Question

I have a panel which displays Form objects. When the Close() method is called on a form, the panel.Controls.Count is modified, the Count is one less. How is this possible?

This is the way I show a form in th panel:

// insertForm is a windows form

insertForm.TopLevel = false;
insertForm.Dock = DockStyle.Fill;
insertForm.FormBorderStyle = FormBorderStyle.None;

this.pnlContent.Controls.Clear();
this.pnlContent.Controls.Add(insertForm);

And when somewhere form.Close() is called, the Controls count of pnlContent is 0. So, how is this possible? What all happens when Close() is called?

Answer 1

Closing a form causes the form to be disposed, which in turn causes it to be removed from its parent’s control-collection.

If you use a decompiler, you will notice two things:

1. When you add a control to a ControlCollection, the control’s owner is set as the ControlCollection's owner. ControlCollection.Add(Control value) calls value.AssignParent(this.owner).
2. When a control is disposed, it removes itself from its owner’s ControlCollection. Essentially, Control.Dispose calls parent.Controls.Remove(this).

Lesson learned: Don’t use a ControlCollection as general purpose collection-type for Controls. It is deeply integrated into WinForms for control-containment and parent-child relationships. Use this type only when you genuinely require a parent control to contain a child control.