Destructors in VB.NET


This article is about understanding the working concept of destructor in VB.NET. I know you all may be thinking why a dedicated article on simple destructor phenomenon. As you read this article you will understand how different is VB.NETdestructor are when compared to C++ destructors. 
In simple terms a destructor is a member that implements the actions required to destruct an instance of a class. The destructors enable the runtime system, to recover the heap space, to terminate file I/O that is associated with the removed class instance, or to perform both operations. For better understanding purpose I will compare C++ destructors with VB.NETdestructors. 

Generally in C++ the destructor is called when objects gets destroyed. And one can explicitly call the destructors in C++. And also the objects are destroyed in reverse order that they are created in. So in C++ you have control over the destructors. 

One may be thinking how VB.NETtreats the destructor. In VB.NET, you can never call them, the reason is one cannot destroy an object. So who has the control over the destructor (in VB.NET)? it's the .Net frameworks Garbage Collector (GC). 

Now few questions arise why GC should control destructors why not us? 
The answer is very simple GC can do better object release than we can. If we do manual memory management one has to take care both allocation and de-allocation of memory. So there is always chance that one can forgot de-allocation. And also manual memory management is time consuming and complex process. So lets understand why VB.NET forbids you from explicitly writing code for destructors.  

  1. If we are accessing unmanaged code usually we forget to destroy an object. This avoids the destructor call and memory occupied by the object will never get released. Let examine this case by taking example, Below figure shows that Memory stacks in which application XYZ loads an unmanaged code of 30 bytes. When applications XYZ ends imagine it forgot to do destroy an object in Unmanaged code so what happens is Application XYZ memory gets deallocated back to the heap but unmanaged code remains in memory. So Memory gets wasted.
  2. If we are trying to release the object while object is still doing some process or i mean object is still active.
  3. If we are trying to release the object that is already been released. 

So lets see how GC will handle above situations: 

  1. If program ends GC automatically reclaims all the memory occupied by the objects in the program.
  2. GC keeps tracks of all the objects and ensures that each object gets destroyed once.
  3. GC ensures that objects, which are being referenced, are not destroyed.
  4. GC destroys the objects only when necessary. Some situations of necessity are memory is exhausted or user explicitly calls System.GC.Collect() method. 

Understanding the complete working of Garbage collector (GC) is a big topic. But I will cover the some its details with respect to our topic. GC is a .net framework thread, which runs when needed or when other threads are in suspended mode. So first GC creates the list of all the objects created in the program by traversing the reference fields inside the objects. This list helps the GC to know how many objects it needs to keep track. Then it ensures that there are no circular references inside this list. In this list GC then checks for all the objects, which have destructor, declared and place them in another list called Finalization List. 

So now GC creates two threads one, which are reachable list and another unreachable, or finalization List. Reachable objects are cleared one by one from the list and memory occupied by these objects are reclaimed back. The 2nd thread, which reads the finalization lists and calls, the each object finalized in separate object. 

Lets see how VB.NET compiler understands the destructor code. Below is a small class created in visual studio .Net, I have created a class called class1 which has a constructor and a destructor.  

Imports System
Namespace ConsoleApplication3
Class Class1
Public Sub New()
End Sub 'New
Overloads Overrides Sub Finalize()
End Sub
'Entry point which delegates to C-style main Private Function
Public Overloads Shared Sub Main()
End Sub
Shared Sub Main(ByVal args() As String)
Dim c As New Class1
End Sub 'Main
End Class 'Class1
End Namespace 'ConsoleApplication3

So after compiling the code open the assemblies in ILDASM.EXE (Microsoft Dissembler Tool) tool and see the IL code. You will see something-unusual code. In above code the compiler automatically translates a destructor into an override of the Object.Finalize() method. In other words, the compiler translates the following destructor:

Class Class1
Overrides Sub Finalize()
End Sub 'Finalize
End Class 'Class1Into following: 

In Source Code Format In IL Code Format
Class Class1
Overrides Sub Finalize () '
End Try
End Sub 'Finalize
End Class 'Class1




.method family hidebysig virtual instance void
        Finalize() cil managed
  // Code size       10 (0xa)
  .maxstack  1
    IL_0000:  leave.s    IL_0009
  }  // end .try
    IL_0002:  ldarg.0
    IL_0003:  call       instance void [mscorlib]System.Object::Finalize()
    IL_0008:  endfinally
  }  // end handler
  IL_0009:  ret
} // end of method Class1::Finalize

The compiler-generated Finalize method contains the destructor body inside try block, followed by a finally block that calls the base class Finalize. This ensures that destructors always call its base class destructor. So our conclusion from this is Finalize is another name for destructors in C#. 

Points to remember

  1. Destructors are invoked automatically, and cannot be invoked explicitly.
  2. Destructors cannot be overloaded. Thus, a class can have, at most, one destructor.
  3. Destructors are not inherited. Thus, a class has no destructors other than the one, which may be declared in it.
  4. Destructors cannot be used with structs. They are only used with classes.
  5. An instance becomes eligible for destruction when it is no longer possible for any code to use the instance.
  6. Execution of the destructor for the instance may occur at any time after the instance becomes eligible for destruction.
  7. When an instance is destructed, the destructors in its inheritance chain are called, in order, from most derived to least derived.


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