TopMost property not working

I have a form that when I check a box (Duplicate #), a form pops up (CableID_DuplicateView), and I want it to remain ontop until the checkbox is unchecked. However this isn't happening.

I have set the TopMost property to true both dynamically and statically;

this.TopMost = true;
this.TopMost = Checkbox.Checked;

But if I click on anything on the previous form, it gets shoved to the back.

enter image description here

How can I keep it ontop?

Note: I want the other form to still be accessible beneath the top form. And yes, this is an Mdi application, does that make a difference on the TopMost property?

Solution

It is impossible to make the windows stay on top in MDI system. You can try to activate the windows, so they pop on the top, but this makes only more harm to other actions. I wouldn't recommend trying that. There is just no working way to make some window TopMost in MDI configuration.