C＃ - 委托谓词<T>

Question

What is the use of delegate Predicate<T> ,Shall i handle it as nested style like

Func<Predicate<T>> ?. Simple example please.

Answer 1

There are several use cases where its useful to allow the caller to provide a function that returns true or false . The Predicate<T> type is a way to do this. This is no different than Func<T, bool> (except Predicate<T> was available in earlier versions of C#).

A simple (and contrived example):

string[] strings = new string[] { "hello", "goodbye", "how are you" };

Array.FindAll(strings, delegate(string s)
                           {
                               return s.StartsWith("h");
                           });

or, lambda style:

Array.FindAll(strings, s => s.StartsWith("h"));

The FindAll method returns an array of all items that match the condition. In this case, all the strings that start with "h". The Predicate is s.StartsWith("h") .

The useful thing here, is that the Array class knows how to search through all its elements, and create a new array containing only the elements that match. This part of the algorithm is common to many pieces of code. The part that is not common, is the matching criteria, which will vary based on the requirements of the specific piece of code. Thus the caller specifies that part of the logic, and passes it in as a Predicate .

You can handle it in a "nested" style. Func<Predicate<T>> means you are declaring a function that returns a Predicate<T> . Are you sure this is what you want? What are you trying to achieve?

Answer 2

// Make a list of integers.
List<int> list = new List<int>();
list.AddRange(new int[] { 20, 1, 4, 8, 9, 44 });
// Call FindAll() using traditional delegate syntax.
Predicate<int> callback = new Predicate<int>(IsEvenNumber);
List<int> evenNumbers = list.FindAll(callback);

// Target for the Predicate<> delegate.
static bool IsEvenNumber(int i)
{
    // Is it an even number?
    return (i % 2) == 0;
}

Answer 3

This defines a function that returns predicate (a function that returns true or false). How could you use this? You can remove the responsibility of which predicate is returned to you by delegating this to another function, who's job is not to return True or False rather to allow you delay the execute of the predicate when you want.