How and when are c# Static members disposed?

Question

I have a class with extensive static members, some of which keep references to managed and unmanaged objects.

For instance, the static constructor is called as soon as the Type is referenced, which causes my class to spin up a blockingQueue of Tasks. This happens when one of the static methods is called, for example.

I implemented IDisposable, which gives me methods to handle disposal on any instance objects I created. However, these methods are never called if the consumer doesn't create any instance objects from my class.

How and where do I put code to dispose of references maintained by the static portion of my class? I always thought that disposal of static-referenced resources happened when the last instance object was released; this is the first time I've ever created a class where no instances may ever be created.

Answer 1

The static variable of your class are not garbage collected until the app domain hosting your class is unloaded. The Dispose() method will not be called, because it is an instance method, and you said that you wouldn't create any instances of your class.

If you would like to make use of the Dispose() method, make your object a singleton, create one instance of it, and dispose of it explicitly when your application is about to exit.

public class MyClass : IDisposable {
    public IList List1<int> {get; private set;}
    public IDictionary<string,string> Dict1 {get; private set;}
    public void Dispose() {
        // Do something here
    }
    public static MyClass Instance {get; private set;}
    static MyClass() {
        Instance = new MyClass();
    }
    public static void DisposeInstance() {
        if (instance != null) {
            Instance.Dispose();
            Instance = null;
        }
    }
}

Answer 2

public class Logger : IDisposable
{

    private string _logDirectory = null;
    private static Logger _instance = null;

    private Logger() : this(ConfigurationManager.AppSettings["LogDirectory"])
    {
        AppDomain.CurrentDomain.ProcessExit += CurrentDomain_ProcessExit;
    }

    private Logger(string logDirectory) 
    {
    } 

    public static Logger Instance
    {
        get
        {
            if (_instance == null)
                _instance = new Logger();
            return _instance;
        }
    }

    private void CurrentDomain_ProcessExit(object sender, EventArgs e)
    {
        Dispose();
    }



    public void Dispose()
    {
        // Dispose unmanaged resources
    }
}

Answer 3

You should dispose this objects manually, there is no way to create a "finalizer" for static resources.

Answer 4

If you really want to have static members which keep references to unmanaged objects just create a method for disposing the unmanaged objects and "force" consumer to use it on exit.

By "force" I mean document your class with a paragraph that states "when" and "why" to use this "dispose" method. Do it either if you are the sole consumer (or your code...) or you plan to distribute your class. Also try to use a somehow descriptive name (to that "dispose" method) such as "DisposeStatics", "AlwaysDispose", "DisposeAtEnd" etc.