NotSpecification.cs

public class NotSpecification<TEntity> : Specification<TEntity>
{
    private readonly ISpecification<TEntity> specification;

	public NotSpecification(ISpecification<TEntity> specification)
	{
		this.specification = specification;
	}

	public override bool IsSatisfiedBy(TEntity entity)
	{
		return !this.specification.IsSatisfiedBy(entity);
	}
}