I think so. But take a look at a built-in class in ASP.NET:
public sealed class HttpPostedFile
{
public Stream InputStream { get; } // Stream implements IDisposable
// other properties and methods
}
Suppose I have an instance of HttpPostedFile called file. Since there is no Dispose method to explicitly invoke, file.InputStream.Dispose() won't be invoked until it's destructed, which I think goes against the original intention of IDisposable. I think the correct implementation should contain a standard IDisposable implementation. So, if one of the members implements IDisposable, the class needs to implement it too.
What are your opinions? It seems to be a bit complicated.
In general, you should implement IDisposable if you own the resource represented by the property - see this question for a discussion on this subject.
I'd say that because HttpPostedFile is instantiated during processing of an HTTP request, it doesn't own the stream, and hence doesn't dispose it. The stream will be disposed when the HTTP request processing finishes.
If your class creates one or more IDisposable objects and holds the only references to them, then your class should almost certainly implement IDisposable and dispose the IDisposable objects it created. If one or more IDisposable objects will be passed into the constructor of your class, then you need to consider a few scenarios:
Your creator may want to keep using the IDisposable after you're done with it, and will certainly know when it's no longer needed (the semantics of your class would let him know you're done with it).
Your creator won't want to use the IDisposable after you're done with it, and may not know when you're going to be done with it.
Your class may be used in some circumstances corresponding to (1) above, and in some circumstances (2), but your creator will know in advance which circumstance applies.
Your creator can't predict whether he's going to want to keep using the object after you're done with it.
For scenario #1, there's no need for you to implement IDisposable, though it might not be a bad idea to implement a do-nothing IDisposable handler and have your consumers use it, in case another scenario applies in future.
For scenario #2, your object should take ownership of the IDisposable, and should Dispose it when done. I don't really like having objects take unconditional ownership of IDisposables; I prefer to implement things as in #3.
There are two ways of handling #3. The one I prefer is for your object to take a parameter (either a Boolean or an enum) along with the IDisposable, indicating whether it is supposed to take ownership of the IDisposable. Your class unconditionally implements IDisposable; the implementation disposes of any objects it has taken ownership of, but not those it hasn't. An alternative is to have two subclasses with a common base class - one subclass implements IDisposable and the other does not. I prefer the former pattern, because it allows for the addition of a method to replace an IDisposable with a new one (of which it may or may not take ownership). For example, if I were implementing a control with an Image property, I would have a SetImage method which with a parameter to specify whether the control should own the passed-in image; that method would Dispose the old image if it owned it, and could then either take ownership of the new image or not.
bool OwnMyImage;
Image MyImage;
void SetImage(Image NewImage, bool TakeOwnership)
{
IDisposable oldDisposable; // Could reuse one variable for multiple IDisposables
if (OwnMyImage)
{
oldDisposable = Threading.Interlocked.Exchange(MyImage, null);
if (oldDisposable != null)
{
oldDisposable.Dispose();
}
}
OwmMyImage = TakeOwnership;
MyImage = NewImage;
}
Scenario #4 is complicated; the best way to handle it is probably for your object to implement IDisposable by raising a Disposed event. Your creator can use that event to do either Dispose the object if you were the last one using it, or adjust a flag or counter so that other code will know the object shouldn't be left undisposed on your behalf.
It depends.
Stream is also implemented by TextStream (possibly on top of StringBuilder), so no unmanaged resources are required.
HttpPostedFile may not use any unmanaged resources at all, so it's safe to postpone deconstruction until the garbage collector sees fit.
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 have just discovered that best practise instructs that where any type implement iDisposable, then you should wrap that in a using statement to ensure the object is disposed of correctly, even in the event of an exception.
My question is, how can you easily tell which objects implement iDisposable? Or should I just wrap everything that I am unsure about in the using statement and then rely on the compiler to tell me at compile time?
Thanks.
You could ...
Look for the presence of a Dispose member
Look at the definition of your type (F12)
Do as you suggest, wrap in a using and see what the compiler says
Although, the best thing is to learn what IDisposable is used for, soon you will understand the types that do and should implement this interface. i.e. external resources, unmanaged type wrappers (GDI graphics objects for example), limited resources (database connections)
IDisposable is implemented for example by objects that give access to unmanaged or expensive resources, like files, database connections and things like that. So to a certain extent, you can guess. For the rest, intellisense tells you if the Dispose() method is available on the object.
how can you easily tell which objects implement iDisposable?
Programatically one can use.
IDisposable disposable = obj as IDisposable;
if(disposable!=null)
{
//this object implements IDisposable
}
else
{
//Not implement IDisposable interface
}
If it's a standard class, then the MSDN documentation page should say if it implements IDisposable or not. Third-party libraries also usually come with documentation. Otherwise, if you're using an IDE like Visual Studio, you can inspect the class (F12 key) and see what interfaces it implements.
If you right click and choose Goto Declaration you should get the object browser there you can se all interfaces implemented by the class.
Otherwise use the intellisense to check if the class has a Dispose() -method, in which case you use Using.
And lastly, if you try to use Using on something thats Not an IDisposable you´ll get a compiler error.
Using the Object Explorer you should be able to traverse the hierarchy to see the root of the object you're trying to use.
The compiler will warn you, though, if the variable that you're trying to use is not IDisposable:
using (int i = 1)
{
// ...
}
will give you an error:
Error 1 'int': type used in a using statement must be implicitly convertible to 'System.IDisposable'
You can check this way also
if (anyobject is IDisposable)
{
//it implemants IDisposable
}
If one can employ a "Using" statement and have it work, one generally should. One type of situation to watch out for is creating an object and passing it as a property of some other object. There are four approaches the framework can take here:
A snapshot is taken of the passed-in IDisposable. The receiving object will take care of disposing the snapshot; the supplier of the IDisposable is responsible for Disposing it, and it may do so at any time after the snapshot is taken.
A snapshot is taken of the passed-in IDisposable, without care of whether it's been disposed or not. The supplier is responsible for Disposing the IDisposable, but could legitimately do so at any time--even before it's passed in. The "Font" properties of controls seem to behave this way. If a Font object is going to be used only to set controls' Font properties, one could Dispose the font as soon as it's created and not have to worry about cleaning it up later.
The receiving object requires that the passed-in object not be Disposed until the receiving object is done with it, whereupon the receiving object will Dispose it.
The receiving object requires that the passed-in object not be Disposed until the receiving object is done with it, but the sending object is still responsible for Disposing it.
Unfortunately, Microsoft seems to use different approaches to IDisposable objects in different parts of the framework. Sometimes the best thing to do is dispose of the object immediately after setting the property, and see if that causes problems when the receiving object tries to use it.
I have been very intriuged by design patterns lately, and specifically following correct design patterns in my classes that implement one or several interfaces.
Let's take an example. When a class implement IDisposable you should follow a specific pattern to make sure that your resources are properly cleaned up, by creating a private Dispose(bool disposing) method that differentiates between if it's called by the finalizer, or if it's called from the public Dispose method. Also, a finalizer should be implemented in this case, and you might also need a private bool variable called isDisposed that is set by the Dispose method, so that any method called after th object is Disposed will call a Exception making it clear that this object is already disposed, instead of the code inside the method crashing because some of the required resouces are disposed, and thereby no longer available.
There are also a lot of other interfaces I routinely implement, but it's not all of them I am sure if the way I implement them is the preferred way of doing it, and I might find out later on that it causes a subtle bug that is hard to find, that would probably have been non-existant if I had followed the proper pattern in the first place.
some examples of interfaces I would like to know the best way of implementing are ISerializable, IComparable, IComparable<>, ICloneable, IEnumerable<>, and so on. All interfaces from the Framework are interesting here, so it should not be limited to those I have listed above.
What I'm after is for different interfaces, the preferred way and hopefully also a link to resource on the internet that explains how and also why a specific pattern should be followed.
I hope to get a good collection of these patterns, as I know that they can greatly improve my code and make it more correct, and follow best-practices
It would be great if there are several patterns for the same interface so we can discuss which one is preferred. That might also cause some of you to maybe move over to a new pattern, or make modifications to your existing patterns to improve your code even further, and that would be great!
Edit
After reading Grzenios comment, I would also urge everyone to give the context the pattern should be applied for. For example the IDIsposable pattern should only be followed if you have some unmanaged resources inside your class that you need to dispose, and not if all the objects you need to dispose implements IDisposable themselves.
Edit 2
I should probably start this myself, since I put the question out here. So I'll describe one pattern I know well, and that is the IDisposable pattern.
This pattern should only be used if your class contain one or more unmanaged resources inside your class, and you hav eto make sure that they get Disposed. in this case in addition to the Dispose method we will need a finalizer in case the user of your class forget to dispose it.
So first thing first. Your class should implement the IDisposable interface, and you will have to define the public Dispose method as goverend by the interface. This method should look like this:
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
This will call the protected Dispose(bool) method that takes care of the actual cleanup.
Also, include a vaiable in your class to indicate if the class is disposed or not:
private bool alreadyDisposed = false;
GC.SuppressFinalize tells the garbage collector that this item does not need to be finalized even if it has a finalizer.
Then you need the protected Dispose method. Make it protected instead of private in case any derived class needs to override it:
protected virtual void Dispose(bool isDisposing)
{
if (alreadyDisposed)
{
return;
}
if (isDisposing)
{
// free all managed resources here
}
// free all unmanaged resources here.
alreadyDisposed = true;
}
The finalizer should also call Dispose(bool) if the user forgets to clean up:
~SomeClass(){
Dispose(false);
}
If some method require a disposed resource to function, make the function like this:
public void SomeMethod()
{
if (alreadyDisposed)
throw new ObjectDisposedException("SomeClass",
"Called SomeMethod on Disposed object");
// Method body goes here
}
That's it. This will make sure that the resources gets cleaned up. Preferable by the user of your class calling Dispose, but by adding a Finalizer as a fallback method.
While you're learning about design patterns, you should also look at some common anti-patterns, you get an idea where the pattern comes from. IDisposable is somewhat of an anti-pattern, a minor version of sequential coupling, in that it requires a user to call dispose, and if he forgets, you're in the shit. One of the main reasons of the "disposable pattern" is to fix this issue.
A preferred technique (of mine anyway), wherever possible, is not to expose an IDisposable object to a user, but expose a single method (call it Using(...) for example), which takes a delegate which would execute the code normally contained in your using(...) { } block. This method can perform your construction, execute the delegate, then dispose the resources it consumed, and omits at least 3 problems with IDisposable: That of the user forgetting to call it, the user calling it more than once, and the user calling it too early - you don't need to bother with that boilerplate "disposable pattern" when there's no IDisposable exposed.
An example, lets say I have a File IO requirement where I need to open the same files regularly (and thus, I can't be waiting on the garbage collector to call Finalize in the event that a user forgets to call Dispose).
class MyFileStream {
FileStream fs;
private MyFileStream(string filename, FileMode mode) {
fs = new FileStream(filename, FileMode.Open);
}
private void Dispose() {
fs.Dispose();
}
public static void Using(string filename, FileMode mode, Action<MyFileStream> use) {
MyFileStream mfs = new MyFileStream(filename, mode);
use(mfs);
mfs.Dispose();
}
public void Read(...) { ... }
}
A caller can then say
var x = default(...);
MyFileStream.Using("filename.txt", FileMode.Open, (stream) => {
x = stream.Read(...);
});
Console.WriteLine(x);
Note that this is rather similar to the using() { } language syntax anyway, only this time you are forced to use it, and it imposes further restrictions. One cannot forget to clear up the resources this way.
Still, this pattern isn't always suitable because you sometimes need the resources to last longer than just a method call or so, but if you find the opportunity to use it, do so.
I've completely gone off topic anyway, so back to what you were asking.
Do not use ICloneable - it says nothing about how an object is being cloned (deep or shallow), if you require such thing, create your own IDeepCloneable or IShallowCloneable interfaces instead.
For IEnumerable<>, it's a rare event that you need to create your own, because there's a rather large collection of existing classes in the framework already, and you can usually get additional features much more simply by implementing extension methods (Such as those in LINQ, or your own, making use of the powerful yield keyword, which will create an IEnumerable<> for you in the background).
I wouldn't say there's any specific pattern for the rest, they're pretty self explanatory.
For example the IDIsposable pattern should only be followed if you have some unmanaged resources inside your class that you need to dispose, and not if all the objects you need to dispose implements IDisposable themselves.
I disagee with the above. I never recommend allowing the GC to do clean up and be implicit about object and resource disposal. It's kind of like waiting for the maid to come around and pick up your wet towels at a hotel; it will happen eventually, but the right thing to do is pick them up and hang them up yourself.
Deterministic disposal of objects and keeping scope of those resources as minimal as possible will make for the leanest and most efficient applications, not to mention that for developers reading the code, it is much more explicit to dispose of resources and reads better. Like a beginning and an end to a story that one can see all in one place. I instantiate the object as late as possible and dispose of it as soon as possible after use minimizing scope. Either explicitly calling .Dispose on the object or utilizing a Using block that calls the .Dispose method automatically are (2) good ways of cleaning up.
There are also a lot of other interfaces I routinely implement, but it's not all of them I am sure if the way I implement them is the preferred way of doing it, and I might find out later on that it causes a subtle bug that is hard to find, that would probably have been non-existant if I had followed the proper pattern in the first place.
The entire purpose of an Interface is to create a guidline for the methods, properties, events, etc. that a class is to implement, but not provide the details on how to do it. That is up to you. There is no "pattern" per se of how to implement specefic Interfaces. Don't get thrown off by the IDisposable Interface. Upon creating the Interface in VS.NET, the plumming code is created for you which is not typical for an Interface, and in reality could be completely changed to what you need.
I think where you might be getting confused is about Implementing Interfaces that are in the .NET Framework, but you need to look at the concept of an Interface and not just the ones available in the Framework. If you really want to see examples of how the Interfaces in the .NET Framework are implemented, look at either the MSDN or decompile other Framework objects that implement those Interfaces to see how Microsoft implements them.
As you grow in OOD, you will begin to see the power behind interfaces and begin to create them for your classes. For example lets say you have an interface named IMachine with a method called .StartEngine(). We just want the implementer of the Interface to define all of the details themself; there is no 'pattern' to follow and we the Interface designer don't need to know or care how it is implemented. For a car maybe the methods implementation involves "Get the keys, place them in the ignition, put in park, press brake pedal, turn ignition..." However for a lawnmower the same methods implementation is "Put gass in the mower, prime the carborateor, pull the clutch, pull the cord..."
So you see you can't just look at Interfaces by the ones in the .NET Framework and how to apply a certain pattern to thier implementation. The implementation details, patterns, etc. are up to the implementing class, and as long as the details of the Interface are sufficed, then thats what matters.
Hope this helps a bit...
I was using the FtpWebResponse class and didn't see a Dispose method. It turns out that the class implements IDisposable, but does so explicitly so that you must first cast your instance to IDisposable before calling Dispose:
// response is an instance of FtpWebResposne
((IDisposable) response).Dispose();
Why would the designer of a class such as this one choose to implement IDisposable explicitly? As Anthony Pegram says, doing things this way masks the fact that the object should be disposed for the average developer who is not consulting the documentation every time he/she uses a class.
This is normally done if the class has a Close method that is the exact same as Dispose. The original Dispose is hidden in an explicit implementation so that the exact same method doesn't have two names.
It's officially recommended here:
Do implement a Close method for cleanup purposes if such terminology is standard, for example as with a file or socket. When doing so, it is recommended that you make the Close implementation identical to Dispose...
Consider implementing interface members explicitly to hide a member and add an equivalent member with a better name.
Occasionally a domain-specific name is more appropriate than Dispose. For example, a file encapsulation might want to use the method name Close. In this case, implement Dispose privately and create a public Close method that calls Dispose.
(P.S. I disagree with this convention.)
Occasionally a class will have a Dispose method that is part of the interface but doesn't actually need to be called because the only resource to dispose of is memory: MemoryStream, for example.
As mentioned by others, if the class has a Close method that does the same thing as Dispose, arguably Dispose only needs to exist to support the "using" pattern so it may as well be explicit.
It's a little weird looking to me too. For what it's worth: the base class (WebResponse) implements a Close() method. Reflector shows that WebResponse's Dispose() method just calls Close() and an Internal OnDispose virtual that does nothing.
I have to confess that it smells a little to me, but I bet that they explicitly implemented IDisposable so that there would not be confusion in Intellisense between calling Close() or Dispose().
In addition to what's been said, I might suggest that implementing IDisposable explicitly encourages use of the using block, as it can be used on any type which implements IDisposable and it is more natural (to most people, anyway) to write this:
using (var response = GetResponse())
{
// do something
}
Than this:
var response = GetResponse();
// do something
((IDisposable)response).Dispose();
I'm not sure that would be a developer's intention in explicitly implementing IDisposable, but it's possible.
This question already has answers here:
Proper use of the IDisposable interface
(20 answers)
Closed 5 years ago.
What is the best practice for when to implement IDisposable?
Is the best rule of thumb to implement it if you have one managed object in the class, or does it depend if the object was created in the class or just passed in? Should I also do it for classes with no managed objects at all?
If you mean unmanaged objects then yes, you should be implementing it whenever you have one or more unmanaged resource you are handling in your class. You should also be using the pattern when you are possibly holding on to objects that are IDisposable themselves, and make sure to dispose of them when your class is disposed.
(agreed that this question has already been asked enough times as to run a small printer out of ink should they be printed...)
While everyone has mentioned (unmanaged) resources, I have another thing to add: I use it when I need to eliminate event handler hookups that would otherwise prevent a class from going out of scope and being garbage collected.
As an example, I have a service which gets injected in to a child view, that child view will subscribe to various async finished type events on the service. The owner of that child view has no idea what the concrete type is, it simply has it as an interface. The service may go out of scope at some arbitrary point in the future, and I don't want it hanging around not being GC'ed. Upon getting rid of that child view, the owner will call Dispose on it to give it the chance to unhook any event handlers. Here is a slightly contrived (and very pseudo code) example, note how the interface for the child view also implements IDisposable.
public class OwnerView {
public OwnerView() {
_childView = new ChildView(myServiceReference);
}
public void CloseChildView() {
if (childView != null) {
_childView.Close();
_childView.Dispose();
}
_childView = null;
}
private IChildView _childView;
}
public class ChildView : IChildView {
public ChildView(MyService serviceRef) {
_serviceRef = serviceRef;
_serviceRef.GetSettingsAsyncFinished += new EventHandler(someEventHandler);
}
public void IDisposable.Dispose() {
_serviceRef -= someEventHandler;
}
private MyService _serviceRef;
}
public interface IChildView : IDisposable {
void DoSomething();
... etc ...
}
There are far more authoritative comments about this from others on SO, like Do event handlers stop garbage collection from occuring? and Garbage collection when using anonymous delegates for event handling. You may also want to check out this codeproject article.
I think the docs are pretty clear about what IDisposable is good for.
The primary use of this interface is
to release unmanaged resources. The
garbage collector automatically
releases the memory allocated to a
managed object when that object is no
longer used. However, it is not
possible to predict when garbage
collection will occur. Furthermore,
the garbage collector has no knowledge
of unmanaged resources such as window
handles, or open files and streams.
It even has an example. In that example the class that implements IDisposable contains a handle. A handle needs to be freed when it no longer used. This is done in the Dispose() method. So the user of that class sees that it implements IDisposable and knows that the class needs to be explictily disposed when it is no longer needed (so the handle can be freed). It is a best practise (i.e. rule) that you should always call Dispose() on IDisosable instances when the instance is no longer needed.
You should implement IDisposable when your class holds resources that you want to release when you are finished using them.
When your class contains unmanaged objects, resources, opened files or database objects, you need to implement IDisposable.
If it has properties which also need to be disposed.