.Show vs .ShowDialog

Either create the login first and then create the main form, OR create the mainform first, then call the login dialog with showdialog (my preference). The one you choose should be based on the modularity of your application. Business logic should not be part of your forms. Base your decision on that. The difference between show() and showdialog() is that show() is modeless and showdialog() is modal. Modeless means that the first form creates the second, but doesn't wait for the second to close. Mainform can create login form and you can immeditely click back on mainform without entering login information. Modal means that when mainform creates the login form, the user cannot click on the main form until the login form closes. If they try, they get a "ding" sound, and the titlebar for the login flashes for a second. The main form never comes to the foreground.