Winforms Dispose Controls Quirk

Here is an interesting quirk with Winforms in C#. Calling Dispose on a Control has a side-effect of removing it from the container it has been added to.

So you can have code like this:

for (int c = 0; c < MainPanel.Controls.Count; c++)
   MainPanel.Controls[0].Dispose();

which correctly removes and disposes every control, even though you have a fixed index of 0. Since the control at index 0 is removed (as well as being marked as Disposed), the next control then becomes index 0. Therefore, it does call Dispose on every control.

So when you look at this code:

foreach (Control item in MainPanel.Controls)
{
   item.Dispose();
}

It looks correct, and would be the code that I would write had I not known about this quirk. However, it doesn’t work as intended. It will only remove and dispose some of the controls since you are modifying the list as you are iterating through it. 

This is the danger of adding “side effects” to your code. It can cause weird behaviour that you wouldn’t expect, and can be overlooked if you are just reading the code.