Introduction to inheritance, polymorphism in C#

using System;
	
class Primary
{
	[STAThread]
	static void Main(string[] args)
	{		
		Animal a1 = new Animal();
		a1.Talk();
		a1.Sing();
		a1.Greet();

		Console.WriteLine();

		Animal a2 = new Dog();
		a2.Talk();
		a2.Sing();
		a2.Greet();

		Console.WriteLine();

		Dog d1 = new Dog();
		d1.Talk();
		d1.Sing();
		d1.Greet();

		Console.WriteLine();

		Color c1 = new Color();
		c1.Fill();
		c1.Fill("red");

		Console.WriteLine();

		Green g1 = new Green();
		g1.Fill();
		g1.Fill("violet");

		Console.WriteLine();

		MicrosoftSoftware m1 = new MicrosoftSoftware();
		//MicrosoftSoftware m2 = new MicrosoftSoftware(300); //won't compile

		Console.WriteLine();

		DundasSoftware du1 = new DundasSoftware(50);

		Console.WriteLine();

		DundasSoftware du2 = new DundasSoftware("test",75);
		
	}
}

class Animal
{
	public Animal()
	{
		Console.WriteLine("Animal constructor");
	}
	public void Greet()
	{
		Console.WriteLine("Animal says Hello");
	}
	public void Talk()
	{
		Console.WriteLine("Animal talk");
	}
	public virtual void Sing()
	{
		Console.WriteLine("Animal song");
	}
};

class Dog : Animal
{
	public Dog()
	{
		Console.WriteLine("Dog constructor");
	}
	public new void Talk()
	{
		Console.WriteLine("Dog talk");
	}
	public override void Sing()
	{
		Console.WriteLine("Dog song");
	}
};

class Color
{
	public virtual void Fill()
	{
		Console.WriteLine("Fill me up with color");
	}
	public void Fill(string s)
	{
		Console.WriteLine("Fill me up with {0}",s);
	}
};

class Green : Color
{
	public override void Fill()
	{
		Console.WriteLine("Fill me up with green");
	}
};

class Software
{
	public Software()
	{
		m_x = 100;
	}
	public Software(int y)
	{
		m_x = y;
	}
	protected int m_x;
};

class MicrosoftSoftware : Software
{
	public MicrosoftSoftware()
	{
		Console.WriteLine(m_x);
	}
};

class DundasSoftware : Software
{
	public DundasSoftware(int y) : base(y)
	{
		Console.WriteLine(m_x);
	}
	public DundasSoftware(string s, int f) : this(f)
	{
		Console.WriteLine(s);
	}
};