Predicate Delegate

A delegate is like a function pointer. A Predicate delegate is a very cool use of Delegates which is used with Generic Lists, Arrays and Collections. Since a Generic List does not specify a data type, that is, the data type of the items in the List is set with a type parameter, how would one write a Find() function for a Generic List? Since nothing is known about the data type being searched for, one cannot know what sort of condition is used to discriminate between one item and another.

The Predicate delegate is a Generic method which takes an object of type T as a parameter. It returns true or false, indicating whether or not the object of type T satisfies the condition it tests. The Predicate delegate is assigned the same type as the List when the List type parameter is passed to the instance of the Generic List created. The author of the code knows what the data type is, and can write a method that works with the type passed to test it and see whether it is whatever the author wants it to be. The Find() method uses the Predicate delegate method created to test each item in the List, and returns the first item in the List for which the Predicate delegate returns true

As an example, let's say that we have a Generic List of type Int32:

private List Numbers = new List();

You want to find the first item in the List that is less than 5. So, you create your method:

private static bool IsLessThanFive(int i) { return (i < 5); }

So, now you call the Find() method:

int result = Numbers.Find(new Predicate(IsLessThanFive));

The IsLessThanFive Predicate delegate is called for each item in the list, passing that item as the parameter, until one of the calls returns true. That is the number that is assigned to result.

In .NET 2.0 it is possible to declare "anonymous method", so we can avoid declaring separate method whatsoever:

int result = Numbers.Find(delegate(int x) { return x < 5; });

New .NET 3.5 syntax is even simpler, we can completely avoid "new Predicate" statement (it will be in fact created by compiler in background):

int result = Numbers.Find(IsLessThanFive);