c# - MEF 属性总是返回 Null

Question

原始源代码

我的 BusinessObjects.dll 文件中有一个简单的业务对象:

namespace BusinessObjects
{
    public class MyClass
    {
        public MyClass()
        {
            DateTime = DateTime.Now;
        }

        public DateTime DateTime { get; set; }
    }
}

在我的 SharedUI.dll 中，我有这个“上下文提供者”类，我用它来保存对当前选定的 MyClass 的引用——记住这是一个简单的例子:)...

namespace SharedUI
{
    public class AppContext
    {
        [Export]
        public MyClass SelectedMyClass { get; private set; }

        public void SetupContext(MyClass myClass)
        {
            SelectedMyClass = myClass;
        }

        public static AppContext Context
        {
            get
            {
                if (context == null)
                {
                    context = new AppContext();
                }
                return context;
            }
        }

        private static AppContext context;
    }
}

我的 MefTest.exe 有这个类:

namespace MefTest
{
    public class Program
    {
        [Import]
        public MyClass MyClass { get; set; }

        private void Compose()
        {
            var ventSystem = new MyClass();
            AppContext.Context.SetupContext(ventSystem);

            var executingAssembly = new AssemblyCatalog(Assembly.GetExecutingAssembly());
            var contextAssembly = new AssemblyCatalog(Assembly.LoadFile(string.Format(@"{0}\SharedUI.dll", Environment.CurrentDirectory)));
            var catalog = new AggregateCatalog(executingAssembly, contextAssembly);

            var container = new CompositionContainer(catalog);

            container.ComposeParts(this);
        }

        private void Run()
        {
            Compose();

            // MyClass is always null in the next line?
            Console.WriteLine(MyClass.DateTime.ToString());

            Console.ReadKey();
        }

        private static void Main(string[] args)
        {
            var p = new Program();
            p.Run();
        }
    }
}

我是 MEF 菜鸟所以请多多包涵 :)

根据 Daniel Plaisted 的建议更新了源代码

MyClass源码是一样的...

SharedUI.dll 现在看起来像这样:

namespace SharedUI
{
    [Export]
    public class AppContext
    {
        [Export(typeof(MyClass))]
        public MyClass SelectedMyClass { get; private set; }

        public void SetupContext(MyClass myClass)
        {
            SelectedMyClass = myClass;
        }
    }
}

MefTest.exe 现在看起来像这样:

namespace MefTest
{
    public class Program
    {
        [Import]
        public MyClass MyClass { get; set; }

        [Import]
        public AppContext AppContext { get; set; }

        private void Compose()
        {
            var executingAssembly = new AssemblyCatalog(Assembly.GetExecutingAssembly());
            var contextAssembly = new AssemblyCatalog(Assembly.LoadFile(string.Format(@"{0}\SharedUI.dll", Environment.CurrentDirectory)));
            var catalog = new AggregateCatalog(executingAssembly, contextAssembly);

            var container = new CompositionContainer(catalog);

            container.ComposeParts(this);

            var myClass = new MyClass();
            AppContext.SetupContext(myClass);
        }

        private void Run()
        {
            Compose();

            // AppContext.SelectedMyClass is NOT null in the next line... which is good I guess :)
            Console.WriteLine(AppContext.SelectedMyClass.DateTime.ToString());

            // MyClass is always null in the next line?
            Console.WriteLine(MyClass.DateTime.ToString());

            Console.ReadKey();
        }

        private static void Main(string[] args)
        {
            var p = new Program();
            p.Run();
        }
    }
}

我做错了什么，因为我无法让它工作？

Answer 1

当 MEF 需要获取类属性上的 Export 时，它将创建该类的实例并调用属性 getter。所以 MEF 正在创建一个新的 AppContext 实例，不同于静态的 AppContext.Context 实例。 MEF 创建的实例没有在其上设置 SelectedMyClass 属性，这就是您的导入最终为空的原因。