第一次看到这个词 Specification 是在ABP框架中，起先没怎么太关注这个。

随着接触DDD领域驱动越来越来多，关于如何集中业务逻辑的问题，再次浮现在我脑中。

好吧，我不知道。

我知道如何使用很挫的方式集中业务逻辑，问题是业务越来越大时，“这种很挫的方式集中业务逻辑” 的这一种方法将会使业务逻辑代码变得乱七八糟。

而这种乱七八糟的代码的终点是 “屎山” 。没错，“屎山” 就是巨难整，看着就让人生厌的东西，如果有位后来者看见这玩意，我相信他会骂上一句，然后......

但愿这次学习和以后的使用 Specification 可以帮助我更上一层楼吧。

ISpecification<T> 规约定义

    public interface ISpecification<T>
    {
        bool IsSatisfiedBy(T entity);

        Expression<Func<T, bool>> ToExpression();
    }

Specification<T>抽象类

    /// <summary>
    /// This base specification implements the IsSatisfiedBy method by compiling the expression from ToExpression.
    ///
    /// This is useful for general specifications to prevent duplicated logic.
    /// Beware that it is not very performant in situations where many specifications are dynamically constructed and combined.   
    /// </summary>
    /// <remarks>
    /// see this project https://github.com/elsa-workflows/elsa-core .
    /// </remarks>
    public abstract class Specification<T> : ISpecification<T>
    {private Func<T, bool>? _predicate;
        public virtual bool IsSatisfiedBy(T entity)
        {
            _predicate ??= ToExpression().Compile();
            return _predicate(entity);
        }
        public abstract Expression<Func<T, bool>> ToExpression();

        public static implicit operator Expression<Func<T, bool>>(Specification<T> specification)
        {
            return specification.ToExpression();
        }
    }

具体例子

例如，创建一Project类

    public class Project
    {
        public string Id { get; set; }

        public string Name { get; set; }

        public int Size { get; set; }

        public ProjectType ProjectType { get; set; }
    }

    public enum ProjectType
    {
        A,
        B,
        C,
    }

创建一个用于判断Project  id的规约类

    public class ProjectIdSpecification : Specification<Project>
    {
        public string ProjectId { get; }

        public ProjectIdSpecification(string projectId)
        {
            ProjectId = projectId;
        }

        public override Expression<Func<Project, bool>> ToExpression()
        {
            return project => project.Id == ProjectId;
        }
    }