Difference Between Destructor, Dispose and Finalize Method

Difference between destructor, dispose and finalize method

Destructor implicitly calls the Finalize method, they are technically the same. Dispose is available with objects that implement the IDisposable interface.

You may see : Destructors C# - MSDN

The destructor implicitly calls Finalize on the base class of the
object.

Example from the same link:

class Car
{
~Car() // destructor
{
// cleanup statements...
}
}

The Destructor's code is implicitly translated to the following code:

protected override void Finalize()
{
try
{
// Cleanup statements...
}
finally
{
base.Finalize();
}
}

Your understanding for the Destructor is right:

From MSDN

The programmer has no control over when the destructor is called
because this is determined by the garbage collector
. The garbage
collector checks for objects that are no longer being used by the
application. If it considers an object eligible for destruction, it
calls the destructor (if any) and reclaims the memory used to store
the object. Destructors are also called when the program exits. It is
possible to force garbage collection by calling Collect, but most of
the time, this should be avoided because it may create performance
issues.

In C# what is the difference between a destructor and a Finalize method in a class?

A destructor in C# overrides System.Object.Finalize method. You have to use destructor syntax to do so. Manually overriding Finalize will give you an error message.

Basically what you are trying to do with your Finalize method declaration is hiding the method of the base class. It will cause the compiler to issue a warning which can be silenced using the new modifier (if it was going to work). The important thing to note here is that you can't both override and declare a new member with identical name at the same time so having both a destructor and a Finalize method will result in an error (but you can, although not recommended, declare a public new void Finalize() method if you're not declaring a destructor).

Finalize vs Dispose

Others have already covered the difference between Dispose and Finalize (btw the Finalize method is still called a destructor in the language specification), so I'll just add a little about the scenarios where the Finalize method comes in handy.

Some types encapsulate disposable resources in a manner where it is easy to use and dispose of them in a single action. The general usage is often like this: open, read or write, close (Dispose). It fits very well with the using construct.

Others are a bit more difficult. WaitEventHandles for instances are not used like this as they are used to signal from one thread to another. The question then becomes who should call Dispose on these? As a safeguard types like these implement a Finalize method, which makes sure resources are disposed when the instance is no longer referenced by the application.

What is the difference between using IDisposable vs a destructor in C#?

A finalizer (aka destructor) is part of garbage collection (GC) - it is indeterminate when (or even if) this happens, as GC mainly happens as a result of memory pressure (i.e. need more space). Finalizers are usually only used for cleaning up unmanaged resources, since managed resources will have their own collection/disposal.

Hence IDisposable is used to deterministically clean up objects, i.e. now. It doesn't collect the object's memory (that still belongs to GC) - but is used for example to close files, database connections, etc.

There are lots of previous topics on this:

  • deterministic finalization
  • disposing objects
  • using block
  • resources

Finally, note that it is not uncommon for an IDisposable object to also have a finalizer; in this case, Dispose() usually calls GC.SuppressFinalize(this), meaning that GC doesn't run the finalizer - it simply throws the memory away (much cheaper). The finalizer still runs if you forget to Dispose() the object.

What is the difference between finalize and dispose in .net?

Finalizers are run by the Garbage Collection before an object that is eligible for collection is reclaimed. Dispose() is meant for cleaning up unmanaged resources, like network connections, files, handles to OS stuff, &c. It works best in conjunction with the using block where the compiler makes sure that Dispose() will be called immediately once you are done with an object – and also ensures that you cannot work with the object anymore once it's disposed.

Note that finalizers don't have to run, so relying on that can be dangerous:

What this means for you: Your programs cannot rely on finalizers keeping things tidy. Finalizers are a safety net, not a primary means for resource reclamation. When you are finished with a resource, you need to release it by calling Close or Disconnect or whatever cleanup method is available on the object. (The IDisposable interface codifies this convention.)

Careful also with the precise time when an object becomes eligible for collection. Read the article linked above – it's neither scope (a weird word which has noting to do with the lifetime of an object – it's “the region of program text in which it is legal to refer to [a named entity] by its unqualified name.”) nor is it strictly reference counting as an object can become eligible for collection even before the last reference to it goes away.

What is the point of Finalize and Dispose methods in .NET? (see details before answering)

The author of that blog post is a bit confused...

In C#, there is no such thing as a "destructor". Only Finalizers and IDisposable.

The ~ClassName() method is not called a "destructor". It is called a finalizer.

Dispose exists to release resources from code, where the finalizer exists to be called from the GC. Very often, the finalizer calls the Dispose() method, but the "Dispose Pattern" sets you up to only handle unmanaged resources from the finalizer.

You see, when the finalizer gets called, you are on a different thread, and any managed object you have is not necessarily valid. Because of this, if you call Dispose() from the finalizer, you should really be calling Dispose(false) which tells the "Dispose Pattern" to only dispose unmanaged resources.

Further, the "Dispose Pattern" suggests that when Dispose(true) is called, you should suppress the finalizer on that object.



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