Specification.cs

public abstract class Specification<TEntity> : ISpecification<TEntity>
{
	public abstract bool IsSatisfiedBy(TEntity entity);

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

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

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