c# Generic method call to static function

Question

I have a generic method in a class as follows

    private static Dictionary<Type, Feed> _singletons = new Dictionary<Type, Feed>();

    public static Feed GetFeed<T>() where T:Feed
    {    
        lock(_padlock)
        {
            if (!_singletons.ContainsKey(typeof(T))
            {                   
                _singletons[typeof(T)] = typeof(T).GetInstance();
            }
            return _singletons[typeof(T)];          
        }
    }

Here, Feed is an interface and Type is of types of classes that implement the Feed interface. GetInstance() is a static method in these classes. Is there something wrong with typeof(T).GetInstance();? It says System.Type does not contain a definition for GetInstance().

Because typeof() returns a Type object. The Type class doesn't have such a method. — BoltClock
– BoltClock, Commented Dec 22, 2010 at 5:24
Yes. That part I get. So what is the alternative to this call? — Aks
– Aks, Commented Dec 22, 2010 at 5:26
The typeof(T) operator returns an instance of System.Type, not an instance of T. Now with that being said, I think you should really consider re-writing whatever you're trying to do. Singleton is a very nasty anti-pattern, and combined with reflection magic, seems like you're setting yourself up for an application of questionable maintainability. Ask yourself what advantage you get with your type dictionary: why would a client write var x = Feeds.GetFeed<MyFeedImpl>() instead of var x = new MyFeedImpl()? — Juliet
– Juliet, Commented Dec 22, 2010 at 5:32
Coolness :) I would still recommend making your factory method non-static, and returning new instances of your classes rather than evil singletons. Factories are meant to put a layer of indirection between an object instance and its constructor, so that the object which gets created can vary independently of whoever uses the factory (i.e. like a LogFactory class that returns a FileLogger or DbLogger based on configuration). Static factory methods prevent reduce testability, and specifying the concrete type returned from factory seems weird since Feeds.GetFeed<X>() == X.GetInstance(). — Juliet
– Juliet, Commented Dec 22, 2010 at 6:00

Igor Zevaka · Accepted Answer · 2010-12-22 05:34:22Z

2

You can use Reflection to call a static method like so:

private static Dictionary<Type, Feed> _singletons = new Dictionary<Type, Feed>();

public static Feed GetFeed<T>() where T:Feed
{    
    lock(_padlock)
    {
        if (!_singletons.ContainsKey(typeof(T))
        {                   
            return typeof(T).GetMethod("GetInstance", System.Reflection.BindingFlags.Static).Invoke(null,null);

        }
        return _singletons[typeof(T)];          
    }
}

answered Dec 22, 2010 at 5:34

Igor Zevaka

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1 Comment

J.D. Over a year ago

You'll need BindingFlags.Public as well.

Albin Sunnanbo · Accepted Answer · 2010-12-22 05:30:56Z

2

The simplest way is to use the new constraint

private static Dictionary<Type, Feed> _singletons = new Dictionary<Type, Feed>();

public static Feed GetFeed<T>() where T:Feed, new()
{    
    lock(_padlock)
    {
        if (!_singletons.ContainsKey(typeof(T))
        {                   
            _singletons[typeof(T)] = new T();
        }
        return _singletons[typeof(T)];          
    }
}

answered Dec 22, 2010 at 5:30

Albin Sunnanbo

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