How to create multiple threaded Forms as MDIChild

Question

Is it possible to have multiple threaded forms as MDIChild? I have an ActiveX control in MdiChild form that can take a lot of processing CPU, and would like to make that one control will not influence another control by using sample code below. But line frmDoc.MdiParent = Me throws cross threading exception.

Dim frmDoc As MDIChild
Dim newThread As New Thread(
    Sub()
        frmDoc = New MDIChild
        frmDoc.MdiParent = Me '<- this line throws cross threading exception.
        Application.Run(frmDoc)
    End Sub
)
newThread.IsBackground = True
newThread.SetApartmentState(ApartmentState.STA)
newThread.Start()

Throws System.InvalidOperationException was unhandled:

Message=Cross-thread operation not valid: 
  Control 'FormMdiApp' accessed from a thread other than the thread it was created on.
Source=System.Windows.Forms

Answer 1

GUI elements must be initialized and accessed in the main event loop only. You can process heavy calculations asynchronously or in background threads.

Answer 2

try BackgroundWorker http://msdn.microsoft.com/en-us/library/system.componentmodel.backgroundworker(v=vs.95).aspx

Perform all the heavy opration in DoWork event and use the ProgressChanged/RunWorkerCompleted event to update the UI elements.

Answer 3

Available options on how to implement this

• Switch to VC++ MFC framework and follow MTMDI Sample: Demonstrates an MFC User Interface Thread

• As it was suggested by @Blorgbeard replace MdiChild with Forms

• Look at Running a WPF control in another thread

Just found a good msdn support article How To Create Windows in a Multithreaded Application

Creating a window can force an implicit AttachThreadInput (), when a parent window is created in one thread and the child window is being created in another thread. When windows are created (or set) in separate threads with a parent-child relationship, the input queues are attached.

More information can be found on Walkthrough: Supporting COM Interop by Displaying Each Windows Form on Its Own Thread

Similar question was asked Spawn a new thread to open a new window and close it from a different thread

But, anfortunatelly non of those have anything about child forms.

Update: Just have found bugs in Walkthrough code; but overall this sample has some good ideas.