What is a predicate in c#?

Question

I am very new to using predicates and just learned how to write:

Predicate<int> pre = delegate(int a){ a %2 == 0 };

What will the predicate return, and how is it useful when programming?

Answer 1

Predicate<T> is a functional construct providing a convenient way of basically testing if something is true of a given T object.

For example suppose I have a class:

class Person {
    public string Name { get; set; }
    public int Age { get; set; }
}

Now let's say I have a List<Person> people and I want to know if there's anyone named Oscar in the list.

Without using a Predicate<Person> (or Linq, or any of that fancy stuff), I could always accomplish this by doing the following:

Person oscar = null;
foreach (Person person in people) {
    if (person.Name == "Oscar") {
        oscar = person;
        break;
    }
}

if (oscar != null) {
    // Oscar exists!
}

This is fine, but then let's say I want to check if there's a person named "Ruth"? Or a person whose age is 17?

Using a Predicate<Person>, I can find these things using a LOT less code:

Predicate<Person> oscarFinder = (Person p) => { return p.Name == "Oscar"; };
Predicate<Person> ruthFinder = (Person p) => { return p.Name == "Ruth"; };
Predicate<Person> seventeenYearOldFinder = (Person p) => { return p.Age == 17; };

Person oscar = people.Find(oscarFinder);
Person ruth = people.Find(ruthFinder);
Person seventeenYearOld = people.Find(seventeenYearOldFinder);

Notice I said a lot less code, not a lot faster. A common misconception developers have is that if something takes one line, it must perform better than something that takes ten lines. But behind the scenes, the Find method, which takes a Predicate<T>, is just enumerating after all. The same is true for a lot of Linq's functionality.

So let's take a look at the specific code in your question:

Predicate<int> pre = delegate(int a){ return a % 2 == 0; };

Here we have a Predicate<int> pre that takes an int a and returns a % 2 == 0. This is essentially testing for an even number. What that means is:

pre(1) == false;
pre(2) == true;

And so on. This also means, if you have a List<int> ints and you want to find the first even number, you can just do this:

int firstEven = ints.Find(pre);

Of course, as with any other type that you can use in code, it's a good idea to give your variables descriptive names; so I would advise changing the above pre to something like evenFinder or isEven -- something along those lines. Then the above code is a lot clearer:

int firstEven = ints.Find(evenFinder);