导入属性始终为空（MEF 导入问题）

Question

我尝试了一段时间使用 MEF 来完成工作，但现在遇到了一个问题，我需要帮助。

描述： 我有 2 个 DLL 和 1 个 EXE 文件。 ClassLibrary1（LoggerImpl.cs、SomeClass.cs） 类库2 (ILogger.cs) WindowsApplicationForms1（WindowsApplicaitonForms1.cs、Program.cs）

我需要任何帮助或指导，为什么这不起作用？

// ClassLibrary1.dll
//SomeClass.cs
 public class SomeClass
    {
        [Import("Logging", typeof(ILogger))]
        public ILogger Log { get; set; } <-- ALWAYS NULL ???

        public void Print()
        {
            Log.Print();
        }

    }

// ClassLibrary1.dll
// LoggerImpl.cs
namespace ClassLibrary1
{
    [Export("Logging", typeof (ILogger))]
    public class LoggerImpl : ILogger
    {
        public void Print()
        {
            Console.WriteLine("print called");
        }
    }
}

// ClassLibrary2.dll
// ILogger.cs
namespace LogNamespace
{
    public interface ILogger
    {
        void Print();
    }
}

// WindowsFormsApplication1.exe
// WindowsFormsApplication1.cs
namespace WindowsFormsApplication1
{
    [Export("Form1",typeof(Form1))]
    public partial class Form1 : Form
    {

        [Import("Logging", typeof(ILogger))]
        public ILogger Log { set; get; }

        private CompositionContainer _container;

        public Form1()
        {
            InitializeComponent();
            Compose();
            Log.Print();

            SomeClass c = new SomeClass();
            c.Print();
        }

        private void Compose()
        {
            var catalog = new AggregateCatalog();

            catalog.Catalogs.Add(new DirectoryCatalog("."));
            catalog.Catalogs.Add(new AssemblyCatalog(System.Reflection.Assembly.GetExecutingAssembly()));
            _container = new CompositionContainer(catalog);

            try
            {
                _container.ComposeParts(this);
            }
            catch (CompositionException compositionException)
            {
                MessageBox.Show(compositionException.ToString());
            }
        }
    }
}

Answer 1

如果您自己创建类的新实例（new SomeClass()），容器将不知道它的任何信息，也不会组合它。

对于由 MEF 组成的部件，它需要由 MEF 创建，或者显式传递给容器。您可以手动告诉 MEF 满足 SomeClass 对象的导入，就像您告诉它满足表单的导入一样：

SomeClass c = new SomeClass();
_container.SatisfyImports(c);
c.Print();

但是，您需要直接访问容器才能执行此操作，因此它在 Form1 类之外无法正常工作。一般来说，更好的方法是导出 SomeClass，并在 Form1 类中为 SomeClass 创建导入：

[Export]
public class SomeClass
{
    [Import("Logging", typeof(ILogger))]
    public ILogger Log { get; set; }

    // etc.
}

public partial class Form1 : Form
{
    [Import("Logging", typeof(ILogger))]
    public ILogger Log { set; get; }

    [Import]
    SomeClass _someClass { get; set; }

    // etc.
}

Answer 2

您需要类似于以下的语句才能将 SomeClass 纳入组合过程中

// ClassLibrary1.dll
//SomeClass.cs
using System;
using System.ComponentModel.Composition;
using System.ComponentModel.Composition.Hosting;
using System.Windows.Forms;
using LogNamespace;

public class SomeClass
{
    [Import("Logging", typeof(ILogger))]
    public ILogger Log { get; set; } //<-- ALWAYS NULL ???

    public SomeClass()
    {
        var catalog = new AggregateCatalog();
        CompositionContainer _container;

        // catalog.Catalogs.Add(new DirectoryCatalog("."));
        catalog.Catalogs.Add(new AssemblyCatalog(System.Reflection.Assembly.GetExecutingAssembly()));
        _container = new CompositionContainer(catalog);

        _container.ComposeParts(this);
    }

    public void Print()
    {
        Log.Print();
    }

}

// ClassLibrary1.dll
// LoggerImpl.cs
namespace ClassLibrary1
{
    [Export("Logging", typeof(ILogger))]
    public class LoggerImpl : ILogger
    {
        public void Print()
        {
            Console.WriteLine("print called");
        }
    }
}

// ClassLibrary2.dll
// ILogger.cs
namespace LogNamespace
{
    public interface ILogger
    {
        void Print();
    }
}

// WindowsFormsApplication1.exe
// WindowsFormsApplication1.cs
namespace WindowsFormsApplication1
{
    [Export("Form1", typeof(Form1))]
    public partial class Form1 : Form
    {

        [Import("Logging", typeof(ILogger))]
        public ILogger Log { set; get; }

        private CompositionContainer _container;

        public Form1()
        {
            InitializeComponent();
            Compose();
            Log.Print();

            SomeClass c = new SomeClass();
            c.Print();
        }

        private void Compose()
        {
            var catalog = new AggregateCatalog();

            // catalog.Catalogs.Add(new DirectoryCatalog("."));
            catalog.Catalogs.Add(new AssemblyCatalog(System.Reflection.Assembly.GetExecutingAssembly()));
            _container = new CompositionContainer(catalog);

            try
            {
                _container.ComposeParts(this);
            }
            catch (CompositionException compositionException)
            {
                MessageBox.Show(compositionException.ToString());
            }
        }
    }
}