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Setting Object References to null

Those of you who have created COM objects using Visual Basic 6.0 were well aware that it was

always preferable to set their references to Nothing when you were finished using them. Under the

covers, the reference count of the COM object was decremented by one, and may be removed from

memory if the object’s reference count equaled 0. In a similar fashion, C/C++ programmers often

set pointer variables to null to ensure they are no longer referencing unmanaged memory.

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Given these facts, you might wonder what the end result is of assigning object references to

null under C#. For example, assume the MakeACar() subroutine has now been updated as follows:

static void MakeACar()

{

Car myCar = new Car();

myCar = null;

}

When you assign object references to null, the compiler will generate CIL code that ensures

the reference (myCar in this example) no longer points to any object. If you were once again to make

use of ildasm.exe to view the CIL code of the modified MakeACar(), you would find the ldnull

opcode (which pushes a null value on the virtual execution stack) followed by a stloc.0 opcode

(which sets the null reference on the allocated Car):

.method private hidebysig static void MakeACar() cil managed

{

// Code size       10 (0xa)

.maxstack  1

.locals init ([0] class SimpleGC.Car myCar)

IL_0000:  nop

IL_0001:  newobj instance void SimpleGC.Car::.ctor()

IL_0006:  stloc.0