Not Modal

[alberto-gilman]

If the ParentHwd property is not assigned, you can return to the main form and interact with it

[fafalone]

This is normal behavior; it's the same with a regular API MessageBox (e.g user32 MessageBoxW)

[alberto-gilman]

Is there a way to make the class act like a msgbox, i.e. locking the application even if it doesn't know what the ParenthWnd is?

[fafalone]

The built in MsgBox is just automatically filling in the form hwnd for you; if you call MsgBox from a cls/bas it doesn't lock the application because there's no associated form.

You need some way of telling the class what hwnd you want to be modal to; and since you have access to all your forms, I'm not sure what kind of scenario would justify something like a flag to have the class enumerate the windows for the current process and take a guess at which is the main window.

[alberto-gilman]

Sorry for the delay in responding.

That's incorrect; the MsgBox function blocks the application, even if there's no associated form.
You can verify this in a project with two forms, Form1 and Form2, and with the start in the following Main method:
Public Sub Main()
Dim f As New Form1
f.Show
Dim g As New Form2
g.Show
MsgBox "HELLO WORLD"
End Sub
Not only the code in Main Sub is blocked until a response is received from the MsgBox, but interaction with either form is also prevented. In contrast, with the cTaskDialog class, you can only block one form at a time, while still being able to interact with the other.