Decoration with several interface

Question

I am always struggling with decoration + interfaces. Say I have the following 'behavior' interfaces :

interface IFlyable { void Fly();}
interface ISwimmable { void Swim();}

A main interface

interface IMainComponent { void DoSomethingA(); void DoSomethingB();}

A decorator on the main interace

    public class Decorator : IMainComponent
    {
        private readonly IMainComponent decorated;
        [..]

        public virtual void DoSomethingA()
        {
            decorated.DoSomethingA();
        }

        public virtual void DoSomethingB()
        {
            decorated.DoSomethingB();
        }
    }

My issue is how to forward all the interfaces implemented by the decorated object to the decorator. A solution is to make the decorator implementation the interfaces :

    public class Decorator : IMainComponent, IFlyable, ISwimmable
    {
        [..]

        public virtual void Fly()
        {
            ((IFlyable)decorated).Fly();
        }

        public virtual void Swim()
        {
            ((ISwimmable)decorated).Swim();
        }

But I don't like it because :

1. It may looks like the "Decorator" implement an interface while it is not the case (Cast exception at run time)
2. This is not scalable, I need to add each new interface (and not forget about this addition)

An other solution is to add "a manual cast" that propagates throw the decoration tree :

    public class Decorator : IMainComponent
    {
        public T GetAs<T>()
            where T : class
        {
            //1. Am I a T ?
            if (this is T)
            {
                return (T)this;
            }

            //2. Maybe am I a Decorator and thus I can try to resolve to be a T
            if (decorated is Decorator)
            {
                return ((Decorator)decorated).GetAs<T>();
            }

            //3. Last chance
            return this.decorated as T;
        }

But the issues are :

1. The caller can be manipulating the wrapped object after a call to GetAs().
2. This can lead to confusion/unwanted behaviour if using a method from IMainComponent after a call on GetAs (something like ((IMainComponent)GetAs()).DoSomethingB(); ==> this may call the implementation of the wrapped object, not the full decoration.
3. The GetAs() method need to be called and exiting code with cast/regular "As" will not work.

How to you approch/resolve this issue ? Is there a pattern addressing this issue ?

PD : my question is for a final C# implementation but maybe the solution is more broad.

Answer 1

You will need to create separate decorators for each interface. An alternative is to use a generic interface for your services and a generic decorator. For example:

public interface ICommandService<TCommand>
{
   Task Execute(TCommand command);
}

public class LoggingCommandService<TCommand> : ICommandService<TCommand>
{
  public LoggingCommandService(ICommandService<TCommand> command, ILogger logger)
  {
    ...
  }

  public async Task Execute(TCommand command)
  {
    // log
    await this.command.Execute(command);
    // log
  }
}