依赖倒置原则（Dependence Inversion Principle）

依赖倒置原则（Dependence Inversion Principle）是程序要依赖于抽象接口，不要依赖于具体实现。简单的说就是要求对抽象进行编程，不要对实现进行编程，这样就降低了客户与实现模块间的耦合。高层模块不应该依赖低层模块，二者都应该依赖其抽象；抽象不应该依赖细节；细节应该依赖抽象。面向对象的开发很好的解决了这个问题，一般情况下抽象的变化概率很小，让用户程序依赖于抽象，实现的细节也依赖于抽象。即使实现细节不断变动，只要抽象不变，客户程序就不需要变化。这大大降低了客户程序与实现细节的耦合度。

例如，

1）一般的反面设计实现

using System;
namespace ConsoleApplication
{
    public class HondaCar
    {
        public void Run()
        {
            Console.WriteLine("本田开始启动");
        }
        public void Turn()
        {
            Console.WriteLine("本田开始转弯");
        }
        public void Stop()
        {
            Console.WriteLine("本田开始停车");
        }
    }
    public class FordCar
    {
        public void Run()
        {
            Console.WriteLine("福特开始启动");
        }
        public void Turn()
        {
            Console.WriteLine("福特开始转弯");
        }
        public void Stop()
        {
            Console.WriteLine("福特开始停车");
        }
    }
    public class BmwCar
    {
        public void Run()
        {
            Console.WriteLine("福特开始启动");
        }
        public void Turn()
        {
            Console.WriteLine("福特开始转弯");
        }
        public void Stop()
        {
            Console.WriteLine("福特开始停车");
        }
    }
    public class AutoSystem
    {
        public enum CarType
        {
            Ford, Honda, Bmw
        };
        HondaCar hcar = new HondaCar();
        FordCar fcar = new FordCar();
        BmwCar bcar = new BmwCar();
        private CarType type;
        public AutoSystem(CarType type)
        {
            this.type = type;
        }
        public void RunCar()
        {
            if (type == CarType.Ford)
            {
                fcar.Run();
            }
            else if (type == CarType.Honda)
            {
                hcar.Run();
            }
            else if (type == CarType.Bmw)
            {
                bcar.Run();
            }
        }
        public void TurnCar()
        {
            if (type == CarType.Ford)
            {
                fcar.Turn();
            }
            else if (type == CarType.Honda)
            {
                hcar.Turn();
            }
            else if (type == CarType.Bmw)
            {
                bcar.Turn();
            }
        }
        public void StopCar()
        {
            if (type == CarType.Ford)
            {
                fcar.Stop();
            }
            else if (type == CarType.Honda)
            {
                hcar.Stop();
            }
            else if (type == CarType.Bmw)
            {
                bcar.Stop();
            }
        }
    }
    class Program
    {
        public static void Main()
        {
            AutoSystem autoSystem = new AutoSystem(AutoSystem.CarType.Honda);
            autoSystem.RunCar();
            autoSystem.TurnCar();
            autoSystem.StopCar();
            Console.ReadKey();
        }
    }
}

2）依赖倒置原则的实现

using System;
namespace ConsoleApplication
{
    public interface ICar
    {
        void Run();
        void Turn();
        void Stop();
    }
    public class BmwCar : ICar
    {
        public void Run()
        {
            Console.WriteLine("宝马开始启动");
        }
        public void Turn()
        {
            Console.WriteLine("宝马开始转弯");
        }
        public void Stop()
        {
            Console.WriteLine("宝马开始停车");
        }
    }
    public class FordCar : ICar
    {
        public void Run()
        {
            Console.WriteLine("福特开始启动");
        }
        public void Turn()
        {
            Console.WriteLine("福特开始转弯");
        }
        public void Stop()
        {
            Console.WriteLine("福特开始停车");
        }
    }
    public class HondaCar : ICar
    {
        public void Run()
        {
            Console.WriteLine("本田开始启动");
        }
        public void Turn()
        {
            Console.WriteLine("本田开始转弯");
        }
        public void Stop()
        {
            Console.WriteLine("本田开始停车");
        }
    }
    public class AutoSystem
    {
        private ICar icar;
        public AutoSystem(ICar icar)
        {
            this.icar = icar;
        }
        public void RunCar()
        {
            icar.Run();
        }
        public void TurnCar()
        {
            icar.Turn();
        }
        public void StopCar()
        {
            icar.Stop();
        }
    }
    class Program
    {
        public static void Main()
        {
            AutoSystem autoSystem = new AutoSystem(new HondaCar());
            autoSystem.RunCar();
            autoSystem.TurnCar();
            autoSystem.StopCar();
            Console.ReadKey();
        }
    }
}