Question

object a = new Dog();

vs

Dog a = new Dog();

In both cases a.GetType() gives Dog.
Both invoke same constructor (with same hierarchy).

Then can you please tell me the difference between these two statements?

Answer 1

Both create a Dog object. Only the second allows you to directly invoke Dog methods or to otherwise treat it like a dog, such as if you need to pass the object to a method as a parameter of type Dog (or something in the Dog hierarchy that is more specific than simply object).

object obj = new Dog(); 
// can only see members declared on object
var type = obj.GetType(); // can do this
Console.WriteLine(obj.ToString()); // also this
obj.Bark(); // Error! Bark is not a member of System.Object

Dog dog = new Dog();
// can do all of the methods declared for Object
dog.Bark(); // can finally use the method defined for Dog