Develop Reference

Hidden calling of a function

Let's say I have two objects, Master and Slave.
Slave has a method named Init();. The thing about Init() is, that I need it to be virtual, because it contains user's initialization code, but I also need it to get called automatically when the Slave is added to Master's List. But the method must not be callable by the user, it has to be automatic.
The first thing that I tried is an event - create an event SlaveInitialized that a Slave object could handle in its OnSlaveInitialized handler. This wouldn't work though, because there's a lot of Slave objects and I have no control over the order in which they get created and need to be initialized.
The second thing that I tried is internal method - internal Init() would be called when the object is added to Master's list and all seems okay, until I realized that by doing so I cannot inherit the method in a public class.
So the third thing I did and that worked is this - I created an internal method called _Init() that simply calls a protected virtual Init(), which solved my problem.
Now I want to ask - do I just have a major strike of being stupid, because I am missing the painfully obvious solution here, or is this the way it's normally done? What is the proper way? I hope I got the point of what I'm asking across, I tried my best to explain the problem.
Thanks for any help
This is the gist of the code I now have. Its point is to have Init() invisibly and automatically called when you add any Slave object to the Master's list via Master.AddSlave();
public class Master
{
private List<Slave> _slaves;
public void AddSlave(Slave slave)
{
// Call the "hidden" init
slave._Init();
_slaves.Add(slave);
}
}
public class Slave
{
internal void _Init()
{
// Call the topmost overloaded method.
Init();
}
protected virtual void Init()
{
}
}
public class SuperSlave : Slave
{
protected override void Init()
{
// Now this method gets called automatically
// when Master.AddSlave adds this object.
}
}

As far as I can tell, there are two basic ways to do this.
Like you already tried, an internal InitInternal() method that calls a protected virtual Init()
A protected internal virtual Init(), which outside of your assembly automatically becomes a protected method.

While this doesn't answer the question the way you might want it, however I feel this could be an approperiate solution.
I would recommend throwing an InvalidOperationException if the Init method gets called multiple times.
Description: The exception that is thrown when a method call is invalid for the object's current state.
In my opinion it should not be your responsibility to police your objects from being abused, so long as documentation is written properly, your objects properties and method names describe what they do, it should be enough for most users (developers) consuming your code to understand how it works.
For those who decide to call Init themselves and then add to the Master object, you can throw that exception so they will know that their method for using the class is incorrect.
EDIT:
Naming the method OnInit might be a good idea, that way the user of the class has an indication that it should not be directly called by themselves.

How do I dispose this code properly?

I have a quite large project which I try to keep as clean and tidy as possible. When I run the code analyzer in Visual Studio I get a reliability error which I find quite annoying. I'd really like to learn how to work around it. Here is a simplified example of what I am doing.
Here is the warning.
Warning 1 CA2000 : Microsoft.Reliability : In method 'MyExampleClassForStackOverflow.AddFeed(string)', call System.IDisposable.Dispose on object 'new FeedClassExamle()' before all references to it are out of scope.
Here is my example code:
class MyExampleClassForStackOverflow : IDisposable
{
public ConcurrentDictionary<string, FeedClassExamle> Feeds { get; set; }
public void AddFeed(string id)
{
//The warning is coming from this code block.
//In the full code, the feed classes collects data on a specific
//interval and feeds them back using events.
//I have a bunch of them and they need to be accessible so I
//store them in dictionaries using keys to effeciently find them.
Feeds.TryAdd(id, new FeedClassExamle());
Feeds[id].Start();
}
public void Dispose()
{
foreach (var item in Feeds)
item.Value.Dispose();
}
}
class FeedClassExamle : IDisposable
{
public void Start()
{
}
public void Dispose()
{
}
}
In order to test the code, use:
using (var example = new MyExampleClassForStackOverflow())
{
}
Any suggestion would be welcome.

The warning exists because the code analysis tools can't determine whether the object will get disposed correctly. The way your code is written, the object will not in fact get disposed correctly, but fixing the code will likely not eliminate the warning.
Fundamentally, what needs to happen is for every the AddFeed method to ensure that something will call Dispose on every FeedClassExample instance it creates. The best approach is to avoid creating a FeedClassExample instance if one already exists in the dictonary under the present ID. Failing that, the AddFeed method should either dispose of any FeedClassExample it creates but then decides not to store in the dictionary, or else swap with the one that is in the dictionary (I'm not sure what methods ConcurrentDictionary supports to do that) and then Dispose the old one. The essential requirement is that at all times outside the actual execution of AddFeed, the dictionary will hold all instances of FeedClassExample that have been created but not destroyed.
It may be informative to add a destructor in your FeedClassExample class which does nothing except log a message. If you are calling Dispose on that class correctly, the destructor will never execute. If you fail to call Dispose, it will. Thus, if the destructor ever executes, you can know you're doing something wrong.

The object isn't getting Disposed of if TryAdd fails, so try doing this explicitly:
public void AddFeed(string id)
{
FeedClassExample fce = new FeedClassExamle();
if (!Feeds.TryAdd(id, fce))
{
fce.Dispose();
}
Feeds[id].Start();
}

Only create the instance if it needs to be added:
if (!Feeds.ContainsKey(id)) {
Feeds.GetOrAdd(id, new FeedClassExamle());
}

Disposing Custom Class : Setting NULL VS .Dispose [duplicate]

This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Calling null on a class vs Dispose()
Just want some information regarding Disposing an Object.
I have create a Employee Class which i inherit from IDISPOSIBLE Interface. Below is the sample Code
public class Employee : IDisposable
{
private Int32 _RunID;
public Int32 RunID { get { return _RunID; } set { _RunID = value; } }
public void Dispose()
{
//Dispose(true);
}
}
Now my Question is, it a Good Coding Practice to Dispose every class we create and implementing/Inheriting it with IDisposible Interface even i have seen many other people code they directly set ObjEmployee = null; so just got confused which is good setting NULL or Implementing it With IDisposible Interface or None Of the Above ?

It depends, do you have managed resources (File handles, sockets, connections, etc....) that need to be gotten rid of along with your object? If yes then you need a Dispose() if your class contains basic types or just information you do not need to dispose and setting to null will give a hint to the GC to clear that memory.

When you set ObjEmployee = null, you only mark the object instance to be ready for the Garbage Collector, but you have no influence over when the actual clean-up would take place, and it may take a while. When you use the Dispose() method, the GC runs immediately and frees the memory the object was using.

The fundamental question, in deciding whether a class should implement IDisposable, is whether instances of that class have taken on the responsibility of seeing that other entities get cleaned up; typically those other entities will be altering their behavior on behalf of the IDisposable object and at the expense of other entities, and the IDisposable object is responsible for letting them know when they no longer need to do so.
For example, if any code, anywhere, uses the C function fopen() to open for read-write access a file which is located on a server, the server will alter its behavior by forbidding anyone else from accessing the file until it receives word that the program that opened it has no more need of it. When the program no longer needs exclusive use of the file, it can call fclose() which will in turn cause the server to be notified that the file should again be available to other applications.
If a method in a C# class calls a routine which calls fopen(), and that routine returns after putting the FILE* in a place the C# program knows about but nothing else does, that method takes on a responsibility for seeing that fclose() must somehow get called with that FILE*. The file needs to be fclosed(), and nothing else in the system has the information or impetus necessary to do so, so the responsiblity falls to that C# class.
If the C# method were to return without storing the FILE* anywhere, the file would never get closed, and nobody else, anywhere in the universe, would be able to use it unless or until the application exits. If the C# method has to exit without yielding exclusive use of the file, it must store the FILE* in a way that will ensure that someone, somewhere, will clean it up after exclusive use is no longer required. The normal pattern would be for the method to store the FILE* in a class field, and for class containing the method to implement IDisposable by copying and blanking that field, seeing if it was non-blank, and if it was non-blank, calling fclose() the stored FILE*.
The important thing to realize is that when an object is destroyed by the garbage collector, the system won't care what's in any of the object fields. It won't even look at them. What matters is whether the object has any unfulfilled responsibilities for ensuring that outside entities, which may not even be on the same machine, are informed when their services are no longer needed.

What are IDisposable alternative uses?

The msdn documentation of the System.IDisposable interface states that
The primary use of this interface is to release unmanaged resources.
I'm wondering what are alternative uses.
For example we also needed the IDisposable interface for other allocated resources, such as event subscription and so.
We used the interface as a marker to allow a class instance to know when it's no more used from clients. Client and infrastructural code explicitly call IDisposable.Dispose() whenever they no more need a logical instance of a class implementing the code.
There's no relation with unmanaged resources wrapped from the interface.
When we choosed the IDisposable interface for such a behaviour we considered it as an alternative (undocumented) use of the interface.
Which are the alternative use of IDisposable you have found?
Are they legittimate? Is the MSDN documentation wrong?

I think your reading of the documentation is wrong. Saying that any usage of IDisposable that is not related to unmanaged resources is undocumented is a bit like saying that any usage of System.Int32 that is not counting things is undocumented. It is an interface and has no implementation, there is no functionality there to even begin distinguishing between what's documented and what's undocumented.
The purpose of IDisposable is simply to provide the developer with a mechanism to deterministically control the lifetime of their objects. It just so happens that this mainly a requirement for dealing with unmanaged resources.
One of the more fancy uses of IDisposable is the using block syntactic sugar. As others have mentioned, using blocks give an operation scope and I think those are quite elegant.
Example 1 - timing blocks
StackOverflow uses mini profiler that uses using blocks to identify nested regions of execution:
using (profiler.Step("Doing complex stuff"))
{
using (profiler.Step("Step A"))
{ // something more interesting here
Thread.Sleep(100);
}
using (profiler.Step("Step B"))
{ // and here
Thread.Sleep(250);
}
}
The alternative to not using using is pretty horrible and I don't even want to mock it up here.
Example 2 - Disposable action
There have been different variations of disposable action pattern making rounds in .NET Domain Driven Design circles. Ayende has one, so does Udi Dahan in his Domain Events implementation, Jimmmy Bogard has a slightly different take on this, still in the context of Domain Events. The crux of the pattern is that you want to perform certain actions in some context, then have the context revert back to what it was before after you are done.
Ayende provides a simple example:
class UsuallyReadOnly {
//.. implementation
public IDisposable AllowModification
{
get
{
_allowModification = true;
return new DisposableAction(()=>{ _allowModification = false; } );
}
}
}
And UsuallyReadOnly's usage:
UsuallyReadOnly foo = new UsuallyReadOnly();
using(foo.AllowModification)
{
foo.Name = "Bar";
}

IDisposable is often used in conjunction with using to activate and deactivate something in a definite scope even if it is not an unmanaged resource. The use you describes sound as a reference counting and for sure is not recommended.

For "resources", substitute "responsibilities". When an object is said to hold an unmanaged resource, what that really means is that there is some task that needs to get done sometime, and the object is the only thing with the information and impetus necessary to do it. The purpose of "Dispose" isn't to get rid of any tangible entity, but rather to allow an object to "put its affairs in order". Someone is putting his affairs in order before his death isn't doing anything to himself, but rather he is ensuring that the things he has to do to persons and things outside himself get done. Likewise with IDisposable.Dispose.

Remember there is also the using pattern which acts a bit like RAII.
using ( DisposableObject obj = new DisposableObject( ) )
{
.....
}
So Dispose gets called when the using block is exited.

One of the more popular uses of the IDisposable interface is transaction scopes. You can use it to wrap some SQL logic in a transaction, and explicitly call Complete() to end the transaction:
using (var scope = new TransactionScope())
{
using (var connection = new SqlConnection(connectString))
{
// perform sql logic
...
scope.Complete();
}
}
You could also use a similar pattern for just about anything that requires a temporary function, such as creating and deleting a temporary file:
public class TempFileProvider : IDisposable
{
public Filename { get; private set; }
public TempFileProvider()
{
Filename = Path.GetTempFileName();
}
public void Dispose()
{
File.Delete(Filename);
}
}
So you could use it like:
using (var tempFileProvider = new TempFileProvider())
{
DoSomethingWithFile(tempFileProvider.Filename);
} // deletes temp file

Have a look at the following question Need an alternative to my IDisposable Hack
There i give a nice example of what i used IDisposable for. :)
Granted, it is not the ideal solution, however, it helped me a lot.

Using Wrapper objects to Properly clean up excel interop objects

All of these questions:
Excel 2007 Hangs When Closing via .NET
How to properly clean up Excel interop objects in C#
How to properly clean up interop objects in C#
struggle with the problem that C# does not release the Excel COM objects properly after using them. There are mainly two directions of working around this issue:
Kill the Excel process when Excel is not used anymore.
Take care to explicitly assign each COM object used to a variable first and to guarantee that eventually, Marshal.ReleaseComObject is executed on each.
Some have stated that 2 is too tedious and there is always some uncertainty whether you forget to stick to this rule at some places in the code. Still 1 seems dirty and error-prone to me, also I guess that in a restricted environment trying to kill a process could raise a security error.
So I've been thinking about solving 2 by creating another proxy object model which mimics the Excel object model (for me, it would suffice to implement the objects I actually need). The principle would look as follows:
Each Excel Interop class has its proxy which wraps an object of that class.
The proxy releases the COM object in its finalizer.
The proxy mimics the interface of the Interop class.
Any methods that originally returned a COM object are changed to return a proxy instead. The other methods simply delegate the implementation to the inner COM object.
Example:
public class Application
{
private Microsoft.Office.Interop.Excel.Application innerApplication
= new Microsoft.Office.Interop.Excel.Application innerApplication();
~Application()
{
Marshal.ReleaseCOMObject(innerApplication);
innerApplication = null;
}
public Workbooks Workbooks
{
get { return new Workbooks(innerApplication.Workbooks); }
}
}
public class Workbooks
{
private Microsoft.Office.Interop.Excel.Workbooks innerWorkbooks;
Workbooks(Microsoft.Office.Interop.Excel.Workbooks innerWorkbooks)
{
this.innerWorkbooks = innerWorkbooks;
}
~Workbooks()
{
Marshal.ReleaseCOMObject(innerWorkbooks);
innerWorkbooks = null;
}
}
My questions to you are in particular:
Who finds this a bad idea and why?
Who finds this a gread idea? If so, why hasn't anybody implemented/published such a model yet? Is it only due to the effort, or am I missing a killing problem with that idea?
Is it impossible/bad/error-prone to do the ReleaseCOMObject in the finalizer? (I've only seen proposals to put it in a Dispose() rather than in a finalizer - why?)
If the approach makes sense, any suggestions to improve it?

Is it impossible/bad/dangerous to do the ReleaseCOMObject in the destructor? (I've only seen proposals to put it in a Dispose() rather than in a destructor - why?)
It is recommended not to put your clean up code in the finalizer because unlike the destructor in C++ it is not called deterministically. It might be called shortly after the object goes out of scope. It might take an hour. It might never be called. In general if you want to dispose unmanaged objects you should use the IDisposable pattern and not the finalizer.
This solution that you linked to attempts to work around that problem by explicitly calling the garbage collector and waiting for the finalizers to complete. This is really not recommended in general but for this particular situation some people consider it to be an acceptable solution due to the difficulty of keeping track of all the temporary unmanaged objects that get created. But explicitly cleaning up is the proper way of doing it. However given the difficulty of doing so, this "hack" may be acceptable. Note that this solution is probably better than the idea you proposed.
If instead you want to try to explicitly clean up, the "don't use two dots with COM objects" guideline will help you to remember to keep a reference to every object you create so that you can clean them up when you're done.

We use the LifetimeScope class that was described in the MSDN magazine. Using it properly cleans up objects and has worked great with our Excel exports. The code can be downloaded here and also contains the magazine article:
http://lifetimescope.codeplex.com/SourceControl/changeset/changes/1266

Look at my project MS Office for .NET. There is solved problem with referencich wrapper objects and native objects via native VB.NET late-binding ability.

What I'd do:
class ScopedCleanup<T> : IDisposable where T : class
{
readonly Action<T> cleanup;
public ScopedCleanup(T o, Action<T> cleanup)
{
this.Object = o;
this.cleanup = cleanup;
}
public T Object { get; private set; }
#region IDisposable Members
public void Dispose()
{
if (Object != null)
{
if(cleanup != null)
cleanup(Object);
Object = null;
GC.SuppressFinalize(this);
}
}
#endregion
~ScopedCleanup() { Dispose(); }
}
static ScopedCleanup<T> CleanupObject<T>(T o, Action<T> cleanup) where T : class
{
return new ScopedCleanup<T>(o, cleanup);
}
static ScopedCleanup<ComType> CleanupComObject<ComType>(ComType comObject, Action<ComType> actionBeforeRelease) where ComType : class
{
return
CleanupObject(
comObject,
o =>
{
if(actionBeforeRelease != null)
actionBeforeRelease(o);
Marshal.ReleaseComObject(o);
}
);
}
static ScopedCleanup<ComType> CleanupComObject<ComType>(ComType comObject) where ComType : class
{
return CleanupComObject(comObject, null);
}
Usage case. Note the call to Quit, which seems to be necessary to make the process end:
using (var excel = CleanupComObject(new Excel.Application(), o => o.Quit()))
using (var workbooks = CleanupComObject(excel.Object.Workbooks))
{
...
}

For what it's worth, the Excel Refresh Service on codeplex uses this logic:
public static void UsingCOM<T>(T reference, Action<T> doThis) where T : class
{
if (reference == null) return;
try
{
doThis(reference);
}
finally
{
Marshal.ReleaseComObject(reference);
}
}