[英]C# Singleton Pattern and MEF
I have a question about the Singleton Pattern and MEF. 我对Singleton Pattern和MEF有疑问。 I'm new in implementing MEF Plugins and I haven't found an answer.
我是实施MEF插件的新手,我还没有找到答案。
Is it possible to provide only one instance of a class through a MEF implemented Plugin? 是否可以通过MEF实现的插件只提供一个类的实例?
My old class is something like this: 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
Can someone provide me an example? 有人可以给我一个例子吗?
Yes it is possible to do so. 是的,有可能这样做。
By default, MEF will always return the same instance of a class when it fill your imports. 默认情况下,MEF在填充导入时将始终返回相同的类实例。 So technically you do not have to do anything if you want it to be a singleton.
所以从技术上讲,如果你想让它成为一个单身人士,你就不需要做任何事情。 This is what MEF calls a Shared Creation Policy.
这就是MEF所谓的共享创建策略。
If you do not want your imports to come from the same instance, you need to specify it, either in your attributes likewise: 如果您不希望导入来自同一实例,则需要在属性中指定它:
[Import(RequiredCreationPolicy = CreationPolicy.NonShared)]
public MyClass : IMyInterface
Or you can override your own CompositionContainer so that will create NonShared instances by default. 或者您可以覆盖自己的CompositionContainer,以便默认情况下创建NonShared实例。
Note that you can also explicitly specify that you want a Shared Creation Policy (singletons): 请注意,您还可以明确指定您需要共享创建策略(单例):
[Import(RequiredCreationPolicy = CreationPolicy.Shared)]
public MyClass : IMyInterface
{
public MyClass() { } // you can have a public ctor, no need to implement the singleton pattern
}
But it is not necessary as Shared (singleton) is already the default value. 但是没有必要,因为共享(单例)已经是默认值。
Here is a link to MEF's documentation : http://mef.codeplex.com/wikipage?title=Parts%20Lifetime which explains what I just talked about. 以下是MEF文档的链接: http : //mef.codeplex.com/wikipage? title = Items% 20Lifetime ,它解释了我刚刚谈到的内容。 You can also find blogs on the subject by searching for : "MEF Creation Policy".
您还可以通过搜索“MEF创建策略”找到有关该主题的博客。
Very bad practice ! 非常糟糕的做法! use static constructor: guaranteed to be executed exactly once
使用静态构造函数:保证只执行一次
Please note that for each T
that you create, the CLR will copy the static data for the new T
. 请注意,对于您创建的每个
T
,CLR将复制新T
的静态数据。
So you are creating a singleton per T
type that you use. 因此,您要创建每个
T
类型的单例。
The best way for singleton is something like this: 单身人士的最佳方式是这样的:
public class Logger {
public static readonly Logger Instace;
static Logger() {
Instace = new Logger();
}
}
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