Lately I was doing a little project that involved a DLL module (that was created with C#) and that I needed to use in my application (that was written in unmanaged C++) For this to work I was using ATL/COM.
I noticed that even though I'm using a _com_ptr_t in my C++ application for handling my core COM interface, C# object's destructor is called only when my application is closed.
Let me give you some source to make things a bit more clearer:
Some of my C# code:
[ComVisible(true)]
[InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIDispatch)]
public interface ITestCOM
{
[DispId(1)]
void Connect([In, MarshalAs(UnmanagedType.U2)] ushort value);
}
[ComVisible(true)]
[ClassInterface(ClassInterfaceType.None)]
[ComSourceInterfaces(typeof(ITestCOMEvents))]
public partial class TestCOM : ITestCOM
{
...
~TestCOM()
{
MessageBox.Show("DESTRUCTOR");
}
...
public void Connect(ushort value)
{
...
}
}
I'm creating the .tlb file using something like this:
"RegAsm.exe TestCOM.dll /tlb:Test.tlb /codebase"
In my C++ header I have:
#import "C:\...\mscorlib.tlb"
#import "......\TestCOM.tlb" named_guids exclude("ISupportErrorInfo")
#include <afxdisp.h>
#include <atlcom.h>
class Unit : public ::IDispEventSimpleImpl<0, Unit, &__uuidof(TestCOM::ITestCOMEvents)>
{
public:
BEGIN_SINK_MAP(Unit)
SINK_ENTRY_INFO(0, __uuidof(TestCOM::ITestCOMEvents), 0x1, OnEventCallback, &OnEventCallbackDef)
END_SINK_MAP()
...
private
TestCOM::ITestCOMPtr mTestCOM;
// NOTE: This would be the same as "_com_ptr_t<_com_IIID<TestCOM::ITestCOM, &__uuidof(TestCOM::ITestCOM)> > mTestCOM;"
}
And my C++ source file I create my "mTestCOM" like this:
mTestCOM.CreateInstance(TestCOM::CLSID_TestCOM)
And basically that's it.. I can use any of my C# "TestCOM" object's functions like this:
mTestCOM->Connect(7);
The question is:
Why my C# TestCOM object's destructor is called only when my application is closed, and not when my C++ "Unit" object is destroyed?
Though I'm not familiar with C# and COM integration, I do know that destructors in C# work very differently to destructors in C++. A C# object is memory-managed and garbage-collected. This means that at some point after the object stops being referenced by the application it belongs to and becomes 'unreachable', the garbage collector will destroy it.
So the first important thing is that the delay between an object being abandoned and the garbage collector destroying it is nondeterministic... it will happen "at some point in the future", which may well be at the point the application terminates.
Secondly, the garbage collector is not always running. There is the concept of "memory pressure", when your application is allocating large chunks of memory and the free memory available to it is running out... at that point, the garbage collector will fire to get rid of old, unreachable objects. If your application does not allocate lots of memory it will not suffer from any memory pressure and the garbage collector will not need to run.
If you want to deterministically clean up some of a managed object's resources, you will need to use something like the IDisposable interface and call the Dispose method explicitly.
This is entirely normal and a side-effect of the garbage collector. Which only collects garbage, and runs the finalizer, when a managed app allocates enough memory to fill up a generation and trigger a collection. If you don't allocate enough then this won't happen until the app terminates.
This should not be a problem, make sure that you don't use the finalizer to do anything other than release unmanaged resources that were not disposed. Writing a finalizer is the wrong thing to do in 99.9% of all cases, finalizers are implementation details of .NET framework classes. Like the SafeHandle classes. There is otherwise no way to let deterministic destruction in the client app produce deterministic disposal in the managed code. If you do need to dispose members then you'll need to expose a method that the client code can call.
Related
I have written the very basic program below, I am new to C#. The destructor ~Program() doesn't get called, so I do not see in the output the 'Destructor called' string. I have checked other similar questions but I don't find the answer to mine. Thanks.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using static System.Console;
namespace LyfeCicleObject
{
class Program
{
public Program()
{
WriteLine("Cons called");
}
~Program()
{
WriteLine("Destructor called");
}
static void Main(string[] args)
{
WriteLine("Main started");
Program p1 = new Program();
{
WriteLine("Block started");
Program p2 = new Program();
WriteLine("Block ended");
}
WriteLine("Main ended");
}
}
}
The short answer -- the reason you're not seeing the "Destructor called" output -- was buried somewhere in the comments:
.NET Core does not run finalizers at the end of the program
(See: Finalizers (C# Programming Guide)).
.NET Framework will attempt to do it but .NET Core just won't do it.
Disclaimer: We have no way of knowing if these statements will continue to hold true; this is how they're implemented and documented as of now.
According to Raymond Chen, though, in his post Everybody thinks about garbage collection the wrong way, it would not be invalid if .NET Framework didn't run finalizers at the end of the program, either. The relevant quote, which says it from a different perspective, is this:
A correctly-written program cannot assume that finalizers will ever run.
So as long as you don't assume that finalizers will run, it shouldn't matter how they're implemented or if an implementation changes.
Before going any further with C#, you're going to have to abandon the idea of destructors in .NET because they simply don't exist. C# uses C++'s destructor syntax for finalizers but the similarities stop there.
The good news is that there is a way to do something close to what you were trying to do, but it takes a paradigm shift, a substantial change in how you think about resource acquisition and release. Whether or not you really need to do it is a totally different question.
Finalizers aren't the only way, or even the best way, to release resources that need to be released in a timely manner. We have the disposable pattern to help with that.
The disposable pattern allows class implementors to opt in to a common mechanism for deterministically releasing resources (not including memory on the managed heap). It includes finalizers but only as a last chance at cleanup if the object wasn't disposed properly, especially if the process isn't terminating.
I'd say the main differences you'll see compared to C++ destructors are:
There's more that the class's implementor must do support the disposable pattern.
The class's consumer also has to opt in to it with a using statement.
What you won't see is that the memory won't necessarily be reclaimed immediately.
If you want to know more about how, read on...
Before I get into any code, it's worth mentioning some points of caution:
A finalizer should never be empty. It causes instances to be kept alive longer and for nothing.
As mjwills stated in a comment, 99.9% of the time, you shouldn't be writing a finalizer. If you find yourself writing one, take a step back and make sure you have a good reason to do it in terms of .NET code and not because you would do it that way in C++.
More often than not, you'll be overriding Dispose(bool) after deriving from a class that implements the disposable pattern rather than creating the base of a class hierarchy that needs to be disposable. For example, the .Designer.cs files in Windows Forms applications override Dispose(bool) in order to dispose of the components field if it's not null.
Okay, code...
The following is an example of a simple class that implements the disposable pattern. It doesn't provide support for child classes so it's marked sealed and Dispose(bool) is private.
public sealed class SimpleDisposable : IDisposable
{
public SimpleDisposable()
{
// acquire resources
}
~SimpleDisposable()
{
Dispose(false);
}
public void Dispose()
{
// Suppress calling the finalizer because resources will have been released by then.
GC.SuppressFinalize(this);
Dispose(true);
}
private void Dispose(bool disposing)
{
if (disposing)
{
// release managed resources
// (you don't want to do this when calling from the finalizer because the GC may have already finalized and collected them)
}
// release unmanaged resources
}
}
Actual cleanup takes place in the Dispose(bool) method. If the parameter is true, it means that disposal is happening via the IDisposable interface (usually a using statement but not necessarily) and it's okay to clean up managed resources as well. If it's false, it means that disposal is happening as part of a GC sweep, so you can't touch managed resources because they may have already been collected.
If you're writing a base class that needs to support the disposable pattern, things change slightly: Dispose(bool) is becomes protected and virtual so it can be overridden by subclasses but is still inaccessible to the consumer.
The following is an example of a base class that supports the disposable pattern for subclasses.
public abstract class BaseDisposable : IDisposable
{
protected BaseDisposable()
{
// acquire resources
}
~BaseDisposable()
{
Dispose(false);
}
public void Dispose()
{
GC.SuppressFinalize(this);
Dispose(true);
}
protected virtual void Dispose(bool disposing)
{
if (disposing)
{
// release managed resources
}
// release unmanaged resoures
}
}
And then the following is a subclass that uses that support. Subclasses don't also need to implement the finalizer or IDisposable.Dispose. All they have to do is override Dispose(bool), dispose of their own resources, and then call the base implementation.
public class DerivedDisposable : BaseDisposable
{
public DerivedDisposable()
{
// acquire resources for DerivedDisposable
}
protected override void Dispose(bool disposing)
{
if (disposing)
{
// release DerivedDisposable's managed resources
}
// release DerivedDisposable's unmanaged resources
// Let the base class do its thing
base.Dispose(disposing);
}
}
So what does it mean to dispose managed and unmanaged resources?
Managed resources are things like other disposable objects and even non-disposable objects (e.g. strings). Some disposable types in the BCL will set such fields to null to ensure that the GC doesn't find active references to them.
When your class is being disposed, the consumer has decided that it and its resources are no longer needed. If your object contains other disposables, it's okay to dispose those objects, and so on down the chain, because it's not happening during garbage collection.
Unmanaged resources are things like file handles, global memory, kernel objects... pretty much anything you allocated by making a call to the operating system. Those aren't affected by the garbage collector and need to be released no matter what, so they're not subject to the disposing test.
If your disposable object used another disposable object that has an unmanaged resource, it's that object's responsibility to implement the Disposable pattern to release its resources, and your responsibility to use it.
Not all objects that implement IDisposable actually have unmanaged resources. Often a base class will support the disposable pattern simply because its author knows that at least one class that derives from it might need to use unmanaged resources. However, if a class doesn't implement the disposable pattern, one of its subclasses can introduce that support if it needs it.
Let's alter your program a bit and make it do what you were expecting, but using the disposable pattern now.
Note: As far as I know, it's not very common to have Main create an instance of the containing Program class. I'm doing it here to keep things as close to the original as possible.
using System;
internal sealed class Program : IDisposable
{
private readonly string _instanceName;
public Program(string instanceName)
{
_instanceName = instanceName;
Console.WriteLine($"Initializing the '{_instanceName}' instance");
}
~Program()
{
Dispose(false);
}
public void Dispose()
{
GC.SuppressFinalize(this);
Dispose(true);
}
private void Dispose(bool disposing)
{
if (disposing)
{
Console.WriteLine($"Releasing the '{_instanceName}' instance's managed resources");
}
Console.WriteLine($"Releasing the '{_instanceName}' instance's unmanaged resources");
}
private static void Main(string[] args)
{
Console.WriteLine("Main started");
Program p0 = new Program(nameof(p0));
using (Program p1 = new Program(nameof(p1)))
{
Console.WriteLine("Outer using block started");
using (Program p2 = new Program(nameof(p2)))
{
Console.WriteLine("Inner using block started");
Console.WriteLine("Inner using block ended");
}
Console.WriteLine("Outer using block ended");
}
Console.WriteLine("Main ended");
}
}
Build and run against .NET Framework 4.7.2 and you get the following output:
Main started
Initializing the 'p0' instance
Initializing the 'p1' instance
Outer using block started
Initializing the 'p2' instance
Inner using block started
Inner using block ended
Releasing the 'p2' instance's managed resources
Releasing the 'p2' instance's unmanaged resources
Outer using block ended
Releasing the 'p1' instance's managed resources
Releasing the 'p1' instance's unmanaged resources
Main ended
Releasing the 'p0' instance's unmanaged resources
Build and run against .NET Core 2.1 and you get the following output:
Main started
Initializing the 'p0' instance
Initializing the 'p1' instance
Outer using block started
Initializing the 'p2' instance
Inner using block started
Inner using block ended
Releasing the 'p2' instance's managed resources
Releasing the 'p2' instance's unmanaged resources
Outer using block ended
Releasing the 'p1' instance's managed resources
Releasing the 'p1' instance's unmanaged resources
Main ended
Instances p1 and p2 were disposed in the reverse order in which they were constructed because of the using statements, and both managed and unmanaged resources were released. This was the desired behavior behind trying to use a "destructor".
On the other hand, .NET Framework and .NET Core did things a bit different at the end, showing the differences I mentioned at the beginning:
.NET Framework's GC called the finalizer for p0 so it only released the unmanaged resources.
.NET Core's GC did not call the finalizer for p0.
If you want to see the output from finalizer:
add new method
initialize variable p1 inside
add GC.Collect()
call new method from Main
profit
If you want to know the reason of current behavior: the p1 variable exists in the scope of the Main method and will be collected by the GC when scope gets unreachable and I suppose it happens when program already stopped that means no GC (memory will be released in another way).
I've recently inherited an asp.net project that is running into memory leaks. In my investigation I noticed there are a lot of custom classes implementing IDisposable, but they aren't employing "using" when called. I've been fixing these issues, but one use case I was curious about was putting static methods on the class. Something like this:
public class ImDisposable : IDisposable{
public static GetList(string search){
//doStuff
}
//implement IDisposable
}
public class UseDisposable{
public void GetList(string search){
var list = ImDisposable.GetList(search);
//do stuff
}
}
I've never seen anything like this done, and I was curious how the GC handles this scenario. Thanks.
Static methods don't rely on any particular instance, and therefore they shouldn't be able to access instance members of your class which might tie up un-managed resources.
You need not consider them when thinking about Disposing of resources
On a related note, The existence of methods has little to do with IDisposable either. It doesn't dispose of the "methods" per-se, It disposes of unmanaged resources that an instance of the class might use. (such as open file streams)
You may be confusing memory leaks and garbage collection... just because you're not calling Dispose() doesn't mean you'll have a leak. You get a leak when:
(1) The class you didn't dispose has a handle to unmanaged resources, e.g. a file stream. Theoretically, if a class implements IDisposable, it has unmanaged resources, but I've seen enough empty dispose methods that I'm jaded. I.E. for static classes/members, unless you have a static variable that needs disposed, that's not your memory leak.
Also, static members would only ever have one instance, regardless of how many classes you instantiate, and you can't really have a leak when no new instances are ever created.
(2) You keep object references when you don't need/mean to... good example would be event listeners, especially those connected as a lambda. Not calling Dispose() on a database connection is also considered a leak, even though Dispose() simply returns the connection to the connection pool.
Also keep in mind that a growing memory footprint is not necessarily a leak. Garbage Collection is only performed when the .NET runtime deems it necessary... if there's still plenty of memory available, there's no need to GC.
Highly simplified overview; I'm mostly saying that nothing static is related to your memory leak. There's a good article on pinning down memory leaks here
Your question is a bit vague.
You may see the book CLR via C# by Richter for explanation about the garbage collection. Though my feeling is that your question is more about the interaction between a static method in a non-static class and the garbage collector. The above aforementioned book covers that too.
In general, a static method that allocates a resource will behave in a similar manor to a non-static method that allocates resources. However, the IDisposable interface will operate on the instance of the class. Therefore, your
static GetList(..)
method should not use resources that need the IDisposable implementation to free.
I am studying how garbage collector works in c#. I am confused over the use of Destructor, Dispose and Finalize methods.
As per my research and understandings, having a Destructor method within my class will tell the garbage collector to perform the garbage collection in the way mentioned in the destructor method which cannot be called explicitly on the instances of the class.
The Dispose method is meant to provide the user to control the garbage collection. The Finalize method frees the resources used by the class, but not the object itself.
I am not sure if I understand it the right way. Please clarify the doubts. Any further links or guides are welcome.
Destructor implicitly calls the Finalize method, they are technically the same. Dispose is available with objects that implement the IDisposable interface.
You may see : Destructors C# - MSDN
The destructor implicitly calls Finalize on the base class of the
object.
Example from the same link:
class Car
{
~Car() // destructor
{
// cleanup statements...
}
}
The Destructor's code is implicitly translated to the following code:
protected override void Finalize()
{
try
{
// Cleanup statements...
}
finally
{
base.Finalize();
}
}
Your understanding for the Destructor is right:
From MSDN
The programmer has no control over when the destructor is called
because this is determined by the garbage collector. The garbage
collector checks for objects that are no longer being used by the
application. If it considers an object eligible for destruction, it
calls the destructor (if any) and reclaims the memory used to store
the object. Destructors are also called when the program exits. It is
possible to force garbage collection by calling Collect, but most of
the time, this should be avoided because it may create performance
issues.
In C# terms, a destructor and finalizer are basically interchangeable concepts, and should be used to release unmanaged resources when a type is collected, for example external handles. It is very rare that you need to write a finalizer.
The problem with that is that GC is non-deterministic, so the Dispose() method (via IDisposable) makes it possible to support deterministic cleanup. This is unrelated to garbage collection, and allows the caller to release any resources sooner. It is also suitable for use with managed resources (in addition to unmanaged), for example if you have a type that encapsulates (say) a database connection, you might want disposing of the type to release the connection too.
Is there a difference in the garbage collection if I define the ~Example part or not?
class Example
{
public void Display()
{
Console.WriteLine("hi");
}
~Example()//**does giving this part or not has any effects in garbage collection**
{
}
}
and
class Example
{
public void Display()
{
Console.WriteLine("hi");
}
}
C# is a garbage collected language. This means that the .NET framework, in which code written in C# is executed, has a mechanism for the memory management. We don't have like in C++ to care about the destruction of the created objects, in order we don't have a high memory footprint or memory leaks. That's the job of Garbage Collector.
As it is stated more formally in MSDN
In general, C# does not require as much memory management as is needed
when you develop with a language that does not target a runtime with
garbage collection. This is because the .NET Framework garbage
collector implicitly manages the allocation and release of memory for
your objects. However, when your application encapsulates unmanaged
resources such as windows, files, and network connections, you should
use destructors to free those resources. When the object is eligible
for destruction, the garbage collector runs the Finalize method of the
object.
However the concept of destructors exist also in C# and as in other languages are used for the destruction of instances of classes. Furthermore a destructor cannot be called. It is invoked automatically.
For instance, if we declare the following class:
class Customer
{
~Customer()
{
// Here we place our clean up statements.
}
}
The destructor will implicitly call the Finalize method on the base class of the object. As it is explained in the above link this code would be translated to the following one by the C# compiler:
protected override void Finalize()
{
try
{
// Here goes our clean up statements.
}
finally
{
base.Finalize();
}
}
This means that the Finalize method is called recursively for all
instances in the inheritance chain, from the most-derived to the
least-derived.
In any case you should keep in mind the following:
The programmer has no control over when the destructor is called
because this is determined by the garbage collector. The garbage
collector checks for objects that are no longer being used by the
application. If it considers an object eligible for destruction, it
calls the destructor (if any) and reclaims the memory used to store
the object. Destructors are also called when the program exits.
If the destructor actually had some code in it that is executing, than yes, of course that would be a difference, however you can't guarantee the time when it would be called etc.
Also, you ask a question about asp.net, yet you post code from a console application?
C# has built in mechanism for freeing up un used objects or invalid objects. It runs periodically and cleans up the memory.
There is no need to write any explicit code/functionality to implement this.
For more details refer to this
In this example link ....
https://stackoverflow.com/a/19737565/2948523
I found some sections like,
~ImprovedClass()
~Inner()
Please help me out what are they ? why and how should I use them in code
class Inner
{
public Inner(IntPtr unkOuter)
{
}
~Inner()
{
}
}
public class ImprovedClass
{
// constructor
public ImprovedClass()
{
}
~ImprovedClass()
{
}
}
This is a Destructor Destructors
They are used to release resources that the object may still be holding onto even though it is no longer in use.
Those are called destructors which are calling automatically at the end of your class instance life on instance. You can write code here to release some resources which have been used by your object. Here are some remarks about destructors:
Destructors cannot be defined in structs.
They are only used with classes.
A class can only have one destructor
Destructors cannot be inherited or overloaded.
Destructors cannot be called. They are invoked automatically.
Here is some guide
http://msdn.microsoft.com/en-us/library/vstudio/66x5fx1b.aspx
~ mark is used for destructor not the constructor.
When you are using unmanaged resources such as handles and database
connections, you should ensure that they are held for the minimum
amount of time, using the principle of acquire late and release early.
In C++ releasing the resources is typically done in the destructor,
which is deterministically run at the point where the object is
deleted. The .NET runtime, however, uses a garbage collector (GC) to
clean up and reclaim the memory used by objects that are no longer
reachable; as this runs on a periodic basis it means that the point at
which your object is cleaned up is nondeterministic. The consequence
of this is that destructors do not exist for managed objects as there
is no deterministic place to run them.