MEF Error message - VS2010

Question

Can anybody help me with explaining this error message please:

system.componentmodel.composition.changerejectedexception

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 exports were found that match the constraint: 
ContractName    Itok.BusinessLogic.Interfaces.IFolderService
RequiredTypeIdentity    Itok.BusinessLogic.Interfaces.IFolderService

Resulting in: Cannot set import 'Itok.Web.Photos.Presenters.DefaultPresenter._folderService (ContractName="Itok.BusinessLogic.Interfaces.IFolderService")' on part 'Itok.Web.Photos.Presenters.DefaultPresenter'.

Element: Itok.Web.Photos.Presenters.DefaultPresenter._folderService (ContractName="Itok.BusinessLogic.Interfaces.IFolderService") --> Itok.Web.Photos.Presenters.DefaultPresenter

Here is the IFolderService.cs:

using System;
using System.Collections.Generic;
using Itok.Entities;

namespace Itok.BusinessLogic.Interfaces
{
    public interface IFolderService
    {
        List<Folder> GetFriendsFolders(Int32 AccountID);
        void DeleteFolder(Folder folder);
        List<Folder> GetFoldersByAccountID(Int32 AccountID);
        Folder GetFolderByID(Int64 FolderID);
        Int64 SaveFolder(Folder folder);
    }
}

And this is the exporting class definition, FolderService.cs:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Itok.BusinessLogic.Interfaces;
using System.ComponentModel.Composition;
using Itok.DataAccess.Interfaces;
using Itok.Common;
using Itok.DataAccess;
using Itok.Interfaces;
using Itok.Entities;

namespace Itok.BusinessLogic
{    
    [Export(typeof(IFolderService))]    
    [Export(typeof(ICache))]
    public class FolderService : IFolderService
    {
        [Import]
        private IFriendRepository _friendRepository;
        [Import]
        private IFolderRepository _folderRepository;
        [Import]
        private ICache _cacheService;

        public FolderService()
        {
            MEFManager.Compose(this);
        }

        public List<Folder> GetFriendsFolders(Int32 AccountID)
        {
            List<Friend> friends = _friendRepository.GetFriendsByAccountID(AccountID);
            List<Folder> folders = _folderRepository.GetFriendsFolders(friends);
            folders.OrderBy(f => f.CreateDate).Reverse();
            return folders;
        }

         public void DeleteFolder(Folder folder)
        {   
            if (_cacheService.Exists(folder.AccountID.ToString()))
            {
                _cacheService.Delete(folder.AccountID.ToString());
            }

            _folderRepository.DeleteFolder(folder);
        }

        public List<Folder> GetFoldersByAccountID(int AccountID)
        {        
            List<Folder> cachedFolders = _cacheService.Get(AccountID.ToString()) as List<Folder>;
            if (cachedFolders != null)
            {
                return cachedFolders;
            }
            else
            {
                cachedFolders = _folderRepository.GetFoldersByAccountID(AccountID);
                _cacheService.Set(AccountID.ToString(), cachedFolders);
                return cachedFolders;
            }
        }

        public Folder GetFolderByID(Int64 FolderID)
        {
            return _folderRepository.GetFolderByID(FolderID);
        }

        public Int64 SaveFolder(Folder folder)
        {
            return _folderRepository.SaveFolder(folder);
        }
    }
}

I thank you prior to any help for saving my time.

Answer 1

The error message means that MEF is looking for a class that is exported with the interface IFolderService but there isn't one in the container.

To investigate this, firstly check that there is a class that exports that interface and if there is, then look into whether that class being picked up by the container or not and thirdly, if neither of those resolve the issue, look into whether the class that is exported with the interface IFolderService has some other imports that can't be satisfied.