所有實體類都會有一些公共屬性，可以把這些屬性定義到一個父類中。比如：抽象類BaseEntity

  public abstract class BaseEntity
    {
        public long Id { get; set; }
        public bool Isdelete { get; set; }=false
        public DateTime CreateTime { get; set; }=DateTime.Now
        public DateTime DeleteTime { get; set; }
        public abstract void Do();
             
     
    }

   public class Person:BaseEntity
    {
        public string Name { get; set; }

        public override void Do()
        {
            Console.WriteLine("eat");
        }
    }

   public class Student:Person
    {
        public string StuNo { get; set; }
        public virtual Class Class { get; set; }
    }

   public class Teacher:Person
    {
        public int Salary { get; set; }
    }

  public  class MyContext:DbContext
    {
        public MyContext():base("conn")
        {
            Database.SetInitializer<MyContext>(null);
        }
        public DbSet<Student> Students { get; set; }
        public DbSet<Teacher>  Teachers { get; set; }
        public DbSet<Class> Classes { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Configurations.AddFromAssembly(Assembly.GetExecutingAssembly());
        }
    }

使用公共父類的好處不僅是寫實體類簡單了，而且可以提供一個公共的實體操作類；

  public  class CommonCRUD<T> where T:BaseEntity  //T是泛型，T必須是繼承BaseEntity的類，泛型約束
    {
        private MyContext ctx;
        public CommonCRUD(MyContext ctxs)
        {
            this.ctx = ctxs;
        }

        public void MarkDeleted(long id)
        {
          T item= ctx.Set<T>().Where(e => e.Id == id).SingleOrDefault();//因為約束了類需要繼承BaseEntity，所以可以點出Id
            if(item== null)
            {
                throw new ArgumentException("找不到id=" + id + "數據");
               
            }
            else
            {
                item.Isdelete = true;
                item.DeleteTime = DateTime.Now;
                ctx.SaveChanges();
            }

        }

        public T GetById(long id)
        {
            T item = ctx.Set<T>().Where(e => e.Id == id && e.Isdelete == false).SingleOrDefault();
            return item;
                 
        }
        public IQueryable<T> GetAll(int start,int Count)
        {
            return ctx.Set<T>().OrderBy(e => e.CreateTime).Skip(start).Take(Count).Where(e => e.Isdelete == false);

        }

        public long GetTotalCount()
        {
            return ctx.Set<T>().Where(e => e.Isdelete == false).LongCount();

        }


    }

測試：

        static void Main(string[] args)
        {
            using (MyContext ctx = new MyContext())
            {
                Teacher t1 = new Teacher()
                {
                    Name = "語文老師",
                    Salary = 1200
                };
                Teacher t2 = new Teacher()
                {
                    Name = "數學老師",
                    Salary = 2400
                };
                Teacher t3 = new Teacher()
                {
                    Name = "英語老師",
                    Salary = 800
                };

                //增加
                /*
                ctx.Set<Teacher>().Add(t1);
                ctx.Set<Teacher>().Add(t2);
                ctx.Set<Teacher>().Add(t3);
                ctx.SaveChanges();
                */

                CommonCRUD<Teacher> crud = new CommonCRUD<Teacher>(ctx);

                //刪除
                //crud.MarkDeleted(1);

                //查詢
                 var Ts=   crud.GetAll(0, 10);
                foreach (var item in Ts)
                {
                    Console.WriteLine(item.Name);
                }
                
            }
            Console.WriteLine("ok");
            Console.ReadKey();
        }

為什么用IQueryable，因為性能比IEnumerable高，用IEnumerable后續是在內存中操作的；

  public class Class:BaseEntity
    {
        public override void Do()
        {
            
        }

        public string Name { get; set; }
    }

測試：

        static void Main(string[] args)
        {
            using (MyContext ctx = new MyContext())
            {
                Class c1 = new Class { Name = "三年二班" };
                Student s1 = new Student(){ Name = "chen", StuNo = "10002", Class = c1 };
                Student s2 = new Student(){ Name = "wang", StuNo = "10003", Class = c1 };
                ctx.Students.Add(s1);
                ctx.Students.Add(s2);
                ctx.SaveChanges();
                CommonCRUD<Student> crud = new CommonCRUD<Student>(ctx);
                //對于IQueryable也可以調用Include，需要using System.Data.Entity;
                var Ts =   crud.GetAll(0, 10).Include(e =>e.Class);
                foreach (var item in Ts)
                {
                    Console.WriteLine(item.Name);
                    Console.WriteLine(item.Class.Name);
                }
            }
            Console.WriteLine("ok");
            Console.ReadKey();
        }

為什么BaseEntity是抽象類
設為抽象類是避免被實例化,讓這個類只能作為基類參與繼承，不能讓他new出來

Set<T>()是 DBSet<T>的一個屬性嗎?
db.Teachers的本質就是db.Set<Teachers>()吧?

Set<T>就是上下文類中的一個方法,可以操作指定實體類來改變數據庫。
db.Teachers和db.Set<Teachers>()等效

總結

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