/*

    What is IEnumerable<> in C#?

    IEnumerable is the parent interface for all non-generic collections in System.Collections namespace like ArrayList, HastTable, etc. that can be enumerated. 

    The generic version of this interface is IEnumerable<T>, a parent interface of all generic collections classes in the System.Collections.Generic namespace 

    like List<> and more. 

​

    In System.Collections.Generic.IEnumerable<T> have only a single method which is GetEnumerator(), that returns an IEnumerator. IEnumerator provides the power 

    to iterate through the collection by exposing a Current property and Move Next and Reset methods if we don’t have this interface as a parent, so we can’t 

    use iteration by foreach loop or can’t use that class object in our LINQ query.

    Let's implement some examples of IEnumerable. In this example, we will search a list using a LINQ query and we will go through all values using a for each loop. 

    Let's have a look at the following code.

*/

​

​

using System;

using System.Linq;

using System.Collections;

using System.Collections.Generic;

// We will implement an IEnumerable interface in our C# class. If we implement the IEnumerable interface then we can loop through all the objects of it.

class Test : IEnumerable

{

    Test[] Items = null;

    int freeIndex = 0;

    public String Name { get; set; }

    public string Surname { get; set; }

    public Test()

    {

        Items = new Test[10];

    }

    public void Add(Test item)

    {

        Items[freeIndex] = item;

        freeIndex++;

    }

    // IEnumerable Member

    // The GetEnumerator method will return an object of the Test class one by one because we are using yield. It is important to implement otherwise it will throw 

    // Compilation error: 'Test' does not implement interface member 'System.Collections.IEnumerable.GetEnumerator()'

    public IEnumerator GetEnumerator()

    {

        foreach (object o in Items)

        {

            if(o == null)

            {

                break;

            }

            yield return o;

        }

    }

}

​

public class Program

{

    public static void Main(String[] args)

    {

        Console.WriteLine("******* IEnumerable Example 1 *********");

        IEnumerable<int> Values = from value in Enumerable.Range(1, 10) select value;

        foreach (int a in Values)

        {

            Console.WriteLine(a);

        }

        Console.WriteLine("\n******* IEnumerable Example 2 *********");

        List<string> List = new List<string>();

        List.Add("Sourav");

        List.Add("Ram");

        List.Add("shyam");

        List.Add("Sachin");

        IEnumerable names = from n in List where (n.StartsWith("S")) select n;

        foreach (string name in names)

        {

            Console.WriteLine(name);

        }

        Console.WriteLine("\n******* IEnumerable Example 3 *********");

        Test t1 = new Test();

        t1.Name = "Sourav";

        t1.Surname = "Kayal";

        Test t2 = new Test();

        t2.Name = "Ram";

        t2.Surname = "Das";

        Test myList = new Test();

        myList.Add(t1);

        myList.Add(t2);

        foreach (Test obj in myList)

        {

            Console.WriteLine("\nName:- " +  obj.Name + "\nSurname :- " + obj.Surname);

        }

    }

}