Learn C#: The Object Class

// Crow inherits from Bird, which inherits from Animal, and it implements IFly:
class Bird : Animal
{ }
class Crow : Bird, IFly
{ }

// All of these references are valid:
Crow crow = new Crow();
Bird b = crow;
Animal a = crow;
IFly f = crow;

Cat c = new Cat();
Animal a = c;

c.Meow(); // OK, 'Meow()' is defined for the type 'Cat'

a.Meow();
// Error! 'Meow()' is not defined for the type 'Animal'

// When you write this code:
class Dog {}
// C# assumes you mean:
class Dog : Object {}

// Even if your class explicitly inherits from a class that is NOT Object, then some class in its class hierachy will inherit from Object. In the below example, Dog inherits from Pet, which inherits from Animal, which inherits from Object:
class Dog : Pet {}
class Pet : Animal {}
class Animal {}

// Since every class inherits from Object, any instance of a class can be referred to as an Object.
Dog puppy = new Dog();
Object o = puppy;

Object obj = new Object();
Console.WriteLine(obj.ToString());
// The example displays the following output:
//      System.Object

public static void Main()
{
	MyBaseClass myBase = new MyBaseClass();
  MyDerivedClass myDerived = new MyDerivedClass();
  object o = myDerived;
  MyBaseClass b = myDerived;

	Console.WriteLine("mybase: Type is {0}", myBase.GetType());
  Console.WriteLine("myDerived: Type is {0}", myDerived.GetType());
  Console.WriteLine("object o = myDerived: Type is {0}", o.GetType());
  Console.WriteLine("MyBaseClass b = myDerived: Type is {0}", b.GetType());
}

// The example displays the following output:
//    mybase: Type is MyBaseClass
//    myDerived: Type is MyDerivedClass
//    object o = myDerived: Type is MyDerivedClass
//    MyBaseClass b = myDerived: Type is MyDerivedClass

Random r = new Random();

// These two lines are equivalent:
Console.WriteLine(r);
Console.WriteLine(r.ToString());