Delegates

Sometimes it would be nice to be able to pass a procedure as a parameter to a method. The classic scenario is when building a generic sort routine, for which you need to provide not only the data to be sorted, but also a comparison routine appropriate for the specific data.

It is easy enough to write a sort routine that sorts Person objects by name, or to write a sort routine that sorts SalesOrder objects by sales date. However, if you want to write a sort routine that can sort any type of object based on arbitrary sort criteria, that gets pretty difficult. At the same time, because some sort routines can get very complex, it would be nice to reuse that code without having to copy and paste it for each different sort scenario.

By using delegates, you can create such a generic routine for sorting; and in so doing, you can see how delegates work and can be used to create many other types of generic routines. The concept of a delegate formalizes the process of declaring a routine to be called and calling that routine.

The underlying mechanism used by the .NET environment for callback methods is the delegate. Visual Basic uses delegates behind the scenes as it implements the Event, RaiseEvent, WithEvents, and Handles keywords.

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