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

Question

I get the need to clean up resources during the teardown of an object, but I have always found the differences between Dispose, Finalize, and the destructor methods a bit confusing.

I found this great article that concisely describes the distinctions between them, that I am going to have to save for future reference:
"Difference between Destructor, Dispose and Finalize methods" - Sanjay Saini http://sanjaysainitech.blogspot.com/2007/06/difference-between-destructor-dispose.html

The fundamental question I am trying to ask here is this.

If a language offers destructors (for example C# [refuted]) what value do Dispose and Finalize add to the equation?

Am I just a curmudgeon that is used to the old school way of doing everything in the destructor, or is there something I am missing that is only possible by breaking the tear-down of an object into three parts?

UPDATE:
As noted in some of the replies, C# does not actually have destructors. The question may be moot at this point in recognition of that. When I read in the above referenced article that C# actually had a separate deconstructor (an error apparently), it threw me for a loop and I started wondering what the point of Dispose and Finalize would be if you had a final destructor to wrap up everything. I suppose that in a GC langauge like C# the concept of a single destructor to provide the denemount for an object doesn't make much sense.

Sorry for the downvotes on some of you guys, but a couple people didn't read the question carefully and thought I was asking about the difference between Dispose and Finalize, which really wasn't the point.

Answer 1

Sorry to answer my own question, but I think I have assembled from a variety of sources, including some of the answers a concise answer to my question addressing the differences in object lifecycles when transitioning to a GC platform.

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Non GC Object Lifecycle:

Initialization: Happens in the constructor which is called when the first reference is added to object.

Cleanup: Happens in destructor which is called when last variable referencing object goes out of scope or is explicitly set to nothing/null/another object.

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GC Object Lifecycle:

Initialization: Happens in the constructor which is called when the first reference is added to object.

Clean-up (first pass): Happens in the Dispose Method, which must be explicitly called by the client or implicitly called at the end of a Using statement. This clean-up is specifcially to free up resources that must be released immediately and not wait on the GC.

Clean-up (Final Pass): Happens in Finalize Method which is called on the GC's timetable, which can be delayed. If the finalize method is implemented, it is called unless the GC.SuppressFinalize has been called previously on the object (normally from within the dispose method) to flag it as already cleaned up.

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Did I get this right? Feel free to edit if I missed something here.

Answer 2

Finalize cannot access managed objects, so managed object should be handled in Dispose. The difference between those two is that you should use Dispose for managed and Finalize for unmanaged. Your Destructor should be calling your Dispose so you've properly cleared memory. Take a look at the following articles:

Code Project - IDisposable: What Your Mother Never Told You About Resource Deallocation Gil Fink - Memory Leak And The IDisposable Pattern

Answer 3

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.

Answer 4

The problem with waiting for the finalizer to run to release resources, is that it does not run in a predictable deterministic manner. The finalizer is called when the GC is ready to destroy the object, and this can occur at anytime in your application.

This is why you have dispose. It allows you to deterministicly release resources at a known point in time, ensuring that resources will be freed up. Otherwise you have no way of controlling when the finalizer will run.

Answer 5

It's hard answering this since you already link to articles that explains the difference.

But let me try anyway.

With garbage collection you have non-deterministic memory management, and since GC will run your finalizer, you have guaranteed, but non-deterministic resource management.

This is good, in the sense that you know things will be cleaned up.

However, it is bad, since you don't know when it will happen.

For instance, if you open a file and lock it while you have it open, not knowing when the file will become available to open again later on is bad, but the guarantee that at some point it will be closed is good.

Dispose and IDisposable serves that bad part by adding deterministic resource management. You choose when to dispose of your resources, closing the file, the network connection, the database connection, whatever. It's your choice, and you will typically dispose of the resource when you no longer need it. Thus, when you no longer need the file, you dispose of the object keeping it open. The object will stay in memory (non-deterministic memory management) for a while longer, but the file will be closed when you say so, ready to be opened again right away.

Thus you gain deterministic handling of resources, and couple that with non-deterministic memory management and you got the best of two worlds.

Answer 6

What looks like a destructor in C# (~Foo()) is really just an different way of spelling "Finalize".

Perhaps in hindsight the choice of syntax wasn't the best because ~Foo() in C# is rather different from ~Foo() in C++.

Answer 7

Only managed objects can be finalized automatically. If you want to offer implicit disposal of unmanaged objects, the Finalizer can be used. If you want to offer explicit control of disposal to a caller of your object, you can allow them to call Dispose.

I like this article.

Answer 8

I'm not sure on the reasoning between Destructors and Finalizers, but Dispose exists so that you can free the resources before the Garbage Collector gets to the object (which may be never!)

Answer 9

c# does not offer destructors, so your question is somewhat moot?