How to dynamically decorate a class?

Question

I've got a simple decorator class like the following. Notice how my public methods all create a new instance of the to-be-decorated class, then forward the call to that instance.

The problem with this class is that whenever IMyService gets updated, I have to update this proxy class too.

public class MyProxyService : IMyService
{
    readonly IMyServiceFactory _realServiceFactory;

    public MyProxyService(IMyServiceFactory realServiceFactory)
    {
        _realServiceFactory = realServiceFactory;
    }

    private IMyService CreateRealService()
    {
        return _realServiceFactory.CreateRealService();
    }

    public int A()
    {
        return CreateRealService().A();
    }

    public int B(int b1)
    {
        return CreateRealService().B(int b1);
    }

    public int C(int c1, int c2)
    {
        return CreateRealService().C(c1,c2);
    }

    public int D(int d1, int d2, int d3)
    {
        return CreateRealService().D(d1,d2,d3);
    }
    public void E()
    {
        CreateRealService().E();
    }
}

I've tried creating a dynamic version using Castle.DynamicProxy, without any luck so far.

Anyone know a good, simple way to dynamically create a decorator like this?

Answer 1

I was able to get this to work using DynamicProxy.ProxyGenerator's CreateInterfaceProxyWithTargetInterface(..) .

I first created a dynamic proxy factory. This returns a proxy object, for which each method will be intercepted by the provided IInterceptor:

public class MyDynamicallyDecoratedServiceClientFactory
{
    readonly ProxyGenerator _proxyGenerator;
    readonly IInterceptor _interceptor;
    public MyServiceClientFactory(IInterceptor interceptor)
    {
        _interceptor = interceptor;
        _proxyGenerator = new ProxyGenerator();
    }
    public IMyService Create()
    {
        IMyService proxy = _proxyGenerator.CreateInterfaceProxyWithTargetInterface<IMyService>(null, _interceptor);
        return proxy;
    }
}

I then implemented the interceptor. Upon each method call, this interceptor will be called, which will create a new IMyService from the provided IMyServiceFactory, and delegate the method call to that new instance.

public class MyServiceClientInterceptor : IInterceptor
{
    readonly IMyServiceFactory _svcFactory;
    public MyServiceClientInterceptor(IMyServiceFactory svcFactory)
    {
        _svcFactory = svcFactory;
    }
    public void Intercept(IInvocation invocation)
    {
        IMyService realService = _svcFactory.Create();
        IChangeProxyTarget changeProxyTarget = invocation as IChangeProxyTarget;
        changeProxyTarget.ChangeInvocationTarget(realService);
        invocation.Proceed();
    }
}

Finally, to make use of all this:

// Create a service factory (the to-be-decorated class)
IMyServiceFactory myRealServiceFactory = /* ... */;

// Create a factory that will create decorated services
MyServiceClientInterceptor interceptor = 
        new MyServiceClientInterceptor(myRealServiceFactory);
MyDynamicallyDecoratedServiceClientFactory svcFactory =  
        new MyDynamicallyDecoratedServiceClientFactory(interceptor);

// Create a service client
IMyService svc = svcFactory.Create();

// Use it!
svcProxy.A();