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

Question

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.)

miroxlav miroxlav · Accepted Answer · 2015-02-23T18:14:23

I have found the issue. Form calling CloseAllForms() was attempting to operate its controls after closing of all forms (including itself) was finished. Checking Disposing Or IsDisposed expression helped to avoid the problem.

I'm staying with closing approach shown by Microsoft, i.e. instead of creating copy of forms collection, iterate on live collection and increase index only if some form has to be excluded from closing (or is invalid).

On the other hand I believe that other approaches mentioned in the question are nearly equivalent. As I found out, problem was elsewhere.

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

1 Answers