I was running VS2013's code analysis on one of my current projects, and came across "CA1001: Types that own disposable fields should be disposable." A simple example that generates the warning (presuming DisposableClass implements IDisposable) is:
class HasDisposableClassField
{
private DisposableClass disposableClass;
}
However, converting the field variable to a property no longer generates the warning, even if the circumstance is that the property will be instantiated by the class:
class HasDisposableClassProperty
{
private DisposableClass disposableClass { get; set; }
public HasDisposableClassProperty()
{
disposableClass = new DisposableClass();
}
}
In the first case it's clear that the class should implement the IDisposable pattern, and dispose of its disposableClass field appropriately. My question: is the lack of a warning for the second case a limitation of the code analysis tool? Should the class still implement IDisposable and dispose of the property, despite the lack of a warning?
Yes, the lack of a warning is a limitation of the analysis tool.
You should definitely still implement IDisposable and clean up after yourself assuming your IDisposable properties aren't being injected from elsewhere.
Yes; you still need to dispose it.
Putting something in a property does not magically dispose it for you.
The missing warning is a bug on Code Analysis (it ignores the backing field because it's compiler-generated)
Implementation of disposable should depend on how the resources that need disposing (whether disposable or non-managed) are created.
If your object receives a resource via injection (constructor, method or property), it probably does not own it and should therefore probably not dispose of it.
How the resource is stored (local variable, field or property (with backing field) is not important), however, you may need to check that your resource hasn't been dispose externally, as your object isn't its owner.
If your class creates a resource directly (via create, allocate, open handle, factory method), it probably does own it and should therefore probably dispose of it.
The problem is that most static code analysis tools have limited rule sets and therefore can not make such distinctions and instead try to cover the cases they consider more common.
Related
Little confused (and not sure) with the kind of C# objects that I'm dealing with the at the moment.
For example:
interface IMyInterface
{
IDictionary<string, ICustomPath> MyPathDictionary { get; }
}
which is implemented by a class the also implements IDisposable
class MyClass:IMyInterface,IDisposable
{
}
IMyInterface myInterface = new MyClass();
I know how to dispose off instance "myInterface" of object MyClass (either by a using statement or explicitly cast instance to IDisposable like
((IDisposable)myInterface).Dispose();
or
((MyClass)myInterface).Dispose(); in the finally block after I'm done with what I'm doing.
But I have something like the following line of code
IExampleInterface exampleInterface = some condition ? myInterface.MyPathDictionary[key]:myInterface.CreateSomething(key);
And MyPathDictionary[key] is a dictionary value (where the key is of type string) and value of type IExampleInterface which is implemented by another class called ExampleClass, which also implements IDisposable like.
class ExampleClass:IExampleInterface,IDisposable
{
}
Now my confusion is caused by the above conditional statement since analysis of my code with a proprietary tool says that myInterface.MyPathDictionary[key] is causing a leak of resources/memory. What I'm not sure of is that when I'm disposing off myInterface explicitly in the finally block of my C# code then shall I explicitly dispose off myInteface.MyPathDiciotnary[key] as well because if I already disposed off myInterface then an invocation of MyPathDictionary[key] on it (myInterface) should automatically be disposed off.
Any suggestions regarding this conundrum will be appreciated.
If MyClass owns these resources then ((IDisposable)myInterface).Dispose(); should dispose of them. It is a common pattern to have a disposable root object recursively call dispose on anything that it owns. That kills the entire object tree which is convenient and intuitive for callers.
The key question is whether the objects contained in MyPathDictionary are owned or not.
Your static analysis tool probably thinks that myInterface.MyPathDictionary[key] is a factory method that creates what it returns. Property get operations are method calls. To the tool this might look like a factory. This is a false positive.
On the other hand if you actually called myInterface.CreateSomething(key) then this probably did create something that must be disposed of. You need to ensure this. Either by always disposing v no matter where its value came from. Or, by differentiating between the two cases.
I'd just wrap v in using and be done with it. Makes it easy to review the code and conclude that it is correct.
If implementations of your interface need to be disposable, the best way to handle it is to change your interface so that it also implements IDisposable. That way its clear to all users of the interface that it needs to be disposed properly. If some implementations don't actually need to dispose anything that's fine, they just have an empty implementation.
I am working on a "learning program" and using the Code Rush refactoring tool with my learning. With the latest update of Code Rush it has been recommending implementing IDisposable to my programs. I know what MSDN says about IDisposable and I have a real basic understanding of what it does but because I don't know all the implications of implementing it I have been ignoring the recommendation. Today I decided to learn more about it and went along with the recommendation.
This is what it added to my program.
class Program : IDisposable
{
static Service _proxy;
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing)
{
if (disposing)
if (_proxy != null)
{
_proxy.Dispose();
_proxy = null;
}
}
~Program()
{
Dispose(false);
}
So my questions is this. Does that do everything I need to do to get the advantage of IDisposable or do I need to do something in code to make it work? I put a break points on that and never reached it through the debugger so either it was not needed or I am not using it the way it was intended. Can someone please shed some light on what this is doing for me or how I should use it so it does do something for me?
In this case, CodeRush is suggesting you implement IDisposable because your class encapsulates an IDisposable resource (it's seeing _proxy, though that's not entirely a good thing since it's static). Code Rush thinks that there is a type that you're using which should be explicitly cleaned up, but you're not providing a way to do it via your class.
That being said, IDisposable is tricky - and this is one case where the generated code is not really a good implementation (even if _proxy were an instance variable). I would recommend not using the destructor in this case. It will cause performance issues in the GC, and in this case, doesn't help with the safety, as the encapsulated resource should handle that the case where you forget to call Dispose() for you. For details, see my series on IDisposable, and, in particular, encapsulating an IDisposable class.
In addition, this class shouldn't implement IDisposable (unless there is some other reason to do so) given the code above, as the _proxy resource is static. Disposing a static resource from an instance is likely to cause problems, at least in a general case. (In this case, it's obviously not problematic, but it's not a good practice...) Typically, a static variable has a very different lifetime than an instance member, so automatically disposing of it would be inappropriate.
In a properly-written program, at any given time, for every object that could possibly have a meaningful implementation of IDisposable, there will be some entity that is responsible for ensuring that IDisposable.Dispose will get called on that object sometime between the last "real use" of that instance and its ultimate abandonment. In general, if an object Foo is going to hold references to objects which implement IDisposable, at least one of the following scenarios should apply:
Some other object will also hold the reference for at least as long as Foo needs it, and will take care of calling Dispose on it, so Foo should let the other object take care of the Dispose.
The object holding the reference will be the last thing to use the IDisposable object in question; if Foo doesn't call Dispose, nothing else will. In that scenario, Foo must ensure that that other object's Dispose method gets called once it (Foo) is no longer needed, and before it is abandoned. The most idiomatic way to handle this is for Foo to implement IDisposable.Dispose, and for its Dispose method to call Dispose on the IDisposable objects to which it holds the last useful references.
There are some scenarios where a class designer might not know whether its class is going to hold the last useful reference to an IDisposable object. In some cases, this issue may be resolved by having a class constructor specify whether a IDisposable passed to the constructor is being "lent" or "given". Other cases may require the use of reference-counting wrappers or other complex techniques.
I receive a warning when I run some code through Visual Studio's Code Analysis utility which I'm not sure how to resolve. Perhaps someone here has come across a similar issue, resolved it, and is willing to share their insight.
I'm programming a custom-painted cell used in a DataGridView control. The code resembles:
public class DataGridViewMyCustomColumn : DataGridViewColumn
{
public DataGridViewMyCustomColumn() : base(new DataGridViewMyCustomCell())
{
}
It generates the following warning:
CA2000 : Microsoft.Reliability : In method 'DataGridViewMyCustomColumn.DataGridViewMyCustomColumn()' call System.IDisposable.Dispose on object 'new DataGridViewMyCustomCell()' before all references to it are out of scope.
I understand it is warning me DataGridViewMyCustomCell (or a class that it inherits from) implements the IDisposable interface and the Dispose() method should be called to clean up any resources claimed by DataGridViewMyCustomCell when it is no longer.
The examples I've seen on the internet suggest a using block to scope the lifetime of the object and have the system automatically dispose it, but base isn't recognized when moved into the body of the constructor so I can't write a using block around it... which I'm not sure I'd want to do anyway, since wouldn't that instruct the run time to free the object which could still be used later inside the base class?
My question then, is the code okay as is? Or, how could it be refactored to resolve the warning? I don't want to suppress the warning unless it is truly appropriate to do so.
If you're using Visual Studio 2010 then CA2000 is completely broken. It may also be broken in other versions of FxCop (a.k.a. Code Analysis), but VS2010 is the only one I can vouch for. Our codebase is giving CA2000 warnings for code like this...
internal static class ConnectionManager
{
public static SqlConnection CreateConnection()
{
return new SqlConnection("our connection string");
}
}
...indicating that the connection is not being disposed before it goes out of scope in the method. Well, yeah, that's true, but it isn't out of scope for the application as it's returned to a caller - that's the whole point of the method! In the same way, your constructor argument isn't going out of scope but is being passed to the base class, so it's a false positive from the rule rather than an actual problem.
This used to be a useful rule, but now all you can really do is turn it off until they fix it. Which is unfortunate, because the (very few) actual positives are things that should be fixed.
There is no safe and elegant way to have a chained constructor pass a new IDisposable object to the base constructor, since as you note it's not possible to wrap the chained constructor call in any sort of try finally block. There is an approach which is safe, but it's hardly elegant: define a utility method something like:
internal static TV storeAndReturn<TR,TV>(ref TR dest, TV value) where TV:TR
{
dest = value; return value;
}
Have the constructor look something like:
protected DataGridViewMyCustomColumn(ref IDisposable cleaner) :
base(storeAndReturn(ref cleaner, new DataGridViewMyCustomCell()))
{
}
Code which needs a new object would then have to call a public static factory method which would call the appropriate constructor within a try/finally block whose main line would null out cleaner just before it finished, and whose finally block would call Dispose on cleaner if it's not null. Provided that every subclass defines a similar factory method, this approach will ensure that the new IDisposable object will get disposed even if an exception occurs between time it's created and the time the encapsulating object is exposed to client code. The pattern is ugly, but I'm not sure any nicer other pattern would assure correctness.
What are your opinions on how disposable objects are implemented in .Net? And how do you solve the repetitiveness of implementing IDisposable classes?
I feel that IDisposable types are not the first-class citizens that they should've been. Too much is left to the mercy of the developer.
Specifically, I wonder if there should'nt have been better support in the languages and tools to make sure that disposable things are both implemented correctly and properly disposed of.
In C# for instance, what if my class that needs to implement the disposable semantics could be declared like this:
public class disposable MyDisposableThing
{
~MyDisposableThing()
{
// Dispose managed resources
}
}
The compiler could in this case easily generate an implementation of the IDisposable interface. The destructor ~MyDisposableThing could be transformed into the actual Dispose method that should release managed resources.
The intermediate C# code would look like this:
public class MyDisposableThing : IDisposable
{
private void MyDisposableThingDestructor()
{
// Dispose my managed resources
}
~MyDisposableThing()
{
DisposeMe(false);
}
public void Dispose()
{
DisposeMe(true);
GC.SuppressFinalize(this);
}
private bool _disposed;
private void DisposeMe(bool disposing)
{
if (!_disposed)
{
if (disposing)
{
// Call the userdefined "destructor"
MyDisposableThingDestructor();
}
}
_disposed = true;
}
}
This would make for much cleaner code, less boilerplate disposing code, and a consistent way of disposing managed resources. Implementing IDisposable by hand would still be supported for edge cases and unmanaged resources.
Ensuring that instances are properly disposed is another challenge. Consider the following code:
private string ReadFile(string filename)
{
var reader = new StreamReader();
return reader.ReadToEnd(filename);
}
The reader variable never outlives the scope of the method but would have to wait for the GC to dispose it. In this case, the compiler could raise an error that the StreamReader object was not explicitly disposed. This error would prompt the developer to wrap it in a using statement:
private string ReadFile(string filename)
{
using (var reader = new StreamReader())
{
return reader.ReadToEnd(filename);
}
}
An oft-stated principle is that "design patterns are needed to address language deficiencies". This is an example of that principle. We need the disposable pattern because the language doesn't give it to you.
I agree that disposability could have been elevated out of the "pattern" world and into the C# language proper, as we did with, say, property getters and setters (which are standardizations of the pattern of having getter and setter methods), or events (which standardize the idea of storing a delegate and calling it when something interesting happens.)
But language design is expensive and there is a finite amount of effort that can be applied to it. Thus we try to find the most useful, compelling patterns to put into the language proper. And we try to find a way that does so in a way that is not merely convenient, but actually adds more expressive power to the language. LINQ, for example, moves the concepts of filtering, projecting, joining, grouping and ordering data into the language proper, which adds a lot of expressive power to the language.
Though this is certainly a good idea, I don't think it meets the bar. It would be a nice convenience, I agree, but it doesn't enable any really rich new scenarios.
In addition to the other answers, there is the problem of how much should this implement and what should people expect from it? Say I declared my class like this:
public disposable class MyClass
{
readonly AnotherDisposableObject resource = new AnotherDisposableObject();
~MyClass()
{
this.resource.Dispose();
}
public void DoStuff()
{
this.resource.SomeMethod();
}
}
Then what would you expect to happen if a caller called DoStuff after the instance had been disposed? Should the compiler automatically insert something like
if (this.disposed) throw new ObjectDisposedException();
at the start of every method because you have declared the class as disposable?
If so then what about cases where methods are explicitly allowed to be called after an object is disposed (e.g. MemoryStream.GetBuffer)? Would you have to introduce a new keyword that indicated this to the compiler, e.g. public useafterdispose void ...?
If not then how do you explain to people that the new keyword implements some of the boiler-plate code for you, but that they still need to write code to check whether the object is disposed in each method? Moreover, how can they even check this, because all the state information about whether the object has been disposed is auto-generated! Now they need to track their own flag in the ~MyClass method which undoes half the work the compiler should be doing for you.
I think as a specific language pattern there are too many holes in the concept, and it only attempts to solve one specific problem. Now what could solve this entire class of problem in a general-purpose fashion is mixins (i.e. a Disposable mixin) and this language feature would be generally reusable for different concepts (e.g. Equatable mixin, Comparable mixin, etc.). That's where my money would go.
Personally, I consider the support for IDisposable to be quite decent in the current version of .NET. The presence of the using keyword pretty much makes it into a first-class construct for me.
I do admit there is a certain amount of boilerplate code involved, but not enough to warrant a new language features. (Auto-implemented properties was a good example of a feature that was begging to be introduced.) You've missed out an important point in your post that this "boilerplate" code is not always what you need. Mainly, you need to dispose unmanaged resources outside of the if (disposing) block.
Of course, the destructor (~MyDisposableThing) and parameterless Dispose() method are genuinely boilerplate and could be eliminated by the user of a language keyword, as you suggest - but again I'm not sure the introduction of an actual new keyword is all that necessary for a few lines of code.
I certainly see the point you are making here, and do sympathise with it to some degree. (I'm sure no coder would complain if your suggestion becamse part of the language specification.) However, it's not likely to convince the .NET development team when there are a rather limited number of lines of code anyway, some of which are arguably fairly context-specific (and thus not boilerplate).
I completely agree that IDisposable needs better language suppprt. Here's my variant of it from a while ago. The details are probably wrong, but C++/CLI serves as a pretty good model for this. Unfortunately it confuses the hell out of C# programmers when I show them examples in C++/CLI. But it already does "the right thing" in terms of implementation; we would just need a new syntax in C#.
Even the simplest Windows Forms application has a Dispose method in it, which is generated by a wizard and is fragile in the face of inexpert changes. The idea of composing components together such that one component can "own" several others is so fundamental that IDisposable is practically unavoidable, and unfortunately it seems to take several pages of most books to explain how to implement it correctly.
The existing using statement takes care of the client side. Where we need more language support is on the implementation side.
Some of the fields of a class are references to things that the class "owns", and some not owned. So we have to be able to mark a field as owned.
Also, it would be a very bad idea to automatically generate a finalizer. Most often, a class will own other objects that implement IDisposable. Not all classes are thread safe, nor should they need to be. If they are called from a finalizer, that happens on another thread, forcing them to be thread safe. This is probably the one area around IDisposable that causes the most confusion - a lot of people read the books and come away with the mistaken impression that you have to write a finalizer on an object that supports IDisposable.
I realize this is an old thread but there is something that has been overlooked.
Dispose pattern in both C# and Java break the fundamental reason for not having a deterministic destructor.
MyObject A = new MyObject()
MyObject B = A;
A.Dispose();
What is the state of B now? What if the owner of B didn't really want it disposed. You now have the same issue in C++ where you have to keep track of all the references of objects you are holding on to.
IDisposable is only truly valid within the context of using() and ensuring resource cleanup if an exception occurs.
There are design patterns to do this but it isn't IDisposable
#Peter Yes I am arguing that the Dipsosable pattern has a fault. When implemented the Disposable pattern isn't normally meant to just dispose OS resources with the idea of being able to continue using the object that was disposed. By using the Disposable pattern outside of a try{} finally{} in Java or using() in .NET you break one of the reasons of having a GC. I am not saying memory will leak. I am saying you can now have other parts of the code that have a reference to an Object that has been disposed. Now the onus is back on the developer to check if an object has been disposed before each call, or at the very least catch a ObjectDisposedException.
Lets look at a silly example:
FileStream stream = new FileStream (#"c:\mylog.txt");
logger.setStream (stream);
Who is supposed to invoke .Dispose()? It may not be obvious that the logger is now aquiring ownership of the file stream. Let's say the stream was created somewhere else outside of the developer knowing it will be set as the logging stream.
if we were to add one line, we would break the logger
using (FileStream stream = new FileStream (#"c:\mylog.txt"))
{ logger.setStream (stream); }
or
FileStream stream = new FileStream (#"c:\mylog.txt");
logger.setStream (stream);
stream.Dispose();
The Disposable pattern does not reference count for resources. The developer now has to be conscious of who owns the object and who is responsible for cleaning it up. The real problem is that when Dispose() is invoked the normal behavior is to invalidate the whole object preventing it from being used.
IMHO, the .net languages have a major shortcoming in their handling of iDisposable, which is that there is no nice way of handling initializers that throw exceptions. Unless one 'leaks' a copy of the object under construction or the disposable objects therein, there's no way to clean up any iDisposable objects which were created (either in an initializer, or a base-level constructor) before an initializer throws.
Two features I would like to see toward that end:
A class declaration that would cause a particular method to be invoked if an exception throws out of its constructor.
A field declaration which would indicate that the field should have .Dispose called upon it if some special private method or keyword is used on the object.
BTW, I would also like to see a declaration available for structure methods which would indicate that the method alters the underlying structure. Use of such methods would be forbidden on structure rvalues, and use of such methods on structure properties would generate a read-modify-write sequence.
Okay you need to understand the difference between managed and unmanaged memory.
Put simply, c++ style destructors wouldn't work in the managed world of c# because there's no guarantee when the object gets garbage collected, hence you would never know when the destructor would get called, which would make things very unpredictable.
As opposed to c++, when destructors get called as soon as the class goes out of scope, so you can guarantee when it gets called.
This is the reason why c# can't have destructors.
I have a class that creates several IDisposable objects, all of these objects are then passed to another 'manager' class in a 3rd party library. As I need some of the objects in later calls I kept a local field reference to the created objects so that I could access them at a later time. When I ran FxCop on the class it said that I should implement IDisposable, due to the disposable objects that I kept a reference to. My questions are:
Should I implement IDisposable for my class? (or is it the manager's responsibility?)
If so, should I only dispose the objects I kept a reference to? or should I find a way to dispose all objects I created.
My code:
public class MyClass
{
ClassThatIsDisposable aReference;
}
public MyClass(ManagerClass manager)
{
ClassThatIsDisposable transient=new ClassThatIsDisposable();
manager.Add(transient);
aReference=new ClassThatIsDisposable();
manager.Add(aReference);
}
public void LaterCall()
{
areference.Method();
}
The class that owns the object is the one that should dispose of them... It sounds like your manager is the owner, so he should dispose of the object.
If you're trying to avoid the FXCop warning then one option is to have MyClass request the disposable object from the manager each time is needs to use it. This way you won't have a member variable holding a reference to it. You could do this my having the Add() method return akey for the object that has been added and then use this key to retrieve the object from the manager when you need to use it.
If you create any disposable objects, you should either dispose them yourself or make sure you're handing them off to another class which will take responsibility for that.
In this case, I'd say it really depends on what the manager class is going to do. If that guarantees that it will dispose of anything that is added to it, you're okay. It does look like a somewhat odd design pattern though - I can't say I've used anything similar myself. That's not to say it's necessarily wrong/bad, but it would at least be worth taking another look to see if there's any way of avoiding this slight confusion of ownership.
Bear in mind that if the manager disposes the object you're holding a reference to, it's likely to be unusable afterwards.