我有关于Singleton模式和MEF的问题.我是新的实施MEF插件,我还没有找到答案.
通过MEF实现的插件,可以提供一个类的一个实例吗？
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
有人可以给我一个例子吗？
解决方法
是的,可以这样做.
默认情况下,当MEF填充导入时,MEF将始终返回类的同一个实例.所以在技术上你不需要做任何事情,如果你希望它是一个单身人士.这就是MEF所说的共享创建政策.
如果您不希望导入来自同一个实例,则需要在属性中指定它：
[Import(requiredCreationPolicy = CreationPolicy.NonShared)] public MyClass : IMyInterface
或者您可以覆盖自己的CompositionContainer,以便默认情况下创建NonShared实例.
请注意,您还可以明确指定您想要共享创建策略(单身)：
[Import(requiredCreationPolicy = CreationPolicy.Shared)] public MyClass : IMyInterface { public MyClass() { } // you can have a public ctor,no need to implement the singleton pattern }
但是,共享(单例)已经是默认值不是必需的.
这是MEF文档的链接：http://mef.codeplex.com/wikipage?title=Parts%20Lifetime,它解释了我刚刚谈到的内容.您还可以通过搜索：“MEF创建策略”来查找有关此主题的博客.
