Can any of existing IoC containers create the lazy

问题:

I study different DI patterns. And now I interested in the lazy life-time implementation. For example, I want to write a proxy class that hides the factory behind a service's interface. Can any of existing IoC containers (.NET) create this kind of proxy class dynamically at runtime?

interface IService
{
    void Foo();
    void Bar();
}

class ServiceFactoryProxy : IService
{
    private readonly Func<IService> _factory;

    public ServiceFactoryProxy(Func<IService> factory)
    {
        if (factory == null) throw new ArgumentNullException("factory");
        _factory = factory;
    }

    public void Foo()
    {
        _factory().Foo();
    }

    public void Bar()
    {
        _factory().Foo();
    }
}

回答1:

Here is how it can be done in Castle:

http://litemedia.info/lazy-loading-property-with-castle-dynamicproxy2

回答2:

Unity does this out of the box. You declare a dependency of type Func<IService> in your constructor (like you did with ServiceFactoryProxy) and tell Unity how it should resolve IService. The container will generate the delegate for you.

---

Update

That proxy looks like an encapsulation of Lazy<T>.

If that is your intention you might want to have a look at this extension for Unity.

As far as I know Unity 3.0 will have support for .NET4 so I expect something similar to turn up as part of Unity with the next release.

---

Update2

It took me a while but I finished the lazy proxy feature. It generates IL code that looks like this

public class MyContract_LazyInstantiationProxy : IMyContract
{
  private readonly Lazy<IMyContract> instance;
  public MyContract_LazyInstantiationProxy(Func<IMyContract> factory)
  {
    Guard.AssertNotNull(factory, "factory");
    this.instance = new Lazy<IMyContract>(factory);
  }
  public IMyContract Instance
  {
    get { return this.instance.Value; }
  }
  public string Foo(Bar bar)
  {
    return this.Instance.Foo(bar);
  }
}

So you get the performance of hand-coded lazy proxies but they will be generated dynamically for you.

Have a look at the TecX project on codeplex. The lazy proxy generation can be found in the source code of TecX.Unity.Proxies. A sample on how to use it is in the according test project TecX.Unity.Proxies.Test.