How to dispose my object? I am doing it by this. Whats is wrong? The Obj.Dispose() function does not appear to be right.
class MainclassForm : Form, IDisposeable
{
public void createanddispose()
{
A obj = new A();
obj.dowork();
obj.Dispose();//does not appear as a function
}
}
You can better use the using statement. Something like
using (MyIDisposableObject obj = new MyIDisposableObject())
{
// object here
}
A good reference to check on MSDN: Avoiding Problems with the Using Statement
The C# "using" statement results in a call to Dispose(). This is the
same as Close(), which may throw exceptions when a network error
occurs. Because the call to Dispose() happens implicitly at the
closing brace of the "using" block, this source of exceptions is
likely to go unnoticed both by people writing the code and reading the
code. This represents a potential source of application errors.
To call Dispose() on an object, your class must be inherited from IDisposeable interface and have an implementation of it:
class A : IDisposable
{
public void Dispose()
{
GC.Collect();
}
}
There are a couple things wrong here, but I believe the root is that you might confusing IDisposable with a Garbage Collector.
IDisposable is an interface that the type A, in your example, may implement. It that were the case, you could be sure that any instance of A had the method Dispose() you're looking for. This is useful for things like Streams, where they need to be closed, but it's not useful for (just as an example) a ComplexNumber type that has no outstanding resources.
In these cases, your managed C# code will handle all the "disposal" that needs to happen. It will get rid of the memory that object is using. That feature comes for free (sort of), and you don't need (or want) to do anything to explicitly invoke it. That's the main difference between managed (C#, Java, etc.) and unmanaged (C++) code.
Essentially, if an object is using more than just memory (like an open file does), IDisposable will be useful. If it is just memory (like a Form, or DateTime, or a majority of other things), there's no need for it.
As for your specific code, you've applied IDisposable to the type that contains your functions. Just as you couldn't call this.AddMinutes(1) to get at DateTime's method, you can't get at this one.
If you do need the added functionality of implementing IDisposable, you'll have to do it on A. Applying IDisposable, like any other interface, to MainclassForm will tell the compiler that you have a Dispose() method on that class, which is true only because the Windows Forms object has one. If it didn't, this would throw a compiler error, and you'd have to add one.
Related
So I have a class which implements IDisposable, and I have several methods (in another class) which follow the pattern below:
public void SomeMethod()
{
DisposableObject disposableObject = new DisposableObject();
// Do some stuff with the object
SomeOtherMethod(disposableObject);
disposableObject.Dispose();
}
While all of these methods do different things, they all call SomeOtherMethod at the end, which does a few more things with the disposable object before it's no longer needed.
When I move disposableObject.Dispose(); into SomeOtherMethod, Visual Studio gives me a message saying:
"Use recommended dispose pattern to ensure that object created by 'new DisposableObject()' is disposed on all paths: using statement/declaration or try/finally"
This message appears regardless of whether or not I pass the disposable object to SomeOtherMethod using the ref keyword.
My question is, will that object be disposed as long as SomeOtherMethod calls Dispose() on it? I'm assuming it will, and Visual Studio continues to send the message simply because it isn't "aware" of what's happening to that object in subsequent methods, but I'd love to get some confirmation on that!
It may be disposed or may be not, depends on the fact whether the execution will reach the Dispose invocation or not and that's because an exception can be thrown before the Dispose is called. Using try finally construction explicitly or implicitly by using keyword ensures that it will be called for any scenario and that's why VS gives you the warning.
will that object be disposed
Sorry, but that’s a meaningless question. The CLR does not keep track of whether an object has had its “dispose” method called or not (see Will the Garbage Collector call IDisposable.Dispose for me? )
As a general rule, it is always much nicer (readable/ maintainable / less-bug-prone / etc) that a method that creates an issue should also be the one that cleans up after itself. As you’ve just found, this pattern also allows automated checking by the compiler - and again, it is also a good rule to ensure that your code compiles cleanly without errors OR WARNINGS.
In this case, the warning is giving you a couple of ways to implement this cleanly; personally, I would prefer the “using” clause (so avoiding having to have an explicit call to “dispose”) like :
public void SomeMethod()
{
using (DisposableObject disposableObject = new DisposableObject() )
{
// Do some stuff with the object
SomeOtherMethod(disposableObject);
}
}
No matter where one call the Dispose(), it is called.
Not using the language keyword using for the disposable pattern, therefore moving the dispose in another method, is an anti-pattern, therefore it is a bad practice and a source of potential problems.
You can only remove the warning by adding the warning number in the project build settings.
The method Dispose() doesn't destroy the object.
The dispose pattern is only for freeing unmanaged resources like windows handle and shared memories.
After a call to Dispose() you still have the object and so the reference to the object that remains referenced in the managed memory.
Dispose() is made to be called once time at the end of object usage and no more.
The compiler send you a warning because you break the standard behavior of the pattern usage that is to use the using keyword.
And breaking standards can be source of problems.
The using of disposable objects standard is made to avoid bugs by letting the compiler generates the try { ... } finally { Dispose() } block to be sure that Dispose() is correctly called in the right place to avoid mistakes.
So avoid calling the Dispose() directly.
Unless you are sure of what you do, prefer using:
public void SomeMethod()
{
using ( DisposableObject disposableObject = new DisposableObject() )
{
// Do some stuff with the object
SomeOtherMethod(disposableObject);
}
}
And your code may be robust.
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 a class that spawns another UI thread and does it's thing. I need to abort that thread and clean up whenever my parent class is destroyed. So how do I know when my parent class is destoryed?
Coming from C++ my first thought was to put this in the destructor. But C# doesn't really have any destructors - only finalizers and dispose - which from what I understand - may or may not be called (I guess it's a mood thing for the GC??).
That's great and simple - if you may or may not want to release your resources.
But where do you put code that ABSOLUTELY POSITIVELY MUST BE EXECUTED whenever an object is destroyed?
You put it in Dispose (implementing the IDisposable interface) and then ensure that Dispose is called when the object is no longer required. There's a language construct that does just this:
using (var foo = new Foo())
{
// Do something with foo.
}
foo.Dispose will be called at the end of the using block. This is equivalent to:
{
var foo = new Foo();
try
{
// Do something with foo.
}
finally
{
foo.Dispose();
}
}
Note that Dispose is not called automatically when the object leaves scope; you need to do it yourself, using either a using block or by calling it explicitly.
You should, however, provide a finalizer in Foo that calls Dispose, so that if the object isn't disposed before the GC gets to it, you aren't left with unreleased resources:
~Foo()
{
Dispose();
}
The idea behind the IDisposable pattern is that it tells you unambiguously when a class needs disposing. Here's an article that describes how to implement it properly (accounting for possible descendant classes).
There is no way to guarentee that something "ABSOLUTELY POSITIVELY [WILL] BE EXECUTED" when an object is destroy (try pulling the plug on your PC --- finalizers ain't gonna be called)
The best you can hope for is the finalizer -- defined in C# using C++ destructor syntax. Although, you be better off implementing IDisposable, and using a using{} block.
Although it is not common to use this method, you can define a "destructor" in C# by using the ~ character, like so:
class Parent
{
~Parent() // destructor
{
// cleanup statements...
}
}
Note:
The destructor implicitly calls Finalize on the base class of the
object.
(source)
You make your class a Component and the containing class a Container.
See: C# - What is a component and how is it typically used?
Other answers seem to be busy with "going out of scope" use case. But "whenever my parent class is destroyed"? If "parent class" is a "containing object", components are the right tool.
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.
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.