using System;
using System.ComponentModel.Composition;
using System.ComponentModel.Composition.Hosting;

namespace MefGenericExportImports
{
    [Export]
    public class Program
    {
        [Import]
        public GenericThing<int> IntGenericThing { get; set; }

        [Import]
        public GenericFuncThing<int> FuncIntGenericThingWorkaround { get; set; }

        static void Main(string[] args)
        {
            using (var catalog = new ApplicationCatalog())
            using (var container = new CompositionContainer(catalog))
            {
                container.GetExportedValue<Program>().Run(args);

                // But this fails: Generics can only go one level deep?
                Console.Error.WriteLine("The following exception is expected:");
                container.GetExportedValue<GenericThing<Func<int>>>();
            }
        }

        void Run(string[] args)
        {
            Console.WriteLine($"One very indirect way of getting default value of int: {IntGenericThing.Value}");
            Console.WriteLine($"Workaround (one way to get null!): {FuncIntGenericThingWorkaround.Value}");
        }
    }

    [Export]
    public class GenericThing<T>
    {
        public T Value { get; set; }
    }

    // Workaround is to have the demander just supply the arguments of Func<T> rather than
    // provide Func<T> as a type parameter.
    [Export]
    public class GenericFuncThing<TResult>
    {
        public Func<TResult> Value { get; set; }
    }
}