Dispose, when is it called?

Question

Consider the following code:

namespace DisposeTest
{
    using System;

    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Calling Test");

            Test();

            Console.WriteLine("Call to Test done");
        }

        static void Test()
        {
            DisposeImplementation di = new DisposeImplementation();
        }
    }

    internal class DisposeImplementation : IDisposable
    {
        ~DisposeImplementation()
        {
            Console.WriteLine("~ in DisposeImplementation instance called");
        }
        public void Dispose()
        {
            Console.WriteLine("Dispose in DisposeImplementation instance called");
        }
    }
}

The Dispose just never get's called, even if I put a wait loop after the Test(); invocation. So that quite sucks. I want to write a class that is straightforward and very easy to use, to make sure that every possible resource is cleaned up. I don't want to put that responsibilty to the user of my class.

Possible solution: use using, or call Dispose myself(basicly the same). Can I force the user to use a using? Or can I force the dispose to be called?

Calling GC.Collect(); after Test(); doesn't work either.

Putting di to null doesn't invoke Dispose either. The Deconstructor DOES work, so the object get's deconstructed when it exits Test()

Ok guys, it's clear now!

Thank you all for your answers! I will add a warning in the comment!

Solution 1

I want to write a class that is straightforward and very easy to use, to make sure that every possible resource is cleaned up. I don't want to put that responsibilty to the user of my class.

You can't do that. The memory management is simply not built to accomodate resources that are not memory specifically.

The IDisposable pattern is intended for developers as a way of telling an object when they are done with it, instead of having the memory management trying to figure that out by using things like reference counting.

You can use the Finalizer as a fallback for users who fail to dispose objects properly, but it doesn't work well as the primary method for cleaning up objects. To work smoothly objects should be disposed properly, so that the more costly Finalizer doesn't ever need to be called.

Solution 2

A couple of important points should be made to address the OP's question:

1. .NET GC is non-deterministic (i.e. you never know nor should you depend on when it happens)
2. Dispose is never called by the .NET Framework; you must call it manually - preferably by wrapping its creation in a using() block.
3. Explicitly setting a disposable object to null without calling Dispose() on it is a bad thing to do. What happens is that you explicitly set the objects "root reference" to null. This effectively means that you cannot call Dispose later AND more importantly, it sends the object to the GC Finalization Queue for Finalization. Causing Finalization by bad programming practice should be avoided at all costs.

Finalizer: Some developers refer to it as a destructor. And in fact it is even called a Destructor in the C# 4.0 Language Spec (section 1.6.7.6) and in previous versions of the current ECMA-334 spec. Fortunately, the 4th Edition (June 2006) correctly defines Finalizers in Section 8.7.9 and attempts to clear up the confusion between the two in Section 17.12. It should be noted that there are important internal differences (no need to go into those gory details here) between what is traditionally known as a destructor and a Destructor/Finalizer in the .NET Framework.

1. If a Finalizer is present, then it will be called by the .NET Framework if and only if GC.SuppressFinalize() is not called.
2. You should NEVER explicitly call a finalizer. Fortunately, C# will not explicitly allow this (I don't know about other languages); though it can be forced by calling GC.Collect(2) for the 2nd generation of the GC.

Finalization: Finalization is the .NET Framework's way to deal with the 'graceful' cleanup and releasing of resources.

1. It only occurs when there are objects in the Finalization Queue.
2. It only occurs when a garbage collection occurs for Gen2 (which is approx 1 in every 100 collections for a well-written .NET app).
3. Up to and including .NET 4, there is a single Finalization thread. If this thread becomes blocked for any reason, your app is screwed.
4. Writing proper and safe finalization code is non-trivial and mistakes can be made quite easily (i.e. accidently allowing exceptions to be thrown from the Finalizer, allowing dependencies on other objects that could already be finalized, etc.)

While this is certainly more info that you asked for, it provides background on how things work and why they work the way they do. Some people will argue that they shouldn't have to worry about managing memory and resources in .NET, but that doesn't change the fact that it needs to be done - and I don't see that going away in the near future.

Unfortunately, the examples above (mistakenly) imply that you need to implement a Finalizer as part of the standard Dispose pattern. However, you should not implement a Finalizer unless you are using UNmanaged code. Otherwise, there are negative performance implications.

I have posted a template for implementing the Dispose pattern here: How do you properly implement the IDisposable pattern?

Solution 3

All the answers are (more or less) correct, here's an example:

static void Test()
{
    using (DisposeImplementation di = new DisposeImplementation())
    {
        // Do stuff with di
    }
}

Manually calling Dispose will also work, but the advantage of the using statement is that the object will also be disposed when you leave the control block because an exception is thrown.

You could add a finalizer that handles the resource disposing in case someone "forgets" to use the IDisposable interface:

public class DisposeImplementation : IDisposable
{    
    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    protected virtual void Dispose(bool disposing)
    {
        if (disposing)
        {
            // get rid of managed resources
        }   
        // get rid of unmanaged resources
    }

    ~DisposeImplementation()
    {
        Dispose(false);
    }
}

See this question for additional information. However, this is just compensating for people not using your class correctly :) I suggest you add a big fat Debug.Fail() call to the Finalizer, to warn the developer of their mistake.

If you choose to implement the pattern, you'll see that GC.Collect() will trigger disposal.

Solution 4

Use this as a pattern/template for your classes

public class MyClass : IDisposable
{
    private bool disposed = false;

    // Implement IDisposable.
    // Do not make this method virtual.
    // A derived class should not be able to override this method.
    public void Dispose()
    {
        Dispose(true);
        // This object will be cleaned up by the Dispose method.
        // Therefore, you should call GC.SupressFinalize to
        // take this object off the finalization queue
        // and prevent finalization code for this object
        // from executing a second time.
        GC.SuppressFinalize(this);
    }

    // Dispose(bool disposing) executes in two distinct scenarios.
    // If disposing equals true, the method has been called directly
    // or indirectly by a user's code. Managed and unmanaged resources
    // can be disposed.
    // If disposing equals false, the method has been called by the
    // runtime from inside the finalizer and you should not reference
    // other objects. Only unmanaged resources can be disposed.
    private void Dispose(bool disposing)
    {
        // Check to see if Dispose has already been called.
        if (!this.disposed)
        {
            // If disposing equals true, dispose all managed
            // and unmanaged resources.
            if (disposing)
            {
                // Dispose managed resources.                
                ......
            }

            // Call the appropriate methods to clean up
            // unmanaged resources here.
            // If disposing is false,
            // only the following code is executed.
            ...........................

            // Note disposing has been done.
            disposed = true;
        }
    }

    // Use C# destructor syntax for finalization code.
    // This destructor will run only if the Dispose method
    // does not get called.
    // It gives your base class the opportunity to finalize.
    // Do not provide destructors in types derived from this class.
    ~MyClass()
    {
        // Do not re-create Dispose clean-up code here.
        // Calling Dispose(false) is optimal in terms of
        // readability and maintainability.
        Dispose(false);
    }
}

And of course as mentioned by others don't forget about using(...){} block.

Solution 5

You will have to call Dispose explicitly or by wrapping the object in a using statement. Example:

using (var di = new DisposeImplementation())
{
}

Possible solution: use using, or call Dispose myself(basicly the same).

Using using is the same as calling Dispose inside a finally block.

View more solutions

Comments

[Thorarin]

There's no such thing as a destructor in C#. They're called finalizers. You can actually call it from the finalizer (see my answer), but there may be special precautions you need to make.

[Cygon]

But take care not to call into any other objects in your finalizer. The order of finalization is undefined (especially with circular and other cross-dependencies being allowed), so the only thing a finalizer is allowed to do is clean up unmanaged resources.

[Rodrick Chapman]

C# does indeed have destructors: see the language spec or msdn.microsoft.com/en-us/library/66x5fx1b.aspx

[Dave Black]

The only time a 'using()' statement should not be used is in the case of a WCF Client/Proxy. I have a post on my blog explaining why and provide a solution for disposing of a WCF Client/Proxy. dave-black.blogspot.com/2012/03/…

[Guffa]

Why the downvote? If you don't explain what you think is wrong, it can't improve the answer.

[James]

"Dispose is never called by the .NET Framework; you must call it manually" - the using statement calls Dispose internally.

[Dave Black]

yes, a 'using' statement is effectively a try/finally with a call to Dispose in the finally clause. My point was that you have to actually write the code with the 'using()' statement or call Dispose yourself. In other words, if you don't wrap the use of the disposable resource within a 'using()' statement, you can either leak a resource and/or place it in the queue for Finalization.

[James]

Yep, my point was the comment is slightly misleading for readers, you could update to say "Dispose is never called by the .NET Framework; you must call it manually or wrap the disposable object with a using statement" or something along those lines...

[Dave Black]

Thank you for your clarification. I've edited the note to include the use of a 'using()' block

[Ben Voigt]

@RodrickChapman: See Dave Black's answer. They use the same ~ClassName naming as C++ destructors, but they aren't destructors, they are finalizers. The C# compiler takes the code in the so-called-but-not-actually-a-destructor and uses that to implement an overload of Object.Finalize() (it adds some stuff automatically too)

[Dave Black]

You forgot to mention to never implement a Finalizer unless you are working with unmanaged resources.

[Califf]

I don't think Cygon is absolutely correct on this, as the MSDN says this: "DO NOT access any finalizable objects in the finalizer code path, because there is significant risk that they will have already been finalized." Note, it says FINALIZABLE, not ANY. Not every object is finalizable by design. (reference: msdn.microsoft.com/en-us/library/b1yfkh5e(v=vs.110).aspx)

[Guffa]

@Califf: Yes, that is correct. However, you should still be careful what you reference from a finalizer, as it can be executed even after the application itself has ended, and the system is just wrapping everything up.

[yoyo]

“A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.” ― Douglas Adams, Mostly Harmless

[yoyo]

Though if you don't need a Finalizer then the whole SuppressFinalize / disposing pattern is redundant.

[Dave Black]

@yoyo - Absolutely not true. Just because you don't need a finalizer doesn't mean you shouldn't implement IDisposable. A Finalizer (as well as the Dispose pattern) is necessary for the cleanup/disposal of unmanaged resources. The Dispose pattern/implementation should be implemented any time you need to cleanup managed resources.

[Dave Black]

@Guffa - just to be clear, the CLR does not use reference counting to track allocations.

[morhook]

I think saying 'clueless developers' might be a little bit hard on the people that are having the question.

[morhook]

On this article they agree with most ideas exposed here. It can serve as a reference to read more about this. codeproject.com/articles/29534/…

[FocusedWolf]

"// Do not provide destructors in types derived from this class." ??? I think you can if they also have resources that need disposing. The way i read to do it was to provide a ~DerivedClass() { Dispose(false); } and a protected override void Dispose(bool disposing) { ... base.Dispose(disposing); }

[Yola]

@morhook exactly! i came to C# from C++ few weeks ago and i'm very dissatisfied that user must explicitly call to clean up resources. Don't call me clueless, with smart pointers C++ is very reliable and easy to use while C# requires some manual chore work which i of course will forget to do some time.