Dialog Box Closes Without Permission

I'm writing an application in which a user can add, change and delete members from a list of members, and the adding and changing is done in a dialog box with some input fields.The problem is that when the user presses the OK button it should test whether some combobox is still empty, since this will otherwise return nothing and that will create an error somewhere else in the program. I do this within the OK_Button_Click event of the dialog box by testing whether the selectedItem is equal to nothing, if not it calls me.close, but if it is equal to nothing it displays a messagebox indicating that the user should still pick something from that combobox.Now the problem: when the OK button is clicked in the messagebox the dialog box also closes, even though the me.close isn't executed. [code]