There are two ways to repaint a form and its contents:
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You can use one of the overloads of the Invalidate method with the Update method.
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You can call the Refresh method, which forces the control to redraw itself and all its children. This is equivalent to setting the Invalidate method to true and using it with Update.
The Invalidate method governs what gets painted or repainted. The Update method governs when the painting or repainting occurs. If you use the Invalidate and Update methods together rather than calling Refresh, what gets repainted depends on which overload of Invalidate you use. The Update method just forces the control to be painted immediately, but the Invalidate method governs what gets painted when you call the Update method.
(Above info quoted from MSDN http://msdn2.microsoft.com/en-us/library/system.windows.forms.control.update.aspx)