在没有找到与约束匹配的有效导出时出错答案

【问题标题】：Getting Error in No valid exports were found that match the constraint在没有找到与约束匹配的有效导出时出错
【发布时间】：2013-08-01 03:04:03
【问题描述】：

在运行以下程序时，我得到了

构图保持不变。由于以下错误，更改被拒绝：合成产生了单个合成错误。下面提供了根本原因。查看 CompositionException.Errors 属性以获取更多详细信息。

1) 找不到与约束匹配的有效导出 '((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"))))'，无效的导出可能已被拒绝。

导致：无法在部件“MEF.Program+Application”上设置导入“MEF.Program+Application.ToolWindows (ContractName="System.Collections.Generic.IEnumerable(MEF.IToolWindow)”)'。元素：MEF.Program+Application.ToolWindows (ContractName="System.Collections.Generic.IEnumerable(MEF.IToolWindow)") --> MEF.Program+Application

代码在这里

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();
        }
    }
}

}

有人有想法吗？

【问题讨论】：

标签： .net c#-4.0 mef

【解决方案1】：

如果要填充列表，则需要将 [Import] 更改为 [ImportMany]。我测试了下面的代码，它可以工作。

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

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

如果你想打印它，你还需要给 SomeToolWindow 添加一个名字。

【讨论】：