Auto-generate NullObjects

NullObjects are a very usefull pattern. Among some other things they can be used as a means of defensive programming. If you have a dependency of a certain type (say ILogger) and don’t want make it an optional dependency using property injection (with the added downside of having to check for null every time you access that property) you can use a NullObject (in this case a NullLogger) to ensure that an instance of your interface is always present. Better prevent errors in the first place than telling people what they did wrong afterwards.

But do you really want to spend the time to implement classes that do … nothing?!

I consider DI containers (and infrastructure in general) a great tool to relieve programmers of the burden of routine tasks. Maybe that’s because I’m lazy and don’t want to do the same work twice.

A while ago Oren Eini announced a challenge for a Null Object Dependency Facility for Castle Windsor. I don’t know wether that challenge ever resulted in any actual code but I think its fun to solve that problem for Unity.

After some fiddling around with Reflection.Emit (again…) I am proud to announce that Unity can automatically generate and inject implementations even for trickier interfaces (like TryGetValue with out parameters). The generated code will never return null for strings or collections but empty strings and collections instead.

The functionality is accessible via an extension method for IUnityContainer.

public static IUnityContainer RegisterNullObject(this IUnityContainer container,
    Type contract, string name, LifetimeManager lifetime, 
    params InjectionMember[] injectionMembers)

Get the source code here (project TecX.Unity folder Proxies and the test suite that shows how to use it in TecX.Unity.Proxies.Test).

Generate lazy proxies on the fly

Imagine you have a service described by a contract IMyContract and an implementation thereof called MyService. That service is needed at several places throughout your application. The instantiation of MyService is expensive (e.g. takes a long time or consumes a lot of resources). In addition, that service is not needed all of the time (e.g. a message handler that calls this service or another depending on the content of a message).

If you apply the DI pattern you could inject a factory into your service consumers. That factory will only be called when a service instance is really needed. But wait! That is a leaky abstraction! You know that your implementation has a certain behavior and thus you change the way your application uses it. Another implementation of IMyContract might not show that behavior. And so you can’t interchange those implementations at will.

One solution would be to use the interception features of Unity or DynamicProxy to achieve some kind of lazy instantiation. But that is neither pretty nor fast.

Another approach would be to hand-craft (or generate using T4-templates) those proxies. Manual coding means a lot of maintenance effort. Templates would still involve some interaction for regenerating the proxies when the interfaces change.

A solution that comes with the performance of hand-crafted and compiled code and the convenience of auto-generation by the container are proxies that are dynamically generated using Reflection.Emit.

A code generator creates classes that are wrappers around a Lazy<T> field and look something like this:

public class MyContract_LazyInstantiationProxy : IMyContract
{
  private 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(IBar bar)
  {
    return this.Instance.Foo(bar);
  }
}

And it’s really easy to configure using the provided extension method for IUnityContainer:

var container = new UnityContainer();
container.RegisterLazyProxy(
  x =>
    {
      x.Contract = typeof(IFooService);
      x.ServiceImplementation = typeof(FooService);
    });

Now you can just inject your service into the consumers and don’t have to care wether or not they are lazily instantiated. No leaky abstraction, no factory overload. Just plain constructor injection. Nice!

Grab the source code for the proxy generation here (project TecX.Unity folder Proxies and the test suite that shows how to use it in TecX.Unity.Proxies.Test).