Question

Consider the following example:

public interface ITask
{
    void Execute();
}

public class LoggingTaskRunner : ITask
{
    private readonly ITask _taskToDecorate;
    private readonly MessageBuffer _messageBuffer;

    public LoggingTaskRunner(ITask taskToDecorate, MessageBuffer messageBuffer)
    {
        _taskToDecorate = taskToDecorate;
        _messageBuffer = messageBuffer;
    }

    public void Execute()
    {
        _taskToDecorate.Execute();
        Log(_messageBuffer);
    }

    private void Log(MessageBuffer messageBuffer)
    {}
}

public class TaskRunner : ITask
{
    public TaskRunner(MessageBuffer messageBuffer)
    {

    }

    public void Execute()
    {
    }
}

public class MessageBuffer
{

}


public class Configuration
{
    public void Configure()
    {
        IWindsorContainer container = null;

        container.Register(
            Component.For<MessageBuffer>()
                .LifeStyle.Transient);

        container.Register(
            Component.For<ITask>()
                .ImplementedBy<LoggingTaskRunner>()
                .ServiceOverrides(ServiceOverride.ForKey("taskToDecorate").Eq("task.to.decorate")));

        container.Register(
            Component.For<ITask>()
            .ImplementedBy<TaskRunner>()
            .Named("task.to.decorate"));

    }

}

How can I make Windsor instantiate the “shared” transient component so that both “Decorator” and “Decorated” gets the same instance?

Edit: since the design is being critiqued I am posting something closer to what is being done in the app. Maybe someone can suggest a better solution (if sharing the transient resource between a logger and the true task is considered a bad design)

Edit2: Castle3 has added support for this (http://docs.castleproject.org/Windsor.Whats-New-In-Windsor-3.ashx) by introducing the “Bound” lifestyle

Answer 1

‘Transient’ explicitly means ‘non-shared’, so what you are asking is conceptually the wrong thing to do. The correct solution is to register Shared as a Singleton instead of Transient:

container.Register(Component.For<Shared>());

(Singleton is the default lifetime in Windsor.)

However, I suspect that behind the stated question lies a much more complex problem. I’m guessing that you need Shared to be Transient because you need it with this lifestyle for a lot of other cases, but exactly when it comes to the relationship between Decorator and Decorated you need to share them.

I still think this sounds like a Design Smell, but there are at least two ways you can achieve this result.

The first option involves prematurely resolving Shared and explicitly supply the resolved instance to the configuration of the two IFoo registrations:

container.Register(Component.For<Shared>().LifeStyle.Transient);

var r = container.Resolve<Shared>();

container.Register(Component
    .For<IFoo>()
    .ImplementedBy<Decorator>()
    .DependsOn(new { resource = r }));
container.Register(Component
    .For<IFoo>()
    .ImplementedBy<Decorated>()
    .DependsOn(new { resource = r }));

The second option is to make a specialized, named registration for Shared that is used only by the IFoo registrations:

container.Register(Component.For<Shared>().LifeStyle.Transient);
container.Register(Component.For<Shared>().Named("shared"));
container.Register(Component
    .For<IFoo>()
    .ImplementedBy<Decorator>()
    .ServiceOverrides(new { resource = "shared" }));
container.Register(Component
    .For<IFoo>()
    .ImplementedBy<Decorated>()
    .ServiceOverrides(new { resource = "shared" }));