Does a class need to implement IDisposable when all members are explicitly disposed?

Question

Trying to understand when implementation of IDisposable is necessary:

I wrote a little example.

 public class FileManager { private FileStream fileStream; public void OpenFile(string path) { this.fileStream = File.Open(path, FileMode.Open, FileAccess.Read); } public void CloseFile(string path) { if ( this.fileStream != null && this.fileStream.CanRead) { this.fileStream.Close(); } this.fileStream.Dispose(); } } // client var manager = new FileManager(); manager.Open("path"); manager.Close("path");

Does this class need to implement IDisposable because it has a managed resource (FileStream) which holds onto an unmanaged resource (a file)? Or do I not have to implement IDisposable because I am cleaning up within the class?

Confused.

What happens when an exception is thrown before manager.Close is called? — user743382
– user743382, Commented Jun 8, 2012 at 21:22
When in doubt, run FxCop - it detects and explains where and when Dispose needs to be implemented. — 500 - Internal Server Error
– 500 - Internal Server Error, Commented Jun 8, 2012 at 21:25
@TimSchmelter - Read that a couple times and it just seems like an argument without any resolution. I don't see any answer there. — O.O
– O.O, Commented Jun 8, 2012 at 21:40
@白ジェームス: Therefor i've posted the link just as addition, not as duplicate or answer. — Tim Schmelter
– Tim Schmelter, Commented Jun 8, 2012 at 22:44

supercat · Accepted Answer · 2012-06-08 21:37:19Z

For every instance of any type which implements IDisposable and might do so in a non-trivial fashion, it must at every moment be possible to identify how that instance will be Disposed. In most cases, this means that each IDisposable instance will have a well-defined owner, which is responsible for calling Dispose. In the case of the FileStream instance created by the class, your class is the owner, since nothing else will be able to Dispose it.

Classes with fields that references to IDisposable instances which they own should almost always implement IDisposable, and use their Dispose method to Dispose the IDisposable objects they own. Your class has such a field; it should thus implement IDisposable.

Whenever possible, a class which requires cleanup should be designed so that calling IDisposable.Dispose on it will suffice to perform any and all such cleanup as may be needed. In some cases, it may be impractical to perform cleanup without using some other method, but those cases are pretty rare. If one can design a class so that Dispose will take care of all necessary cleanup, one should do so.

@白ジェームス: Glad it's useful. I get somewhat irked at people who suggest that classes shouldn't implement IDisposable if they use some other method for cleanup, or if their cleanup entails actions not typically associated with IDisposable. The whole point of IDisposable is to be able to say that (1) Someone who creates an object requiring cleanup shouldn't have to do research to figure out how to clean it up--just call IDisposable.Dispose; (2) a class which owns IDisposable objects throughout its lifetime shouldn't generally have to worry...
@白ジェームス: ...how it's going to find out when consumers are done with it, because whoever created it should either call IDisposable.Dispose, or hand off responsibility to some other object whose contract specifies that IDisposable.Dispose will get called. Of course, it's possible that after an entity hands off disposal responsibility, the entity that was given the responsibility will abandon the object without calling Dispose, but if that happens, it is the latter entity, not the former, which is broken.

Jon Senchyna · Accepted Answer · 2012-06-08 21:23:51Z

You will probably want to implement IDisposable, in the event that you (or another developer) use your FileManager class and forget to close it. Note how the example for IDisposable puts a call to Dispose(false) in the finalizer.

Ivo · Accepted Answer · 2012-06-08 21:23:55Z

Why are you passing the path to the close method? In your case, it seems your manager can open different files if they are close before opening another one, so you won't want to dispose that object.

IMHO, I would prefer to implement it this way:

public class FileManager : IDisposable { private string path; private FileStream fileStream; public FileManager(string path) { this.path = path; } public void OpenFile() { this.fileStream = File.Open(path, FileMode.Open, FileAccess.Read); } public void CloseFile() { if ( this.fileStream != null && this.fileStream.CanRead) { this.fileStream.Close(); this.fileStream.Dispose(); } } public void Dispose(){ this.CloseFile(); } } // client var manager = new FileManager("path")){ manager.OpenFile(); //Do other stuff manager.CloseFile()

or

using( var manager = new FileManager("path")){ manager.OpenFile(); //Do other stuff }

This is a little sample I wrote in about 5 minutes to help with my question. I don't plan on using this.
The answer, as usual, is "it depends". So, for the code you've provided, this is my answer. If you have another code, I can help you with the design to see if it will be good to implement IDisposable or not. There's no black and white about it. Sometimes it will make sense to implement IDisposable and sometimes it won't.

Dan Puzey · Accepted Answer · 2012-06-08 21:24:41Z

In the case that you call your Close method, you don't need to separately dispose. However, general practise in this case would be to implement IDisposable because there's no guarantee that a consumer of the class would call Close.

You can only reliably leave out IDisposable if a resource is created and then disposed within the same method, because that's the only way you can be sure that the resource is definitely disposed of after use.

stmax · Accepted Answer · 2012-06-08 21:31:10Z

You should implement IDisposable.

Imagine:

var manager = new FileManager(); manager.Open("path"); // <- throws for some reason manager.Close(); // <- then this never gets called

Of course you could now put a try/finally around it like that:

try { var manager = new FileManager(); manager.Open("path"); } finally { manager.Close(); }

... but that's exactly what using and IDisposable have been invented for, with which you can comfy write:

using (var manager = new Manager()) { manager.OpenFile("path"); } // and CloseFile will automagically be called here.

+1 it was not my intention with the example code to handle what-if scenarios, but good point nonetheless.
@白ジェームス error handling is totally about what-if scenarios.. You always have to assume that at some point while or after opening a resource an exception could be thrown, and you have to provide a way for the user to gracefully handle it. You can either "make him" do that with the try/finally, or allow him to use a "using" by implementing IDisposable. People will thank you. :)
I think I'm trying to say this is example code and only used to help illustrate my question. In the example code, .Close is being called and there are no exceptions :).

Tigran · Accepted Answer · 2012-06-08 21:22:41Z

Don't see any real benefits in implementing IDisposable here, if not declarative ones. If someone sees your class implements IDisposable he understands that there are somewhere resources that have to be cleaned up after use. It's just a built-in .net convension of declaring types like this.

If you don't use that pattern, you are free to do that , but you violate suggested and mainly followed by community guidelunes for .net type declaration.

I wouldn't implement IDisposable only because it's easier to use with nested using, but for the reasons I explained.

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