Question

I will give very simple example.

class Implementation: IMyInterface
{
    string mArg;

    public Implementation(string arg)
    {
        mArg = arg;
    }

    public DoStuff(object param)
    {
        // snip
    }
}

class Decorator: IMyInterface
{
    IMyInterface mWrapped;

    public Decorator(IMyInterface wrapped)
    {
        mWrapped = wrapped;
    }

    public DoStuff(object param)
    {
        var result = mWrapped.DoStuff(param);

        // snip

        return result;
    }
}

Now the argument I need for Implementation constructor I get from user in run-time.

IMyInterface GetMyObject()
{
    string arg = AskUserForString();

    return mContext.Resolve // HOW?
}

So what is the proper way to set this up and resolve to Decorated instance?

The example is simple. But imagine there are more layers (decorators/adapters) and the innermost implementation needs parameter I get in run-time.

Answer 1

I am assuming you are using autofac, as it is in the tags of the question.
You can try something along these lines:

public class ImplementationFactory
{
    private readonly IComponentContext container;

    public ImplementationFactory(IComponentContext container)
    {
        this.container = container;

    }
    public IMyInterface GetImplementation(string arg)
    {
        return container.Resolve<IMyInterface>(new NamedParameter("arg", arg));            
    }
}

And the registration/resolving part:

var builder = new ContainerBuilder();
builder.RegisterType<ImplementationFactory>();
builder.RegisterType<Implementation>().Named<IMyInterface>("implementation");

builder.Register((c, args) => new Decorator(c.ResolveNamed<IMyInterface>("implementation", args.First())))
    .As<IMyInterface>();

var container = builder.Build();

var factory = container.Resolve<ImplementationFactory>();
var impl = factory.GetImplementation("abc"); //returns the decorator

If you have multiple decorators, you can chain them in the order you want within the registration:

builder.Register((c, args) => new Decorator(c.ResolveNamed<IMyInterface>("implementation", args.First())));
builder.Register((c, args) => new SecondDecorator(c.Resolve<Decorator>(args.First()))).As<IMyInterface>();

Here is a good article about decorator support in autofac as well.