using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var instanceOfSampleClass = new SampleClass();

        var listFromInstanceOfSampleClass = instanceOfSampleClass.GetList();

        Console.WriteLine("Before updating list");

        foreach (var item in instanceOfSampleClass.GetList())

            Console.WriteLine(item);

        // These two lines update the list unknowingly\accidentally, and starts affecting list inside listFromInstanceOfSampleClass.

        // We probably don't want to allow updating the internal list without explicitly calling a method on 

        // the class since this breaks encapsulation and can cause side effects.

        listFromInstanceOfSampleClass.Add(2);

        listFromInstanceOfSampleClass.Add(3);

        Console.WriteLine("After updating list");

        foreach (var item in instanceOfSampleClass.GetList())

            Console.WriteLine(item);

    }

}

​

​

public class SampleClass

{

    private List<int> myList;

    public SampleClass()

    {

        myList = new List<int>(){1};

    }

    public List<int> GetList()

    {

        return myList;

    }

}