从另一个组件创建确定类对象的 [英] Decide class of object to be created from another assembly

问题描述

我有一个公开一个公共类（命名为A）要创建使用反射（Assembly.CreateInstance）其他程序.NET程序集。现在，它工作正常。

I have a .net assembly that exposes a public class (named A) to be created other programs using reflection (Assembly.CreateInstance). For now, it works fine.

但现在我有两个子类，比如A1和A2，而我需要创建一个或另一个，这取决于运行时检查。客户不能修改，应该相信返回的对象是A级的。

But now I have two subclasses of A, say A1 and A2, and I need to create one or the other, depending on a runtime check. The client cannot be modified, it should believe that the object returned is of class A.

如果我是使用COM而不是.NET，这将是很容易：只需将支票入IClassFactory.CreateInstance和完成。但是，有没有办法做到这一点。NET中？

If I were using COM instead of .net, it would be easy enough: simply add the check into the IClassFactory.CreateInstance and be done. But is there a way to do this in .net?

请注意：使用代理对象的明显的解决方案，或类似的，是不实际的，因为A类有几百个方法，我不想重新路由所有的人

Note: the obvious solution of using a proxy object, or similar, is not practical, because class A has several hundred methods (!) and I don't want to reroute all of them.

推荐答案

首先，应该指出，这将是更容易，如果您的客户端可以使用您提供的，而不是劫持工厂方法 Activator.CreateInstance 。请考虑前充分阅读...

Firstly, it should be noted that it would be easier if your client could use a factory method that you provide rather than having to hijack Activator.CreateInstance. Please consider that fully before reading on...

下面是一个使用 ContextBoundObject 的例子 - 它使用一个柜台的运行时检查 B 之间决定 C 为实际的对象，即使来电询问一个 A 。请注意，这也适用，如果他们使用新的A（）。

Here's an example using ContextBoundObject - it uses a counter as the "runtime check" to decide between B and C as the actual object, even though the caller asked for an A. Note that this also works if they used new A().

private static void Main()
{
    // subvert Activator.CreateInstance
    for(int i = 0 ; i < 100 ; i++)
    {
        object obj = Activator.CreateInstance(typeof (A));
        Console.WriteLine(obj.GetType().Name);
    }
    // subvert "new()"
    for(int i = 0 ; i < 100 ; i++)
    {
        object obj = new A();
        Console.WriteLine(obj.GetType().Name);
    }
}

class AProxyAttribute : ProxyAttribute
{
    int flipper;
    public override MarshalByRefObject CreateInstance(Type serverType)
    {
        if (serverType == typeof(A))
        {
            return ((flipper++)%2) == 0 ? (A) new B() : (A) new C();
        }
        return base.CreateInstance(serverType);
    }
}
[AProxy]
class A : ContextBoundObject { }
// the two subclasses
class B : A {}
class C : A{}

所以，是的，你的可以的是卑鄙的，谎言给调用者，让他们回去的东西除了他们要求; p这是相当邪恶code，虽然。因人而异等。

So yes, you can be dastardly and lie to the caller by giving them back something other than they asked for ;p This is fairly evil code, though. YMMV etc.

这篇关于从另一个组件创建确定类对象的的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！