Form.Parent and StartPosition.CenterParent

Question

I need to show a form exactly in front of another form, this lead me to the following question.

How come a form can have a start position as CenterParent while having the field this.Parent equals to null?

It must know the parent in order to position itself correctly, which it does, but the Parent field is not set. This is odd. Am I missing something?

        Form2 f = new Form2();
        f.ShowDialog();

Thats all I do on the child form. The parent is set to default windows position. No matter where I move the parent form, the child is shown in the center of the parent.

Answer 1

The information about the owner is passed to the created dialog via the API call (you can see that in Reflector within the ShowDialog(IWin32Window owner) method):

UnsafeNativeMethods.SetWindowLong(new HandleRef(this, base.Handle), -8, new HandleRef(owner, handle));

When there is no owner specified in ShowDialog call the owner variable is calcualated via the GetActiveWindow API call:

IntPtr activeWindow = UnsafeNativeMethods.GetActiveWindow();
IntPtr handle = (owner == null) ? activeWindow : Control.GetSafeHandle(owner);

To get access to the Owner f dialog form you can use the GetWindowLong API call:

IntPtr ownerHandle = NativeMethods.GetWindowLong(nonModalForm.Handle, -8);