if we have a class abc{ int a, int b} and we create object of class. where does variable a ,b and object of class store

The instance of abc, the object itself, goes to the heap. As to the reference to this object, it depends on what obj is. If this is the variable, it is on stack. But if, for example, it is an instance member of some class, it also goes to the heap with the instance of the class. If this is a static member, it goes to the area of static memory.

If something is unclear here, you need to understand that if you have some member or variable of reference type, you are dealing with two different objects: the object itself (always on heap) and the reference (managed pointer) pointing to this object. This is important, because you can have two different references to the same object, one reference, zero references, and so on.

—SA

In this case, obj is stored in the heap.

public class abc
{
  int a; 
  int b;
}
abc obj = new abc();

You may consider obj as a handle or a pointer(if you know C++) to the data. The data pointed to by obj is stored in the heap. Treat obj as an integral data. Consider the following code,

abc a = new abc();
abc b = a;

Now, both a and b are handles to the same data. Here this assignment is not so costly, logically, it just copies an integral value. Unlike C++, in managed languages you can't get the address of the object, as the garbage collector shall move the object. But you can use the GetHashCode function. So,

Console.WriteLine(a.GetHashCode() + " is equal to " + b.GetHashCode());