We start learning about object oriented programming in this part of my C# tutorial. We’ll cover Classes, Methods, Fields, Constructors, Static Classes, Methods and Fields, Structs and Nullable Types. I’m going to cover OOP over a few videos to make sure everyone completely understands everything.

All of the code follows the video below. For best results, print it out and take notes in your own words and then ask me if you have any questions.

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Code & Transcript

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

using System;

namespace CSharpTutA.cs
{
    class Program
    {

        static void Main(string[] args)
        {
            // Create a Rectangle
            Rectangle rect1;

            // Add values to it and run the Area method
            rect1.length = 200;
            rect1.width = 50;
            Console.WriteLine("Area of rect1 : {0}",
                rect1.Area());

            // Use a constructor to create a Rectangle
            Rectangle rect2 = new Rectangle(100, 40);

            // If you assign one Rectangle to another
            // you are setting the values and not
            // creating a reference
            rect2 = rect1;
            rect1.length = 33;

            Console.WriteLine("rect2.length : {0}",
                rect2.length);

            // ----- OBJECT ORIENTED PROGRAMMING -----
            // A class models real world objects by
            // defining their attributes (fields) and
            // capabilities (methods)
            // Then unlike with structs you can 
            // inherit from a class and create more
            // specific subclass types

            // Add a class Project -> Add Class

            // Create an Animal object
            // You could also assign values like
            // fox.name = "Red"
            Animal fox = new Animal()
            {
                name = "Red",
                sound = "Raaaw"
            };

            // Call the static method
            Console.WriteLine("# of Animals {0}",
                Animal.GetNumAnimals());

            // You can also create static utility
            // classes Project -> Add Class
            Console.WriteLine("Area of Rectangle : {0}",
                ShapeMath.GetArea("rectangle", 5, 6));


            // ----- NULLABLE TYPES -----
            // Data types by default cannot have a
            // value of null. Often null is needed
            // when you are working with databases
            // and you can create a null type by 
            // adding a ? to the definition
            int? randNum = null;
            
            // Check for null
            if(randNum == null)
            {
                Console.WriteLine("randNum is null");
            }

            // Another check for null
            if (!randNum.HasValue)
            {
                Console.WriteLine("randNum is null");
            }


            Console.ReadLine();

        }

        // ----- STRUCTS -----
        // A struct is a user defined type that
        // contain multiple fields and methods

        struct Rectangle
        {
            public double length;
            public double width;

            // You can create a constructor method
            // that will set the values for fields
            public Rectangle(double l=1, double w=1)
            {
                length = l;
                width = w;
            }

            public double Area()
            {
                return length * width;
            }
        }
    }

}

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

using System;

namespace CSharpTutA.cs
{
    class Animal
    {
        // Attributes (fields) that all Animals have
        // public means can be directly changed
        // after an object has been created
        public string name;
        public string sound;

        // A constructor sets default values for 
        // fields when an object is created
        // This is the default constructor if no
        // parameters are sent
        public Animal()
        {
            name = "No Name";
            sound = "No Sound";
            numOfAnimals++;
        }

        // You can create additonal constructors
        // but since we are definig defaults you
        // don't have to
        public Animal(string name = "No Name")
        {
            // This refers to this objects name
            // instead of the name passed into
            // the constructor
            this.name = name;
            this.sound = "No Sound";
            numOfAnimals++;
        }

        public Animal(string name = "No Name",
            string sound = "No Sound")
        {
            this.name = name;
            this.sound = sound;
            numOfAnimals++;
        }

        // Capabilities (methods) that all Animals have
        public void MakeSound()
        {
            Console.WriteLine("{0} says {1}",
                name, sound);
        }

        // static fields and methods belong to the class
        // A static field has the same value for all
        // objects of the Animal type
        static int numOfAnimals = 0;

        public static int GetNumAnimals()
        {
            return numOfAnimals;
        }
    }
}

---------- ShapeMath.cs ----------

using System;

    public static class ShapeMath
    {
        // This class can only contain static methods
        // and constant values
        public static double GetArea(string shape = "",
            double length1 = 0,
            double length2 = 0)
        {
            if (String.Equals("Rectangle",
                shape,
                StringComparison.OrdinalIgnoreCase))
            {
                return length1 * length2;
            }
            else if (String.Equals("Triangle",
              shape,
              StringComparison.OrdinalIgnoreCase))
            {
                return length1 * (length2 / 2);
            }
            else if (String.Equals("Circle",
              shape,
              StringComparison.OrdinalIgnoreCase))
            {
                return 3.14159 * Math.Pow(length1, 2);
            }
            else
            {
                return -1;
            }
        }

    }