Difference between A a =new A() and A a=null

Question

In C#,

A a = new A();
A a = null;
A a;

How does these 3 lines work with respect to memory?

I know the first line will create a memory in heap, but what about rest two lines?

How it work if, A a ; is a field and local variable.

Answer 1

1. Creates a new instance of A and assigns it to the variable a.
2. Does nothing. It just assigns null to a reference a. If a is not used, the compiler might optimize it away.
3. Does nothing too. It will revert to A a = default(A); which is the same as 2 since default(A) is null. For method variables it will show you a warning or error if you don't assign it. This one can too be optimized away if not used.