Predicate Delegates in C#

Question

Can you explain to me:

• What is a Predicate Delegate?
• Where should we use predicates?
• Any best practices when using predicates?

Answer 1

Simply -> they provide True/False values based on condition mostly used for querying. mostly used with delegates

consider example of list

List<Program> blabla= new List<Program>();
        blabla.Add(new Program("shubham", 1));
        blabla.Add(new Program("google", 3));
        blabla.Add(new Program("world",5));
        blabla.Add(new Program("hello", 5));
        blabla.Add(new Program("bye", 2));

contains names and ages. Now say we want to find names on condition So I Will use,

    Predicate<Program> test = delegate (Program p) { return p.age > 3; };
        List<Program> matches = blabla.FindAll(test);
        Action<Program> print = Console.WriteLine;
        matches.ForEach(print);

tried to Keep it Simple!

Answer 2

The predicate-based searching methods allow a method delegate or lambda expression to decide whether a given element is a “match.” A predicate is simply a delegate accepting an object and returning true or false: public delegate bool Predicate (T object);

   static void Main()
        {
            string[] names = { "Lukasz", "Darek", "Milosz" };
            string match1 = Array.Find(names, delegate(string name) { return name.Contains("L"); });
            //or
            string match2 = Array.Find(names, delegate(string name) { return name.Contains("L"); });
            //or
            string match3 = Array.Find(names, x => x.Contains("L"));


            Console.WriteLine(match1 + " " + match2 + " " + match3);     // Lukasz Lukasz Lukasz
        }
        static bool ContainsL(string name) { return name.Contains("L"); }

Answer 3

What is Predicate Delegate?

1) Predicate is a feature that returns true or false.This concept has come in .net 2.0 framework. 2) It is being used with lambda expression (=>). It takes generic type as an argument. 3) It allows a predicate function to be defined and passed as a parameter to another function. 4) It is a special case of a Func, in that it takes only a single parameter and always returns a bool.

In C# namespace:

namespace System
{   
    public delegate bool Predicate<in T>(T obj);
}

It is defined in the System namespace.

Where should we use Predicate Delegate?

We should use Predicate Delegate in the following cases:

1) For searching items in a generic collection. e.g.

var employeeDetails = employees.Where(o=>o.employeeId == 1237).FirstOrDefault();

2) Basic example that shortens the code and returns true or false:

Predicate<int> isValueOne = x => x == 1;

now, Call above predicate:

Console.WriteLine(isValueOne.Invoke(1)); // -- returns true.

3) An anonymous method can also be assigned to a Predicate delegate type as below:

Predicate<string> isUpper = delegate(string s) { return s.Equals(s.ToUpper());};
    bool result = isUpper("Hello Chap!!");

Any best practices about predicates?

Use Func, Lambda Expressions and Delegates instead of Predicates.

Answer 4

A delegate defines a reference type that can be used to encapsulate a method with a specific signature. C# delegate Life cycle: The life cycle of C# delegate is

• Declaration
• Instantiation
• INVACATION

learn more form http://asp-net-by-parijat.blogspot.in/2015/08/what-is-delegates-in-c-how-to-declare.html

Answer 5

Just a delegate that returns a boolean. It is used a lot in filtering lists but can be used wherever you'd like.

List<DateRangeClass>  myList = new List<DateRangeClass<GetSomeDateRangeArrayToPopulate);
myList.FindAll(x => (x.StartTime <= minDateToReturn && x.EndTime >= maxDateToReturn):

Answer 6

There's a good article on predicates here, although it's from the .NET2 era, so there's no mention of lambda expressions in there.