Difference between .Owner property and ShowDialog(IWin32Window owner)?

Question

I presume a winform's owner can be set explicitly via the .Owner property OR by passing the owner in the overloaded method ShowDialog(IWin32Window owner)

I am unable to understand why these methods exhibit different behavior when working with MDI forms .

I have created an MDIParent and an MDIChild.

I also have a simple winform MyDialogBox that displays its owner on load.

MessageBox.Show("Dialog's owner is " + this.Owner.Name);

Method A - In the load of MDIChild I have the following code, which causes the MyDialogBox's owner to be set to MDIChild

MyDialogBox box = new MyDialogBox();
box.Owner = this; // Set owner as MDIChild
box.ShowDialog();

Method B - Alternatively, in the load method of MDIChild I have the following code, which causes the MyDialogBox's owner to be set to MDIParent

MyDialogBox box = new MyDialogBox();
box.ShowDialog(this); // Pass MyMDIChild as owner

I also read the following here

Only the MDI parent form can own another form, be it a MDI child, a modal dialog or a form where the parent was set as the Owner param.

If so Method A should not work at all, isn't it ?

What am I missing? Why doesn't method B set the owner to MDIChild ?

Answer 1

Looking at the differences of these 2 options using Reflector, it seems that they have a slightly different implementation: box.Owner = this just assign the provided value of this to the internal owner field. However, when calling ShowDialog(IWin32Window) , the implementation performs the following call, prior to assigning the value:

owner = ((Control) owner).TopLevelControlInternal;

This might result in assignment of the MDIParent.

( Note : I'm far from being an expert regarding MDI, so I might be wrong here).