Visual Basic 2008 - New Form on a different thread

Question

My “form1” is just a simple page with buttons that launch different forms which do all the work, the "form1" code for the first four buttons is below.

What I want is for each form to run in a separate thread.

Public Class Main
    Private Sub btnDownLoadStockPrices_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnDownLoadStockPrices.Click
        LoadStocksFromDownloadSite.Visible = True
    End Sub

    Private Sub btnLoadOptionsIntoDatabase_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnLoadOptionsIntoDatabase.Click
        LoadOptionsIntoDatabase.Visible = True
    End Sub

    Private Sub btnVerifyDatabases_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnVerifyDatabases.Click
        VerifyDatabase.Visible = True
    End Sub

    Private Sub btnAnalyzeStock_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnAnalyzeStock.Click
        AnalyzeSingleStock.visible = True
    End Sub
End Class

I’ve found plenty of code examples to have different buttons on a single form run a subroutine in a separate thread, but for some reason I can’t seem to apply it to an entire form.

I think it’s something simple, like I need to tie each new thread as a handle to each forms “load” routine, but I just can’t get it to work. I don’t care about “synchronizing” threads at all, as each form is really a completely different functional program.

Any help would be much appriciated!

Answer 1

This isn't very common; generally it's best to limit all UI stuff to a single thread. But if you're convinced that you need each form to run on a separate thread, you must take into account the Windows API event handling model. The [over]-simplified version is that each form must have its own message loop to remove event messages from the queue and process them, so if you want to open a form on a new thread, you need to create that message pump.

The easiest way to do that is using the Application.Run method, and let the .NET Framework handle creating that message pump for you. For example:

Dim frm As Form1 = New Form1()
Application.Run(frm)

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From looking at the code shown in your question, I can't discern any possible reason why those forms would need to run on separate threads. You can call the Show method of multiple forms so that they will be displayed on the screen at the same time. They won't block each other as long as you don't use the ShowDialog method, which displays each as a modal dialog. This is the way so many applications display multiple toolbox windows and other kinds of forms on the screen at the same time.

If you need to do some type of processor-intensive calculation, you still don't need to run each on a separate thread. Spin up a background thread (the BackgroundWorker class makes this very simple) and update the appropriate form's UI using the Invoke method.

Answer 2

You can certainly do this on Win32 but I don't know how well this maps over to .net.

The essential issue is that window handles have thread affinity. So you really need all interaction with them to happen in that thread. Essentially this means that you create all the window handles associated with that form in its thread. You also need to run a message loop in the thread.

The reason that people usually run all the UI out of the main thread and handle long-running actions in separate threads is that it is easier that way. You should ask yourself again why you want to do it this non-standard way.

I suspect you are not quite seeing the full picture. The need for threads in a desktop app principally arises when you have long running actions. Usually you want to keep your UI responsive and providing feedback for the long running action. Doing so leads to threads.

However, in your proposed solution you now have a multitude of extra threads and complexity, and you are still faced with the original problem! A long running action on one of your forms will hang it unless you perform that action in a separate thread, and once again we reach the standard solution to the problem.