高层模块不应该依赖底层模块，两都都应该依赖抽象

一个三层的Demo

/// <summary>
/// 人员实体类
/// </summary>
class Person
{public int ID { get; set; }public string Name { get; set; }public bool Sex { get; set; }
}
/// <summary>
/// 表示层
/// </summary>
class UI
{public void Main(){ShowPerson();}    public static void ShowPerson(){Console.WriteLine("输入要查询的人员ID");var id = int.Parse(Console.ReadLine());var bll = new BLL();var person = bll.FindPerson(id);Console.WriteLine($"ID:{person.ID},Name:{person.Name},Sex:{(person.Sex ? "男" : "女")}");}
}
/// <summary>
/// 业务逻辑层
/// </summary>
class BLL
{public Person FindPerson(int id){var dal = new DAL();return dal.QueryPersonByID(id);}
}
/// <summary>
/// 数据访问层
/// </summary>
class DAL
{public Person QueryPersonByID(int id){return new Person { ID = id, Name = "张三丰收", Sex = true };}
}

高层模块指的是调用方：

UI类ShowPerson方法中bll.FindPerson

BLL类FindPerson方法中dal.QueryPersonByID

底层模块指的是实现模块：

BLL类FindPerson()方法

DAL类QueryPersonByID()方法

这是直接用法，为了降低耦合，响应需求变化，要把这种强依赖切降低，遵照“依赖倒置”，增加抽象层，用抽像层隔离高层和底层模块，本代码段中用一个ObjectHub来集中管理实例化过程，如下代码：

 #region 实体类/// <summary>/// 人员实体类/// </summary>class Person{public int ID { get; set; }public string Name { get; set; }public bool Sex { get; set; }}#endregion#region 对象仓库/// <summary>/// 对象仓库/// </summary>class ObjectHub{public static Dictionary<string, object> Container { get; private set; }static ObjectHub(){//通过这里来解耦Container = new Dictionary<string, object>{{ nameof(IDAL),  new DAL() },{ nameof(IBLL),new BLL() }};}}#endregion#region 表示/// <summary>/// 表示层/// </summary>class UI{static void Main(string[] args){var ui = new UI();ui.ShowPerson();}public void ShowPerson(){//依赖业务逻辑层接口var bll = ObjectHub.Container[nameof(IBLL)] as IBLL;Console.WriteLine("输入要查询的人员ID");var id = int.Parse(Console.ReadLine());var person = bll.FindPerson(id);Console.WriteLine($"ID:{person.ID},Name:{person.Name},Sex:{(person.Sex ? "男" : "女")}");}}#endregion#region 业务逻辑/// <summary>/// 业务逻辑层接口/// </summary>interface IBLL{Person FindPerson(int id);}/// <summary>/// 业务逻辑层/// </summary>class BLL : IBLL{public Person FindPerson(int id){//依赖业务逻辑层接口var dal = ObjectHub.Container[nameof(IDAL)] as IDAL;return dal.QueryPersonByID(id);}}#endregion#region 数据访问/// <summary>/// 数据访问层接口/// </summary>interface IDAL{Person QueryPersonByID(int id);}/// <summary>/// 数据访问层/// </summary>class DAL : IDAL{public Person QueryPersonByID(int id){return new Person { ID = id, Name = "张三丰收", Sex = true };}}#endregion

原来是高层模块依赖底层模块，现在依赖倒置，怎么倒置了呢？个人理解：高层和底层之间出现了一个抽像，高层依赖抽象，按理抽象依赖底层，但这里恰恰是底层和也依赖抽象，打破了一个依赖链，形成一个倒置。