C# Singleton Pattern and MEF

Question

我有一个关于Singleton模式和MEF的问题。我是实施MEF插件的新手，我还没有找到答案。

是否可以通过MEF实现的插件只提供一个类的实例？

我以前的课是这样的：

  #region Singleton
  /// 
  /// This class provide a generic and thread-safe interface for Singleton classes.
  /// 
  /// The specialized singleton which is derived
  /// from SingletonBase<T>
  public abstract class Base where T : Base
  {
    /* the lock object */
    private static object _lock = new object();
    /* the static instance */
    private static T _instance = null;
    /// 
    /// Get the unique instance of .
    /// This property is thread-safe!
    /// 
    public static T Instance
    {
      get
      {
        if (_instance == null)
        {
          lock (_lock)
          {
            if (_instance == null)
            {
              /* Create a object without to use new (where you need a public ctor) */
              object obj = FormatterServices.GetUninitializedObject(typeof(T));
              if (obj != null)  // just 4 safety, but i think obj == null shouldn't be possible
              {
                /* an extra test of the correct type is redundant,
                 * because we have an uninitialised object of type == typeof(T) */
                _instance = obj as T;
                _instance.Init(); // now the singleton will be initialized
              }
            }
          }
        }
        else
        {
          _instance.Refresh();  // has only effect if overridden in sub class
        }
        return _instance;
      }
    }

    /// 
    /// Called while instantiation of singleton sub-class.
    /// This could be used to set some default stuff in the singleton.
    /// 
    protected virtual void Init()
    { }
    /// 
    /// If overridden this will called on every request of the Instance but
    /// the instance was already created. Refresh will not called during
    /// the first instantiation, for this will call Init.
    /// 
    protected virtual void Refresh()
    { }
  }
  #endregion
  #region class
  public class xy : Base
  {
    private bool run;
    public xy()
    {
      this.run = false;
    }
    public bool isRunning()
    {
      return this.run;
    }
    public void start()
    {
      // Do some stuff
      this.run = true;
    }
  }
  #endregion

有人能给我举个例子吗？

Answer 1

是的，可以这样做。

默认情况下，MEF在填充导入时将始终返回类的同一实例。所以从技术上讲，如果你想让它成为一个单例，你不必做任何事情。这就是MEF所说的共享创造政策。

如果你不希望你的导入来自同一个实例，你需要指定它，或者在你的属性中同样指定：

[Import(RequiredCreationPolicy = CreationPolicy.NonShared)]
public MyClass : IMyInterface

或者，您可以覆盖自己的CompositionContainer，以便在默认情况下创建NonShared实例。

请注意，您还可以明确指定您想要共享创建策略（singletons）：

[Import(RequiredCreationPolicy = CreationPolicy.Shared)]
public MyClass : IMyInterface
{
    public MyClass() { } // you can have a public ctor, no need to implement the singleton pattern 
}

但这并不是必须的，因为Shared（singleton）已经是默认值了。

以下是MEF文件的链接：http://mef.codeplex.com/wikipage?title=Parts%20Lifetime这就解释了我刚才所说的。你也可以通过搜索找到关于这个主题的博客："MEF创建政策"。

Answer 2

非常糟糕的做法！使用静态构造函数：保证只执行一次

请注意，对于您创建的每个T，CLR将复制新T的静态数据。

因此，您正在为所使用的每个T类型创建一个singleton。

单例的最佳方式是这样的：

public class Logger {
    public static readonly Logger Instace;
    static Logger() {
        Instace = new Logger();
    }
}