ProxySample/ProxyableConsole/IProxyable_TResult.cs

IProxyable_TResult.cs

using System;

namespace ProxyableConsole
{
    public interface IProxyable<TResult>
    {
        TResult Execute();
    }

    class BodyProxyable<TResult> : IProxyable<TResult>
    {
        Func<TResult> execute;

        public BodyProxyable(Func<TResult> execute)
        {
            this.execute = execute ?? throw new ArgumentNullException(nameof(execute));
        }

        public TResult Execute() => execute();
    }

    class AspectProxyable<TResult> : IProxyable<TResult>
    {
        IProxyable<TResult> source;
        Func<Func<TResult>, TResult> execute;

        public AspectProxyable(IProxyable<TResult> source, Func<Func<TResult>, TResult> execute)
        {
            this.source = source ?? throw new ArgumentNullException(nameof(source));
            this.execute = execute ?? throw new ArgumentNullException(nameof(execute));
        }

        public TResult Execute() => execute(source.Execute);
    }

    public static class Proxyable
    {
        public static IProxyable<TResult> Body<TResult>(Func<TResult> execute) => new BodyProxyable<TResult>(execute);

        public static IProxyable<TResult> Aspect<TResult>(this IProxyable<TResult> source, Func<Func<TResult>, TResult> execute) => new AspectProxyable<TResult>(source, execute);
    }
}

Program.cs

using System;

namespace ProxyableConsole
{
    class Program
    {
        static void Main(string[] args)
        {
            var result = Proxyable.Body(() =>
                {
                    Console.WriteLine("Body");
                    return 123;
                })
                .Aspect(f =>
                {
                    Console.WriteLine("Before");
                    var r = f();
                    Console.WriteLine("After");
                    return r;
                })
                .Execute();
        }
    }
}