Class Variable Vs Object

An object of a particular class is an instance of that class stored in heap memory. This instance contains slots for all the object's fields as well as some overhead needed by the CLR.

In contrast a variable of a particular class is a storage area which contains a reference to an object or null if it doesn't currently refer to an object. It doesn't contain the object itself as I said earlier.

For example:

class MyClass

{

   public int MyInt;

   public bool MyBool;

}

// ...

void SomeMethod()

{

   MyClass mc = new MyClass();

   mc.MyInt = 3;

   MyClass mc2 = null;

}

Here the expression: new MyClass() creates a new instance of MyClass and returns a reference (i.e. a pointer) to where it's stored on the heap. Initially, the memory used is wiped clean which results in the two fields having their default values (all bytes zero). So, MyInt is zero and MyBool is false.

The reference is then assigned to the local variable 'mc' of type MyClass. This local variable is stored on the stack of the current thread. 'mc' is then used to change the value of the MyInt field to 3.

'mc2' is also a local variable of type MyClass but it doesn't currently refer to an object as it's been assigned a null reference.