面向方面的设计模式布谷鸟&x27；使用PostSharp在C#中添加鸡蛋

我正在尝试使用Postsharp在C#中实现AO设计模式布谷鸟蛋

这种设计模式的思想正如其名称所述，是用其他对象替换现有对象。在AspectJ中，它是通过以下方式完成的：

public aspect MyClassSwapper {
  public pointcut myConstructors(): call(MyClass.new());
  Object around(): myConstructors() {
      return new AnotherClass();
  }
}

public class AnotherClass extends MyClass {
    . . .
}

[Serializable]
[MulticastAttributeUsage(MulticastTargets.Method, Inheritance = MulticastInheritance.None)]
public class MyClassSwapperAttribute : MethodInterceptionAspect, IInstanceScopedAspect
{
    private readonly Type cuckooType;

    [NonSerialized] private object cuckooEgg;

    public MyClassSwapperAttribute(Type cuckooType)
    {
        this.cuckooType = cuckooType;
    }

    private MyClassSwapperAttribute()
    {
        // this creates an "empty" aspect instance applied on the cuckoo's egg itself
    }

    public object CreateInstance(AdviceArgs adviceArgs)
    {
        if (adviceArgs.Instance.GetType() == cuckooType)
            return new MyClassSwapperAttribute();

        return this.MemberwiseClone();
    }

    public void RuntimeInitializeInstance()
    {
        if (this.cuckooType == null)
            return;

        // for each instance of the target class we create a new cuckoo's egg instance
        this.cuckooEgg = Activator.CreateInstance(cuckooType);
    }

    public override void OnInvoke(MethodInterceptionArgs args)
    {
        if (this.cuckooEgg == null)
        {
            base.OnInvoke(args);
        }
        else
        {
            // delegate the method invocation to the cuckoo's egg, that derives from the target class
            args.ReturnValue = args.Method.Invoke(this.cuckooEgg, args.Arguments.ToArray());
        }
    }
}

[MyClassSwapper(typeof (AnotherClass))]
class MyClass
{
    // ...
}

我是Postsharp的新手，所以我想问一下，Postsharp中是否有能够执行类似操作的工具—重写基类构造函数的返回值并返回子类的对象？

虽然Postsharp允许您应用于构造函数，但您仍然无法替换返回的对象实例。事实上，C#中的构造函数不返回值。有相关的问题和建议

实现布谷鸟蛋模式的另一种方法是拦截原始类的方法，并在另一个类实例上调用适当的方法。在这种情况下，

另一个类

甚至不必从

MyClass

派生，或者您可以将不同的方法委托给不同的替换类

实现模式的示例实现可以如下所示：

public aspect MyClassSwapper {
  public pointcut myConstructors(): call(MyClass.new());
  Object around(): myConstructors() {
      return new AnotherClass();
  }
}

public class AnotherClass extends MyClass {
    . . .
}

[Serializable]
[MulticastAttributeUsage(MulticastTargets.Method, Inheritance = MulticastInheritance.None)]
public class MyClassSwapperAttribute : MethodInterceptionAspect, IInstanceScopedAspect
{
    private readonly Type cuckooType;

    [NonSerialized] private object cuckooEgg;

    public MyClassSwapperAttribute(Type cuckooType)
    {
        this.cuckooType = cuckooType;
    }

    private MyClassSwapperAttribute()
    {
        // this creates an "empty" aspect instance applied on the cuckoo's egg itself
    }

    public object CreateInstance(AdviceArgs adviceArgs)
    {
        if (adviceArgs.Instance.GetType() == cuckooType)
            return new MyClassSwapperAttribute();

        return this.MemberwiseClone();
    }

    public void RuntimeInitializeInstance()
    {
        if (this.cuckooType == null)
            return;

        // for each instance of the target class we create a new cuckoo's egg instance
        this.cuckooEgg = Activator.CreateInstance(cuckooType);
    }

    public override void OnInvoke(MethodInterceptionArgs args)
    {
        if (this.cuckooEgg == null)
        {
            base.OnInvoke(args);
        }
        else
        {
            // delegate the method invocation to the cuckoo's egg, that derives from the target class
            args.ReturnValue = args.Method.Invoke(this.cuckooEgg, args.Arguments.ToArray());
        }
    }
}

[MyClassSwapper(typeof (AnotherClass))]
class MyClass
{
    // ...
}

此实现需要从

MyClass

派生另一个类

MyClass

，并且必须在

MyClass

本身上应用该属性，例如：

public aspect MyClassSwapper {
  public pointcut myConstructors(): call(MyClass.new());
  Object around(): myConstructors() {
      return new AnotherClass();
  }
}

public class AnotherClass extends MyClass {
    . . .
}

[Serializable]
[MulticastAttributeUsage(MulticastTargets.Method, Inheritance = MulticastInheritance.None)]
public class MyClassSwapperAttribute : MethodInterceptionAspect, IInstanceScopedAspect
{
    private readonly Type cuckooType;

    [NonSerialized] private object cuckooEgg;

    public MyClassSwapperAttribute(Type cuckooType)
    {
        this.cuckooType = cuckooType;
    }

    private MyClassSwapperAttribute()
    {
        // this creates an "empty" aspect instance applied on the cuckoo's egg itself
    }

    public object CreateInstance(AdviceArgs adviceArgs)
    {
        if (adviceArgs.Instance.GetType() == cuckooType)
            return new MyClassSwapperAttribute();

        return this.MemberwiseClone();
    }

    public void RuntimeInitializeInstance()
    {
        if (this.cuckooType == null)
            return;

        // for each instance of the target class we create a new cuckoo's egg instance
        this.cuckooEgg = Activator.CreateInstance(cuckooType);
    }

    public override void OnInvoke(MethodInterceptionArgs args)
    {
        if (this.cuckooEgg == null)
        {
            base.OnInvoke(args);
        }
        else
        {
            // delegate the method invocation to the cuckoo's egg, that derives from the target class
            args.ReturnValue = args.Method.Invoke(this.cuckooEgg, args.Arguments.ToArray());
        }
    }
}

[MyClassSwapper(typeof (AnotherClass))]
class MyClass
{
    // ...
}