Working with C#. I have an abstract class that I use to read/write settings to an xml file. When the class is no longer needed I want to perform one last write operation to the xml file before the class gets disposed. I tried putting this code in a destructor method ~myClass() {} but it throws an exception saying that the safe handle is closed. I am guessing this means that the class is already disposed or partially disposed. So if the destructor is not the correct place to do this where is the correct place? Do I need to implement IDisposable or something?
There is no such thing as a destructors in C#. Please refer to this question's accepted answer from Jon Skeet who explains and provide a good article on the topic.
Sample use of a C# Destructor
Besides, perhaps should you consider to implement the IDisposable interface in your class so that you could perform your manoeuvre.
There are many threads on finalizers on SO just read a few of them.
The basic point is that you should never rely on finalizers since their execution order is undefined or might not run at all. You should use IDisposablefor any managed cleanup(your case) and SafeHandles/CriticalFinalization for native resources.
It sounds like today you are attempting to write out to the settings file in your object's finalizer using a SafeHandle field. This is not the right approach.
Once the object hits the CLR finalizer thread, all bets are off about other managed resources the object can see. It's very legal and possible for the SafeHandle field to already be finalized and hence invalid by the time the containing class`s finalizer runs.
The standard way to work with a scenario like this is to implement IDisposable and use the Dispose method (which is intended to be called by user code) to properly dispose of managed resources such as the file.
For example:
abstract class MyClass : IDisposable {
private SafeFileHandle _handle;
public void Dispose() {
WriteTheFile();
_handle.Dispose();
}
}
If you need to have your finalizer give you a chance to write some stuff before the file is closed, you need to ensure that a strong reference to the file will outlive your object. One way to do this would be to store a strong reference to the file in a static field. When your object's finalizer runs, you can write the data, close the file, and null out the static field. Note that if your finalizer fails to null out the field, for whatever reason, the file may be kept open indefinitely.
Related
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.
I am a beginner in C#, I don't know why this isn't working, I just want to set this object to null by calling Dispose() method.
Why this is not possible?
class MyClass:IDisposable
{
public void Dispose()
{
this = null;
}
}
The purpose of the Dispose method isn't to clean up that class, but to clean up the disposable dependencies that the class is holding on to so that it can be disposed of normally by the garbage collector.
I'd suggest reading up more on the Dispose pattern and how to implement it in C#.
A bit of pedantry: The Dispose method is not a destructor, nor is it a finalizer.
Your question really seems to boild down to 'How do I delete stuff in C#'. The short answer is you can't, that's the job of the Garbage Collector System.GC. The IDisposable inerface is used to ensure that imporant resources that don't belong to .Net (like files, or network/database connections, but not a .Net class), are cleaned up on demand.
The IDisposable interface is important because it allows you to use the using pattern. That is, if your class implements IDisposable it can be used in a using block. Consider for instance:
class MyClass : IDisposable {
public void Dispose() { }
}
this class can now be used like this:
using (MyClass instance = new MyClass())
{
} // instance.Dispose() is called here at the end of the block.
The point of the using block is that when the block ends, it the compiler will throw in a call to Dispose() which you can use to get rid of important resources like files and database connections. For example, all of the Stream classes, like FileStream and what not implement IDisposable because it's a bad idea to leave a file open. Instead, you wrap all of your access in a using block, and then you are guaranteed that FileStream.Dispose will close the file out. Consider:
using (FileStream myFile = File.OpenRead("..."))
{
// Read the content of the file.
} // The file is guaranteed to be closed here. Cool!
This is much neater than doing something like this:
FileStream stream = File.OpenRead(" ... ");
stream.Close(); // Yes, you closed it manually, but it's error prone. What if you forget to do this?
Now what you're thinking of is a term called "Finalization", that is when the class is actually destroyed. This happens when the garbage collector (the System.GC class) actually destroys objects and cleans up their memory. Consider:
public class MyClass {
// This method, the 'Finalizer' will be called when the class is destroyed.
// The 'finalizer' is essentially just the name of the class with a '~' in front.
~MyClass() {
Console.WriteLine("Destroyed!");
}
}
public class Program {
public static void Main() {
MyClass referenceHeld = new MyClass(); // Reference held
new MyClass(); // No reference held on this class
WeakReference sameAsNoReference = new WeakReference(new MyClass()); // Equivalent to no reference.
System.GC.Collect(); // Force the garbage collector to collect
Console.ReadLine();
}
}
In short, the Garbage collector is the part of the runtime that cleans up stuff that isn't being used. What does it mean to not be used? It means that there are no references attached to the object. For example, if you run the program above, you'll notice that the word "Destroyed" gets printed on the screen twice. That's because two of the instances of MyClass created in the Main function are not pointed to by reference (A WeakReference is essentially the same thing as no reference). When we call GC.Collect() the garbage collector runs and cleans up the references.
That said, you should NOT call GC.Collect on your own. You can, for experimentation and education of course, but most people will tell you that the garbage collector does a fine job of keeping things clean on it's own. It doesn't make sense to have a bunch of GC.Collect scattered throughout your code, because that's the whole point of having a Garbage collector - to not have to worry about cleaning things up yourself.
So in short, you really can't destroy objects on your own, unless you call GC.Collect() (which you shouldn't do). The IDisposable interface allows you to work with the using pattern, ensuring that important resources are released (This is not the same thing as destroying the object though! All IDisposable does is ensure that Dispose() is called when the using block exits so you can clean up important stuff, but the object is still alive - an important distinction).
A class cannot make itself null because it has no control of who is referencing it.
For example, if you have a variable with an object in your code, that object cannot make itself null in YOUR code, it can only set it's OWN members to null.
In addition to this, think of a case where multiple classes reference the same object, where would he be null?
This is the kind of things the containing class should do.
The simple answer is that the "this" keyword is read only and can't be set.
A longer more fundamental answer is that you can't set objects themselves to null in c#, however you can set the reference to an object to null. When you set an object's reference to null and nothing else references that object, that is when the object is in a state to be garbage collected. (This is a simplification of what actually occurs.)
For example:
Object oTest = new Object;
oTest = null;
In the example above the object still exists after its reference oTest has been set to null. Its simply waiting for the garbage collector to come around and delete it.
So in your code it looks like your trying to set all of your references to your object equal to null, even if that may not be what your intending. This can't be done from the object itself. You need to make sure that all of the reference to your object are manually set to null or they are guaranteed to leave scope in your program.
null (C# Reference)
this (C# Reference)
You cannot modify your this pointer. In your example, your Dispose method doesn't need to do anything, so could be omitted altogether (along with updating the class to no longer implement IDisposable)
Dispose pattern is only needed when you use non-CLR resources, like graphics contexts or low level io. There are edge cases when you need to free up resources now, but as you say you are a beginner you really should not bother (yet).
Setting this to nil does not help. Consider this
MyClass sample = new MyClass();
sample.Dispose();
// at this point, sample still has a value
When you want to get rid of an object in C#, all you need is to let all the references go out of scope, or set them to nil. (Multiple variables can refer to the same instance). The runtime will free the object (and its subobjects) automatically because nobody is using it anymore.
Roughly speaking, you can think of them as pointers (technically they are not, but we are trying to explain the principle here)
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.
Is there a way to detect whether or not an object has called GC.SuppressFinalize?
I have an object that looks something like this (full-blown Dispose pattern elided for clarity):
public class ResourceWrapper {
private readonly bool _ownsResource;
private readonly UnmanagedResource _resource;
public ResourceWrapper(UnmanagedResource resource, bool ownsResource) {
_resource = resource;
_ownsResource = ownsResource;
if (!ownsResource)
GC.SuppressFinalize(this);
}
~ResourceWrapper() {
if (_ownsResource)
// clean up the unmanaged resource
}
}
If the ownsResource constructor parameter is false, then the finalizer will have nothing to do -- so it seems reasonable (if a bit quirky) to call GC.SuppressFinalize right from the constructor. However, because this behavior is quirky, I'm very tempted to note it in an XML doc comment... and if I'm tempted to comment it, then I ought to write a unit test for it.
But while System.GC has methods to set an object's finalizability (SuppressFinalize, ReRegisterForFinalize), I don't see any methods to get an object's finalizability. Is there any way to query whether GC.SuppressFinalize has been called on a given instance, short of buying Typemock or writing my own CLR host?
This is not possible, the GC just doesn't provide this information. Good reason for that, it isn't just two states that the object can be in. It also might already be on the finalization queue or it might already have been finalized.
A custom CLR host isn't going to help you with that, the hosting interface don't provide any hooks into the gc. You can check if SuppressFinalize has been called when it should have simply by checking this in the finalizer. Log it (quickly). You can't prove the opposite.
Fwiw, the .NET frame classes don't do this, they just let the finalizer run anyway.
If you want to confirm that finalization has been suppressed if your object doesn't own the resource, perhaps you could have the finalizer assert that it owns the resource? The test would have to do GC.Collect and GC.WaitForPendingFinalizers, but production code wouldn't have anything extra except the assert (which could be omitted from the production build). One slight caveat with the assert: if the thread that creates the object dies between creating the object and setting the ownership status, the finalizer may run inappropriately.
That having been said, I wonder whether it would be better to have an abstract ResourceWrapper type, with separate subtypes OwnedResourceWrapper and SharedResourceWrapper, that own or do not own the resource in question. Then the subtype that doesn't own resources wouldn't need to have a finalizer in the first place. Note that it may be useful for SharedResourceWrapper to implement IDisposable as a no-op.
This may help (reductio absurdum). A trick would be do some log (this could be a static state) in finalizers and if somebody is absent, he has called the suppress finalize but still you cannot be sure when.
This is working if you are the author of the type.
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.