries to do here. In fact, you're probably swallowing an exception that would have rather obtusely informed you of this situation.

The simple fix is to set the SynchronizingObject property of the Process class to one of your UI components (like the form, or the specific control you want to output to). This forces all event handlers to execute on the same thread that created that component. At that point, your code should work fine, because you're not trying to do any cross-thread UI access. (Message boxes are not vulnerable to this because any thread can display a message box. You're not trying to access an existing UI object that is bound to another thread.)

Alternatively, you could handle the marshalling yourself in the event handler method through the use of delegates and the BeginInvoke method, but this seems like unnecessary work to me.