How to Create MDI Child forms asynchronously? - vb.net

Question

I have Parent VB.Net form that has to create some MDI childs.
Each MDI has to connect to a database to do some staff which may take some time...
I'm trying to create the MDI child asynchronously using the following code but it stills execute synchronously :
ParentForm.BeginInvoke(Sub()
CreateMDIChildForm()
End sub)
This code executes but it's still synchronous, and the parent form freezes.
I tried to create the MDI in a separate thread but then I can't join the form created to the parent form.
Does anyone have an idea please ?
Thanks.

Answer 1

I can reproduce this with the following code:
Public Class Form1
Private Sub Open(sender As System.Object, e As System.EventArgs) Handles Button1.Click
Me.BeginInvoke(Sub() Me.CreateMDIChildForm())
End Sub
Private Sub CreateMDIChildForm()
Dim f As New Form2()
MsgBox("OK")
End Sub
Public Class Form2
Inherits Form
Sub New()
Thread.Sleep(5000)
End Sub
End Class
End Class
And the fix would be something like this:
Public Class Form1
Private Sub Open(sender As System.Object, e As System.EventArgs) Handles Button1.Click
If (Me.operationCompleted Is Nothing) Then
Me.operationCompleted = New SendOrPostCallback(AddressOf Me.CreateMDIChildFormCompleted)
End If
Me.asyncOperation = AsyncOperationManager.CreateOperation(Nothing)
Dim thread As New Thread(AddressOf Me.CreateMDIChildForm)
thread.Start()
End Sub
Private Sub CreateMDIChildForm()
Dim f As New Form2()
Me.asyncOperation.PostOperationCompleted(Me.operationCompleted, f)
End Sub
Private Sub CreateMDIChildFormCompleted(args As Object)
Dim f As Form = DirectCast(args, Form)
'TODO: Set mdi child, show window etc.
MsgBox("OK")
End Sub
Private asyncOperation As AsyncOperation
Private operationCompleted As SendOrPostCallback
Public Class Form2
Inherits Form
Sub New()
Thread.Sleep(5000)
End Sub
End Class
End Class

Answer 2

The recommended method to fix issues like this is not to try to load forms in separate threads, but, rather, to do only the time-consuming work in a separate thread.
In this circumstance, what that means is that you would load the MDI child forms normally, all on the same UI thread, but inside the child forms, they would each start a separate thread to perform the DB-related work which is causing the hangups. That way, the DB stuff won't cause the form-loading to hang up. The form will load and show itself quickly and then the database work will be done after the form is already visible.
You may need to disable some, or all, of your controls on the child form until the DB work is complete. You may also want the child form to display some sort of spinning animation to show that it is still loading. The easiest way to implement separate threads in your UI, for things like this, is to use a BackgroundWorker component. You will find it in the Components section of your form designer tool box.

Answer 3

Solution :
Dim Thread as new Thread(sub()
ParentForm.BeginInvoke(sub()
CreateMDIChild()
end sub)
end sub)
Thread.Start()
With this, the MDI child form is still created by the parent form but everything is done in a separate thread.