Advantages of using delegates?

Question

I\'m looking to implement the Observer pattern in VB.NET or C# or some other first-class .NET language. I\'ve heard that delegates can be used for this, but can\'t figure ou

Answer 1

Here is an example that might help.

There is an application that uses a large set of data. A feature is needed that allows the data to be filtered. 6 different filters can be specified.

The immediate thought is to create 6 different methods that each return the data filtered. For example

public Data FilterByAge(int age)

public Data FilterBySize(int size)

.... and so on.

This is fine but is a very limited and produces rubbish code because it's closed for expansion.

A better way is to have a single Filter method and to pass information on how the data should be filtered. This is where a delegate can be used. The delegate is a function that can be applied to the data in order to filter it.

public Data Filter(Action filter)

then the code to use this becomes

Filter(data => data.age > 30);

Filter(data => data.size = 19);

The code data => blah blah becomes a delegate. The code becomes much more flexible and remains open.