Programming .NET Windows Applications

Apply Button

All dialog buttons discussed so far set the dialog box's DialogResult property to a value, which not only has the effect of returning that value to the parent form, but also of closing the dialog box. Sometimes you'll want the parent form to process the changes made in the dialog box while leaving the dialog box open. Many programs implement an Apply button for this purpose.

An Apply button typically is disabled, i.e., grayed out, when the dialog first opens. As soon as any change is made in one of the dialog controls, the button is enabled. Clicking the Apply button makes available to the parent form all the changes made in the dialog box, but leaves the dialog box open. The Apply button is then disabled until the next change in the dialog box is made.

You might be tempted to implement the Apply button by creating a public method in the parent Form class that performs whatever work needs to be done when the Apply button is clicked. The Apply button would simply execute that public method when it was clicked, passing any required information as arguments.

This is not a clean way to implement an Apply button, however, because the dialog box class needs to know too much about the parent class. Also, if other classes invoke your dialog box, the dialog box must know their methods as well. This design tightly couples the dialog class to the forms that invoke it, which is generally an indication of poor design.

A preferable design, which decouples the dialog class from the ...