using System;

public class Program

{

    class Base {}

    class Derived: Base {}

    public interface ISpecification<in T>

    {

        /// <summary>

        /// Gets the expression that encapsulates the criteria of the specification.

        /// </summary>

        Func<T, bool> Predicate { get; }

​

        /// <summary>

        /// Evaluates the specification against an entity of <typeparamref name="T"/>.

        /// </summary>

        /// <param name="entity">The <typeparamref name="T"/> instance to evaluate the specificaton

        /// against.</param>

        /// <returns>Should return true if the specification was satisfied by the entity, else false. </returns>

        bool IsSatisfiedBy(T entity);

    }

    public class Specification<T> : ISpecification<T>

    {

        public Specification(Func<T, bool> predicate)

        {

            this.Predicate = predicate;

        }

​

        public Func<T, bool> Predicate

        {

            get;

            private set;

        }

​

        public bool IsSatisfiedBy(T x)

        {

            return Predicate.Invoke(x);

        }

​

        public static Specification<T> operator &(Specification<T> left, ISpecification<T> right)

        {

            return new Specification<T>((x) => left.Predicate(x) && right.Predicate(x));

        }

        public static Specification<T> operator &(ISpecification<T> left, Specification<T> right)

        {

            return new Specification<T>((x) => left.Predicate(x) && right.Predicate(x));

        }

    }

    public static void Main()

    {

        new Specification<Base>((x) => true) & new Specification<Base>((x) => true);

    }

}