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GUI for .NET Primer
Events : Event handlers

Event handlers

.NET supports an event model for classes. Like ABL class-based objects, .NET objects define events as members. An ABL application can respond to any .NET event using an ABL event handler. These event handlers are associated with specific .NET events. .NET forms and controls fire events when an action occurs. As a result all subscribed handlers get called.
ABL allows you to subscribe and unsubscribe event handler methods or internal procedures to .NET events. You can use the Subscribe( ) method on a .NET event of a particular .NET class instance to subscribe event handlers written in ABL. .NET supports multiple subscribers attached to one event. Normally two parameters are passed to the event handler—the object firing the event and an object that provides event-specific arguments as properties.
In this example, the FormObjectRef object subscribes to the owner of the Load event, passing it the name of the event handler, the FormObjectRefLoad( ) method:

  (sender AS System.Object, e AS System.EventArgs).
You can use the Unsubscribe( ) method to detach an event handler from a control.
A .NET event handler can be either a method in a class (.cls) or an internal procedure. The main difference between using methods and internal procedures as .NET event handlers is that ABL verifies method event handler signatures at compile time (using strong typing), and verifies internal procedure event handler signatures at run time.