Getting Error in No valid exports were found that match the constraint

Question

While running the following Program i am getting the

The composition remains unchanged. The changes were rejected because of the following error(s): The composition produced a single composition error. The root cause is provided below. Review the CompositionException.Errors property for more detailed information.

1) No valid exports were found that match the constraint '((exportDefinition.ContractName == "System.Collections.Generic.IEnumerable(MEF.IToolWindow)") AndAlso (exportDefinition.Metadata.ContainsKey("ExportTypeIdentity") AndAlso "System.Collections.Generic.IEnumerable(MEF.IToolWindow)".Equals(exportDefinition.Metadata.get_Item("ExportTypeIdentity"))))', invalid exports may have been rejected.

Resulting in: Cannot set import 'MEF.Program+Application.ToolWindows (ContractName="System.Collections.Generic.IEnumerable(MEF.IToolWindow)")' on part 'MEF.Program+Application'. Element: MEF.Program+Application.ToolWindows (ContractName="System.Collections.Generic.IEnumerable(MEF.IToolWindow)") --> MEF.Program+Application

Code is here

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ComponentModel.Composition;
using System.ComponentModel.Composition.Primitives;
using System.ComponentModel.Composition.Hosting;
using System.Collections;

//Without MEF
namespace MEF
{
    public interface IToolWindow
    {
        string Name { get; set; }
    }
    public interface IMenuService
    {
        IMenu GetMenu(string menuName);
    }

    public interface IMenuItem
    {
        string Name { get; set; }
        string Title { get; set; }
        Action Handler { get; set; }
    }

    public interface IMenu
    {
        string Name { get; set; }
        string Title { get; set; }
        string ToolWindow { get; set; }
        IEnumerable<IMenuItem> Items { get; set; }
        IMenuItem GetItem(string itemName);
    }

    //public interface IMenuItem<> :IMenuItem{}
class Program
{
    static void Main(string[] args)
    {
        var container = new CompositionContainer();
        var compositionBatch = new CompositionBatch();

        var someToolWindow = new SomeToolWindow();
        var application = new Application();
        var menuService = new MenuService();

        compositionBatch.AddPart(application);
        compositionBatch.AddPart(someToolWindow);
        compositionBatch.AddPart(menuService);


        container.Compose(compositionBatch);

        //application.ToolWindows = new List<IToolWindow>{someToolWindow};
        //someToolWindow.MenuService = menuService;


        foreach(var window in application.ToolWindows)
            Console.WriteLine(window.Name);
        Console.WriteLine(someToolWindow.MenuService);
        Console.ReadLine();
    }



    public class Application
    {
        [Import]
        public IEnumerable<IToolWindow> ToolWindows{get;set;}

        public IToolWindow GetWindow(string windowName)
        {
            return ToolWindows.Where(w =>w.Name == windowName).FirstOrDefault();
        }
    }

    [Export(typeof(IToolWindow))]
    public class SomeToolWindow : IToolWindow
    {
        [Import]
        public IMenuService MenuService { get; set; }
        public string Name { get; set; }
        public string Title { get; set; }

    }

    [Export(typeof(IMenuService))]
    public class MenuService:IMenuService
    {
        public IEnumerable<IMenu> Menus {get;set;}
        public IMenu  GetMenu(string menuName)
        {
            return Menus.Where(m => m.Name == menuName).FirstOrDefault();
        }
    }
}

}

Any one have a idea?

Acceleration

Answer 1

You need to change the [Import] to [ImportMany] if you want to populate a list. I tested the code below and it works.

   public class Application
   {
       [ImportMany]
       public IEnumerable<IToolWindow> ToolWindows { get; set; }

       public IToolWindow GetWindow(string windowName)
       {
           return ToolWindows.Where(w => w.Name == windowName).FirstOrDefault();
       }
   }

Also you will need to add a name to SomeToolWindow if you want it to print it.

Subdeacon