The Code Analysis-friendly way to dispose of objects

Question

As part of our Visual Studio 2010 (primarly C# 4.0) development standards, we have Code Analysis turned on. As I am reviewing recently submitted code for a new project, I am seeing a ton of

CA2000 : Microsoft.Reliability: In method 'XYZ', object 'ABC' is not disposed along all exception paths. Call System.IDisposable.Dispose on object 'ABC' before all references to it are out of scope.

warnings. The problem is that nothing I do seems to eliminate the warnings - and I've spent hours scouring the web and trying everything I can.

First, let me be clear that I am not talking about putting a simple using block in to properly dispose of a local variable - that's not an issue. In my case, these warnings are appearing when the object is either returned by the method or assigned to another object within the method.

Here is a code sample that contains four such warnings:

public void MainMethod()
{
    var object1 = CreateFirstObject();    // Warning here
    var object2 = CreateSecondObject();   // Warning here

    SomeCollectionProperty.Add(object1);
    SomeCollectionProperty.Add(object2);
}

private SomeObject CreateFirstObject()
{
    var theObject = new SomeObject()      // Warning here
    {
        FirstProperty = "some value",
        // ...
    };

    return theObject;
}

private SomeOtherObject CreateSecondObject()
{
    var theObject = new SomeOtherObject() // Warning here
    {
        FirstProperty = "a different value",
        // ...
    };

    return theObject;
}

I've commented the lines where the warnings occur.

I've tried refactoring both Create methods as described in the MSDN article (here) but the warnings still appear.

UPDATE I should note that both SomeObject and SomeOtherObject implement IDisposable.

Also, while object initializers may be a component of the problem, keep in mind that the initializers are isolated to the two private methods and have nothing to do with the warnings in MainMethod.

Can anyone show me how to properly implement these methods to eliminate the CA2000 warnings?

Twelvemo

I know this isn't really going to help, but... I find that CA2000 generates so many well-meaning false positives that the occasional genuine problem becomes very difficult to spot. For this reason I usually suppress it altogether. – Eisk

I don't see why it should generate all those warnings. Unless SomeObject and SomeOtherObject are using unmanaged resources and/or have open file handles (for example), this warning is not necessary imho. I know it doesn't help your problem, but if this would happen to me, I'd also suppress them altogether. – Plimsoll

While it touches on a problem with object initializers, this questions has a larger context. Plus, in that example, the problem is solved by putting a using statement around the initializer which cannot be accomplished in this case because the object is returned from the method and cannot be disposed or the variable in MainMethod will be set to null. – Twelvemo

That said, looking at the two private methods, is it safe to say that this warning will always appear when using object initializers? This is importants as recent updates to our dev standards encourage using object initializers for 'cleaner' code. – Twelvemo

And, even if I refactor the two private methods, I am still left with two warnings inside MainMethod. – Twelvemo

Eh, its close, and you already have an answer. But, I'll make a deal with you. I'll reopen if you promise to never, ever, place a number's value (in parens) after spelling it out. The word "four" actually means 4. We can figure this out. You don't have to say "four (4) pencils." – Hinder

Habit from writing too much commercial documentation over the years. And after being told to do it a certain way enough times by the editor, it becomes second nature. :-) – Twelvemo

Answer 1

The problem that is being detected by CA2000 in this case is that a disposable instance may be "orphaned" if an exception occurs before it is passed out of the method. For example, a "correct" implementation of CreateFirstObject would look something like the following:

private SomeObject CreateFirstObject()
{
    var theObject = new SomeObject();
    try
    {
        theObject.FirstProperty = "some value";
    }
    catch
    {
        theObject.Dispose();
        throw;
    }

    return theObject;
}

---

Given what you have described concerning your desired behaviour for MainMethod, its "correct" implementation might look something like this:

public void MainMethod()
{
    var object1 = CreateFirstObject();
    try
    {
        SomeCollectionProperty.Add(object1);

        var object2 = CreateSecondObject();
        try
        {
            SomeCollectionProperty.Add(object2);
        }
        catch
        {
            object2.Dispose();
            throw;
        }
    }
    catch
    {
        object1.Dispose();
        SomeCollectionProperty.Remove(object1); // Not supposed to throw if item does not exist in collection.

        throw;
    }
}

Terranceterrane

You are correct that this works for the two private methods. As I stated in a comment above, we have been strongly encouraging everyone to use initializers but if CA will always generate this warning, we need to reconsider. – Twelvemo

How about the two warnings in MainMethod? – Twelvemo

Yes, this warning will always be generated when using an initializer for a disposable type since the initializer is just C# syntactic sugar for the old-style "new then set properties" instructions. In the generated IL, there is an instantiation call followed by one or more property setter calls, any of which might generate an exception. – Terranceterrane

That said, I tend to agree with LukeH that the exception scenario problems reported by CA2000 are just plain too noisy, and I find that the cure is a bit worse than the problem in many/most cases. I also tend to disable the rule for that reason. – Terranceterrane

I'm not actually seeing any CA2000 problems being reported for MainMethod. I could still propose a fix for the underlying orphaning problem, but I'm not sure what you want to have happen in the case of an exception. For example, if there's an exception creating object2 or adding it to the collection, should object1 be allowed to be to the collection? – Terranceterrane

I see your point but, yes, in this case they both would need to be added or the entire operation fails. I'm sure this could be different in other cases so it is something we'll have to keep an eye on. – Twelvemo

As for the two warnings in MainMethod, I think we have more than just the minimum rule set enabled which may be why I'm seeing them. Based on all of the discussion thus far, it looks like CA is concerned about what happens to 'object1' if CreateSecondObject or either Add statement fails. Likewise for 'object2' if one of the Adds fails. I'm guessing that the only way to work-around these warnings is to wrap try-catch blocks around the statements much in the same way you did in the CreateFirstObject method, yes? – Twelvemo

I do have CA2000 enabled, but it's not picking up problems in MainMethod. Perhaps your actual code is different from your sample in some meaningful way? You are correct that the fix involves disposing when an exception occurs -- I'll add a sample to my answer. However, this is an actual resolution of a real problem, not just a work-around for an inconvenient code analysis warning. ;) – Terranceterrane

Yes, that does eliminate all of the warnings. Now I'll have to decide if all of it is worth it! Thx! – Twelvemo

Answer 2

One way to get rid of the warning is to suppress it in code:

[SuppressMessage(
    "Microsoft.Reliability",
    "CA2000:DisposeObjectsBeforeLosingScope",
    Justification = "Factory method")]

But this is not a real solution of the problem.

A solution is described here: How to get rid of CA2000 warning when ownership is transferred?

In the mentioned link it is basically stated to add the object to a collection implementing ICollection<T>, but I haven't tested that.

Genus

Yea, I read that thread before posting here. First, imho, the solution is incomplete and impractical. I agree with Damien's comments and will never be able to sell to anyone else here that I need to implement ICollection<T> on non-collection classes just because I want to suppress CA warnings. Seems counterintuitive to what the CA rules are supposed to accomplish. – Twelvemo

While I agree with and have suppressed the message in many cases in the past, we are making a push to improve our coding habits and gaining a better understanding of this rule in the hopes that we can make small changes to our code to eliminate the warnings is preferred (and why I posted). Considering why the rule exists, I think it is a valid argument to say that our code will only be more stable and memory-friendly if we satisfy the rule. – Twelvemo

Answer 3

What happens if you wrap the returned objects in main in a using block, or implement a finally to dispose of the objects?

Do the SomeOtherObjects need to implement IDisposable?

Alcus

This is not possible, because he adds them to a collection: SomeCollectionProperty.Add(object1); – Genus

Good point .... Assuming that IDisposable is implemented because of the need to dispose of some unmanaged object -- would it not be a safe assumption that these still need to be cleaned up at some point? If that is the case, then he could make SomeColllectionProperty a derived type that implements IDisposable and is responsible for walking through any child objects and calling dispose on them. – Alcus

Thank you @Peter, that is the issue I'm running into. – Twelvemo

@jtm001, SomeObject and SomeOtherObject may not necessarily be my code but provided by the BCL or 3rd party tools. Do you know for a fact that having a disposable collection would eliminate the warning? Unfortunately, there will be times when I don't control the collection code either. – Twelvemo

Answer 4

What is needed is to implement a pattern similar to a "using" block, but disabling the disposal of the object in the scenario where it will be returned successfully. The approach offered by Nicole Calinoiu is reasonable, though I prefer to avoid catching exceptions that are simply going to bubble up. My preferred expression of the code, given the constraints of the C# language, would be to use an InitializedSuccessfully flag, and then have a "finally" block which takes care of the disposal if InitializedSuccessfully has not been called.

If a class is going to contain many IDIsposable objects, and the set of such objects will be fixed once construction is complete, it may be useful to define an IDisposable manager class which will hold a list of IDisposable objects. Have the constructors of your class accept a DisposableManager object as a parameter, and place all of the objects it constructs into the list generated thereby (it may be helpful for your class to have an instance method:

T regDisposable<T>RegDispose(T newThing) where T:IDisposable
{
  myDisposableManager.Add(newThing);
  return newThing;
}

To use it, after myDisposableManager had been initialized, simply say something like: var someDisposableField = RegDispose(new someDisposableType());. Such a pattern would offer two big benefits:

1. All the main class would have to do in its Dispose implementation would be `if (myDisposableManager != null) myDisposableManager.Dispose();` The act of setting an IDisposable object in the constructor (using RegDispose) would also provide for its cleanup.
2. The code calling the constructor for the main object could, if the constructor throws an exception, call the Dispose method on the DisposableManager object it had created and passed in. This would ensure timely cleanup of the partially-created object, something which is otherwise all but impossible.

In vb, it's possible for a base-class constructor to expose a constructor parameter as a field which is available to field initializers. Thus, one could nicely use the RegDispose pattern in field initializers as well as in the explicit constructor. In C# that is not possible. It is possible to use [threadstatic] fields for that purpose, but some caution would be required to ensure that any such fields that get set also get unset. A constructor gets called from within something like a threadpool thread could generate a memory leak. Also, threadstatic fields cannot be accessed nearly as efficiently as normal ones, and I don't know any way in C# to avoid having to re-fetch the thread-static field many times--once for each IDisposable object registered.

Guideboard

Seems a bit like using a 12 foot pole to catch a 6 foot snake, but I'll reconsider if I continue to have issues. – Twelvemo

Answer 5

The pattern we use and that clears the warning most of the time is

DisposableObject retVal;
DisposableObject tempVal;

try{
  tempVal = new DisposableObject();
  tempVal.DoStuff();
  retVal = tempVal;
  tempVal = null;
} finally{
  if (tempVal != null) { tempVal.Dispose();} //could also be writtent tempVal?.Dispose();
}

return retVal;

Unfortunately, there are still some cases where the warning won't go so we then delete the warning locally with a justification saying the disposable is covered with pattern.

This is very briefly mentioned on Microsoft Documentation.

Kowalewski