This is what I understand about IDisposable and finalizers from "CLR via C#", "Effective C#" and other resources:
While I understand the reasoning for and agree with all of the above, there is one scenario where I think it makes sense to break these rules: a singleton class that is responsible for unmanaged resources (such as providing a single point of access to particular files).
I believe it is always wrong to have a Dispose() method on a singleton because the singleton instance should live for the life of the application and if any client code calls Dispose() then you are stuffed. However, you want a finalizer so that when the application is unloaded the finalizer can clean up the unmanaged resources.
So having a singleton class with a finalizer that does not implement IDisposable seems to me to be a reasonable thing to do, yet this type of design is counter to what I understand are best practices.
Is this a reasonable approach? If not, why not and what are the superior alternatives?
If you find yourself wanting to Dispose them then you probably need to refactor your solution a bit. If the problem is around the freeing of resources during an AppDomain unload then put the responsibility of freeing the resource with the same object responsible for managing the life time of the AppDomain .
If your class creates unmanaged resources, then you can implement IDisposable so that these resources will be cleaned up properly when the object is disposed of. You override Dispose and release them there.
It is used where only a single instance of a class is required to control the action throughout the execution. A singleton class shouldn't have multiple instances in any case and at any cost. Singleton classes are used for logging, driver objects, caching and thread pool, database connections.
A singleton is a class that allows only a single instance of itself to be created and gives access to that created instance. It contains static variables that can accommodate unique and private instances of itself.
I'd first mention that Object Oriented design patterns and their consequences do not always influence every language decision, even in Object Oriented languages. You can certainly find classic design patterns that are easier to implement in one language (Smalltalk) as opposed to another (C++).
That being said, I'm not sure I agree with the premise that a singleton instance should only be disposed at the end of an application. Nothing in the design pattern descriptions that I've read for Singleton (or Design Patterns: Elements of reusable Object-Oriented Software) mention this as a property of this pattern. A singleton should ensure that only one instance of the class exists at any one moment in time; that does not imply that it must exist for as long as the application exists.
I have a feeling that in practice, many singletons do exist for most of the life of an application. However, consider an application that uses a TCP connection to communicate with a server, but can also exist in a disconnected mode. When connected, you would want a singleton to maintain the connection information and state of the connection. Once disconnected, you may want to keep that same singleton - or you may dispose of the singleton. While some may argue that it makes more sense to keep the singleton (and I may even be among them), there is nothing in the design pattern itself that precludes you from disposing of it - if a connection is remade, the singleton can be instantiated again, as no instance of it exists at that moment in time.
In other words, you can create scenarios where it is logical for singletons to have IDisposable.
If the unmanaged resource is released only on application exit you don't even need to worry with a finalizer since the process unload should deal with this for you anyway.
If you have multiple app domains and you want to deal with an app domain unload that's one possible issue but possibly one you don't need to care about.
I second those saying that this design is possibly not the right thing to do (and will make it harder to fix is subsequently you find that you do actually need two instances) Create the object (or a lazy loading wrapper object) in your entry point and pass it through the code to where it is needed making it clear who is responsible for providing it to whom, then you are free to change the decision to use only one with little effect to the rest of the code (which uses what it gets given)
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