In this part of my C# Tutorial we learn about Inheritance, Aggregations, Virtual, Override, Inner Classes, New, Base, Polymorphism and much more.

For the best results print out the code below and take notes as you pause your way through the video. All of the code and a transcript of the video follows below the video.

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Code & Transcript

---------- Animal.cs ----------

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

namespace CSharpTutA.cs
{
    // If this class was sealed you couldn't inherit 
    // from it sealed class Animal
    class Animal
    {
        private string name;

        // A protected field can be changed by
        // a subclass directly
        protected string sound;

        // Inheritance has a "is-a" relationship, 
        // while an aggregation or delegate 
        // represents a "Has-a" relationship
        // like we have here with the AnimalIDInfo
        // object
        protected AnimalIDInfo animalIDInfo = new AnimalIDInfo();
  
        public void SetAnimalIDInfo(int idNum, string owner)
        {
            animalIDInfo.IDNum = idNum;
            animalIDInfo.Owner = owner;
        }

        public void GetAnimalIDInfo()
        {
            Console.WriteLine($"{Name} has the ID of {animalIDInfo.IDNum} and is owned by {animalIDInfo.Owner}");
        }

        // Added virtual so that this method can
        // be overridden by subclasses
        // You must add override to the method in 
        // the subclass
        public virtual void MakeSound()
        {
            Console.WriteLine($"{Name} says {Sound}");
        }

        public Animal()
        :this("No Name", "No Sound") { }

        public Animal(string name)
            :this(name, "No Sound") { }

        public Animal(string name, string sound)
        {
            Name = name;
            Sound = sound;
        }

        public string Name
        {
            get { return name; }
            set
            {
                if (!value.Any(char.IsDigit))
                {
                    name = "No Name";
                } else {
                    name = value;
                }
            }
        }

        public string Sound
        {
            get { return sound; }
            set
            {
                if(value.Length > 10)
                {
                    sound = "No Sound";
                }
                sound = value;
            }
        }

        // You can create inner classes that are 
        // normally helper classes for the outer 
        // class because it can access private
        // members of the outer class
        public class AnimalHealth
        {
            public bool HealthyWeight(double height,
                double weight)
            {
                double calc = height / weight;

                if ((calc >= .18) && (calc <= .27))
                {
                    return true;
                }
                else return false;
            }

        }

    }
}

---------- Dog.cs ----------

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

namespace CSharpTutA.cs
{
    // When you inherit from another class you
    // receive all of its fields and methods
    // You cannot inherit from multiple classes
    class Dog : Animal
    {
        // You can add additional properties
        // or fields
        public string Sound2 { get; set; } = "Grrrrr";

        // You can overwrite methods by adding new
        /*
        public new void MakeSound()
        {
            Console.WriteLine($"{Name} says {Sound} and {Sound2}");
        }
        */

        // Add override so that the correct method
        // is called when a Dog is created as an
        // Animal type
        public override void MakeSound()
        {
            Console.WriteLine($"{Name} says {Sound} and {Sound2}");
        }


        // Create a constructor that has the base 
        // constructor initialize everything except 
        // Sound2
        public Dog(string name = "No Name", 
            string sound = "No Sound", 
            string sound2 = "No Sound 2")
            :base(name, sound){
                Sound2 = sound2;
            }
    }
}

---------- Program.cs ----------

using System;

namespace CSharpTutA.cs
{
    class Program
    {

        static void Main(string[] args)
        {
            Animal whiskers = new Animal()
            {
                Name = "Whiskers",
                Sound = "Meow"
            };

            Dog grover = new Dog()
            {
                Name = "Grover",
                Sound = "Woof",
                Sound2 = "Grrrrr"
            };

            // Demonstrate changing the protected
            // field sound
            grover.Sound = "Wooooof";

            whiskers.MakeSound();
            grover.MakeSound();

            // Define the AnimalIDInfo
            whiskers.SetAnimalIDInfo(12345, "Sally Smith");
            grover.SetAnimalIDInfo(12346, "Paul Brown");

            whiskers.GetAnimalIDInfo();

            // Test the inner class
            Animal.AnimalHealth getHealth = new Animal.AnimalHealth();

            Console.WriteLine("Is my animal healthy : {0}", getHealth.HealthyWeight(11, 46));

            // You can define 2 Animal objects but have
            // one actually be a Dog type. 
            Animal monkey = new Animal()
            {
                Name = "Happy",
                Sound = "Eeeeee"
            };

            Animal spot = new Dog()
            {
                Name = "Spot",
                Sound = "Wooooff",
                Sound2 = "Geerrrr"
            };

            // The problem is that if you call a 
            // function in Animal it won't call
            // the overridden method in Dog unless
            // the method that might be overridden
            // is marked virtual
            spot.MakeSound();

            // This is an example of how Polymorphism
            // allows a subclass to override a method
            // in the super class and even if the 
            // subclass is defined as the super class
            // type the correct method will be called


            Console.ReadLine();

        }
        
    }

}

---------- AnimalIDInfo.cs ----------

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

namespace CSharpTutA.cs
{
    class AnimalIDInfo
    {
        public int IDNum { get; set; } = 0;
        public string Owner { get; set; } = "No Owner";

    }
}