Specification Pattern Template

Specification.cs

namespace Specifications
{
  public interface ISpecification<T>
	{
		bool IsSatisfiedBy(T candidate);
		ISpecification<T> And(ISpecification<T> other);
		ISpecification<T> Or(ISpecification<T> other);
		ISpecification<T> Not();
	}

	public abstract class CompositeSpecification<T> : ISpecification<T>
	{
		public abstract bool IsSatisfiedBy(T candidate);

		public ISpecification<T> And(ISpecification<T> other)
		{
			return new AndSpecification<T>(this, other);
		}

		public ISpecification<T> Or(ISpecification<T> other)
		{
			return new OrSpecification<T>(this, other);
		}

		public ISpecification<T> Not()
		{
			return new NotSpecification<T>(this);
		}
	}

	public class AndSpecification<T> : CompositeSpecification<T>
	{
		private readonly ISpecification<T> _left;
		private readonly ISpecification<T> _right;

		public AndSpecification(ISpecification<T> left, ISpecification<T> right)
		{
			_left = left;
			_right = right;
		}

		public override bool IsSatisfiedBy(T candidate)
		{
			return _left.IsSatisfiedBy(candidate) && _right.IsSatisfiedBy(candidate);
		}
	}

	public class OrSpecification<T> : CompositeSpecification<T>
	{
		private readonly ISpecification<T> _left;
		private readonly ISpecification<T> _right;

		public OrSpecification(ISpecification<T> left, ISpecification<T> right)
		{
			_left = left;
			_right = right;
		}

		public override bool IsSatisfiedBy(T candidate)
		{
			return _left.IsSatisfiedBy(candidate) || _right.IsSatisfiedBy(candidate);
		}
	}

	public class NotSpecification<T> : CompositeSpecification<T>
	{
		private readonly ISpecification<T> other;

		public NotSpecification(ISpecification<T> other)
		{
			this.other = other;
		}

		public override bool IsSatisfiedBy(T candidate)
		{
			return !other.IsSatisfiedBy(candidate);
		}
	}
}