C＃Singleton模式和MEF [英] C# Singleton Pattern and MEF

问题描述

我有一个关于Singleton模式和MEF的问题。我在MEF实施新的插件，我还没有找到答案。

I have a question about the Singleton Pattern and MEF. I'm new in implementing MEF Plugins and I haven't found an answer.

是否有可能通过MEF实现插件只提供一个类的实例？

Is it possible to provide only one instance of a class through a MEF implemented Plugin?

我的老班是这样的：

  #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

有人可以提供我一个例如？

Can someone provide me an example?

推荐答案

是的，它是可以这样做。

Yes it is possible to do so.

在默认情况下，MEF将始终返回一个类的同一个实例时，它填补你的进口。所以，在技术上，如果你希望它是一个单身，你不必做任何事情。这是MEF调用共享创建策略。

By default, MEF will always return the same instance of a class when it fill your imports. So technically you do not have to do anything if you want it to be a singleton. This is what MEF calls a Shared Creation Policy.

如果你不想让你的进口来自于同一个实例，则需要指定它，无论是在你的属性也同样：

If you do not want your imports to come from the same instance, you need to specify it, either in your attributes likewise:

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

或者，您可以覆盖自己CompositionContainer中，这样会默认创建非共享的实例。

Or you can override your own CompositionContainer so that will create NonShared instances by default.

请注意，您还可以显式地指定要共享创建策略（单身）：

Note that you can also explicitly specify that you want a Shared Creation Policy (singletons):

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

但它不是必需的，因为共享（单） ，已经是默认值

But it is not necessary as Shared (singleton) is already the default value.

下面是MEF的文档的链接： http://mef.codeplex.com/wikipage?title=Parts%20Lifetime 这也解释了我刚才讲的。您也可以通过搜索找到关于这一主题的博客：MEF创建策略

Here is a link to MEF's documentation : http://mef.codeplex.com/wikipage?title=Parts%20Lifetime which explains what I just talked about. You can also find blogs on the subject by searching for : "MEF Creation Policy".

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