How to delete dynamic array ?

[Ilya Yaroshenko]

case 1:
  delete ar;
case 2:
  ar.destroy();
case 3:
  GC.free(ar.ptr);
case 4:
  ar = null;// (assumed that ar is only one pointer to the same 
array)

What is the difference?

How to free memory to the system immediately?

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Thank You & Best Regards,
Ilya