Delegate, Func, Action, Predicate

Delegate is a reference type that holds the reference of a class method. Any method which has the same signature as delegate can be assigned to delegate. Delegate is like function pointer in C++.
To read more delegates-c-sharp.html

Different Flavors of Delegate

1. Func<TParameter, TOutput>
2. Action<TParameter>
3. Predicate<in T>

1- Func is logically similar to base delegate implementation. The difference is in the way we declare. At the time of declaration, we need to provide the signature parameter & its return type.

Func<string, int, int> tempFuncPointer;

Func is always used when you have return object or type from method. If you have void method, you should be using Action.

2- Action is used when we do not have any return type from method. Method with void signature is being used with Action delegate.

Action<string, int> tempActionPointer;

Similar to Func delegate, the first two parameters are the method input parameters. Since we do not have return object or type, all the parameters are considered as input parameters.

3- Predicate is a function pointer for method which returns boolean value. It is used to represent a set of criteria and determine if the argument matches the criteria. Declaration for the same looks like this:

Predicate<Employee> tempPredicatePointer;

Example Code: 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace TestAppConsole
{
    public class Program
    {
        public static void Main(string[] args)
        {
            Action<int> myAction = new Action<int>(DoSomething);
            myAction(123);           // Prints out "123"
                                     // can be also called as myAction.Invoke(123);

            Func<int, double> myFunc = new Func<int, double>(CalculateSomething);
            Console.WriteLine(myFunc(5));   // Prints out "2.5"


            Predicate<int> myPred = new Predicate<int>(CheckVal);   
            Console.WriteLine(myPred(1));    // Prints out "true"
           
            Console.ReadLine();
        }

        static void DoSomething(int i)
        {
            Console.WriteLine(i);
        }

        static double CalculateSomething(int i)
        {
            return (double)i / 2;
        }

        static bool CheckVal(int i)
        {
            return true;
        }

    }
}

In short:

Action is a delegate (pointer) to a method, that takes zero, one or more input parameters, but does not return anything.

Func is a delegate (pointer) to a method, that takes zero, one or more input parameters, and returns a value (or reference).

Predicate is a special kind of Func often used for comparisons.