C# - Ensure A Windows Form Is "fully" Closed?

I have a form in a Windows form application that I want to display on top of a main form, close it, and then immediately show a dialog box using MessageBox.Show(). But the first form is still showing when the message box is shown, and it does not disappear until I click OK on the message box. I tried waiting to show the message box in an event handler for the form's VisibleChanged event and even calling Refresh() on both the form and the main form. Is there a way I can determine when the first form has fully disappeared before displaying the message box?

Edit:Here is some code that demonstrates how the forms are being shown.

static class Program
{
// The main form is shown like this:
static void Main()

[code]....