Ninject and Decorators

Question

Given:

public interface IBatchProcess
{
    void Run();
}

and multiple implementation of:

public class BatchProcessOne : IBatchProcess { ... }
public class BatchProcessTwo : IBatchProcess { ... }
public class BatchProcessThree : IBatchProcess { ... }

and a tracing decorator:

public class BatchProcessTraceDecorator : IBatchProcess
{
    private readonly IBatchProcess _inner;

    public BatchProcessTraceDecorator( IBatchProcess inner )
    {
        _inner = inner;
    }

    public void Run()
    {
        Trace.TraceInformation( "Starting batch process..." );
        _inner.Run();
        Trace.TraceInformation( "Batch process complete." );
    }
}

How can I bind the decorator and all of the implementations such that when I call kernel.GetAll I get 3 instances of the tracing decorator, each with a different inner batch process?

I know about Ninject Interception and do not want to use proxy based solutions for this for various reasons. At the moment it looks like I need to play around with the activation strategy for IBatchProcess instances such that they are resolved and then I can decorate and return them but I am hoping I have simply missed something in the binding api.

Answer 1

If you don't need to inject dependencies into the decorator there is a very simple solution to your problem. Simply use the OnActivation binding methods. That would look like

Bind<IBatchProcess>().To<ConcreteBatchProcess>()
                     .OnActivation((ctx, process) => 
                         new BatchProcessDecorator(process));

If you need to inject stuff into the decorator you can use the same trick but resolve the decorator by

Bind<IBatchProcess>().To<ConcreteBatchProcess>()
                     .OnActivation((ctx, process) => 
                         ctx.Kernel.Get<BatchProcessDecorator>(new ConstructorArgument("process", process)));

Hope that helps