Pipes and filters à la Steve Bate. No, wait, it´s not pipes and filters anymore. It´s an assembly line processing work pieces. And you can assemble the assembly line using the + operator.

assemblyLineFor.cs

public class AssemblyLineFor<T>
{
    public interface IFilter
    {
        T Process(T message);
    }


    private readonly IList<Func<T, T>> _stages = new List<Func<T, T>>();


    public static AssemblyLineFor<T> operator +(AssemblyLineFor<T> assemblyLine, IFilter singletonFilter)
    {
        return assemblyLine.Register(singletonFilter);
    }

    public static AssemblyLineFor<T> operator +(AssemblyLineFor<T> assemblyLine, Type perMessageFilterType)
    {
        return assemblyLine.Register(perMessageFilterType);
    }

    public static AssemblyLineFor<T> operator +(AssemblyLineFor<T> assemblyLine, Func<T, T> filter)
    {
        return assemblyLine.Register(filter);
    } 



    protected AssemblyLineFor<T> Register(IFilter singletonFilter)
    {
        _stages.Add(singletonFilter.Process);
        return this;
    }

    protected AssemblyLineFor<T> Register(Type perMessageFilterType)
    {
        _stages.Add(msg => {
                        var f = Activator.CreateInstance(perMessageFilterType) as IFilter;
                        return f.Process(msg);
                    });
        return this;
    }

    protected AssemblyLineFor<T> Register(Func<T, T> filter)
    {
        _stages.Add(filter);
        return this;
    }


    public T Process(T message)
    {
        return _stages.Aggregate(message, (current, stage) => stage(current));
    }
}

sample.cs

class Program
{
    static void Main()
    {
        var a = new AssemblyLineFor<string>();
        a = a + new SingletonFilter3() 
              + typeof(PerMessageFilter3) 
              + (msg => msg + "L;");

        var result = a.Process("world:");

        Console.WriteLine(result);
    }
}


class SingletonFilter3 : AssemblyLineFor<string>.IFilter
{
    public string Process(string message)
    {
        return message + "SF" + this.GetHashCode() + ";";
    }
}

class PerMessageFilter3 : AssemblyLineFor<string>.IFilter
{
    public string Process(string message)
    {
        return message + "PMF" + this.GetHashCode() + ";";
    }
}