How to correctly implement CloseAllForms() in WinForms MDI environment?

I have tested multiple ways to close all MDI child forms, but they seem to be unstable. In many cases I'll get ObjectDisposedException.

What I tried:

'collect copy of app forms array
Dim formsToClose As Form() = My.Application.OpenForms.OfType(Of Form).ToArray()

'iterate over collection, skip special forms
For i As Integer = LBound(formsToClose) + 1 To UBound(formsToClose)
    Dim f As Form = formsToClose(i)
    If f Is Nothing OrElse frmSpecial.IsMyInstance(f) Then Continue For
    Try
        clsWinForms.ForceCloseForm(f)
    Catch ex As Exception
        'some code here
    End Try
Next i

ForceCloseForm() does Form.Close() after it switched off form validation.

• This approach randomly crashes with ObjectDisposedException.


• Running with index from last to 0 crashes with even higher probability.


• While-loop-based trick from Microsoft (operating on live collection, keeping index constant on success) crashes the same way, too.


• If I insert test for f.Disposing and f.IsDisposed, there is no improvement.

How can I close all open forms in stable way?

(If you want to give some code excerpt, you can choose C# or VB.NET.)