Entity Framework：数据库初始化的三种机制

直接看代码

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using System.Data.Entity;

namespace Study.CodeFirst
{
    class Program
    {
        static void Main(string[] args)
        {
            TestDropCreateDatabaseAlways();

            TestCreateDatabaseIfNotExists();

            TestDropCreateDatabaseIfModelChanges();
        }

        private static void TestDropCreateDatabaseAlways()
        {
            //此处每次运行都会创建新的数据库，每次的Id都不一样
            Database.SetInitializer<SecurityContext>(new SecurityContextDatabaseInitializerForDropCreateDatabaseAlways());

            using (SecurityContext context = new SecurityContext())
            {
                Console.WriteLine(context.Users.First().Id);
                Console.WriteLine(context.Users.First().Name);
            }
        }

        private static void TestCreateDatabaseIfNotExists()
        {
            //此处只有第一次运行才会创建新的数据库，每次的Id都一样。
            Database.SetInitializer<SecurityContext>(new SecurityContextDatabaseInitializerForCreateDatabaseIfNotExists());

            using (SecurityContext context = new SecurityContext())
            {
                Console.WriteLine(context.Users.First().Id);
                Console.WriteLine(context.Users.First().Name);
            }
        }

        private static void TestDropCreateDatabaseIfModelChanges()
        {
            //测试此处需要修改User模型
            //此处只有修噶模型运行才会创建新的数据库，模型变更后，每次的Id都一样。
            Database.SetInitializer<SecurityContext>(new SecurityContextDatabaseInitializerForDropCreateDatabaseIfModelChanges());

            using (SecurityContext context = new SecurityContext())
            {
                Console.WriteLine(context.Users.First().Id);
                Console.WriteLine(context.Users.First().Name);
            }
        }
    }

    public class User
    {
        public Guid Id { get; set; }

        public string Name { get; set; }

        //public int Age { get; set; }
    }

    public class SecurityContext : DbContext
    {
        public DbSet<User> Users
        {
            get
            {
                return this.Set<User>();
            }
        }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Entity<User>();

            base.OnModelCreating(modelBuilder);
        }
    }

    public class SecurityContextDatabaseInitializerForDropCreateDatabaseAlways : DropCreateDatabaseAlways<SecurityContext>
    {
        protected override void Seed(SecurityContext context)
        {
            context.Users.Add(new User() { Id = Guid.NewGuid(), Name = "DropCreateDatabaseAlways" });

            base.Seed(context);
        }
    }

    public class SecurityContextDatabaseInitializerForCreateDatabaseIfNotExists : CreateDatabaseIfNotExists<SecurityContext>
    {
        protected override void Seed(SecurityContext context)
        {
            context.Users.Add(new User() { Id = Guid.NewGuid(), Name = "CreateDatabaseIfNotExists" });

            base.Seed(context);
        }
    }

    public class SecurityContextDatabaseInitializerForDropCreateDatabaseIfModelChanges : DropCreateDatabaseIfModelChanges<SecurityContext>
    {
        protected override void Seed(SecurityContext context)
        {
            context.Users.Add(new User() { Id = Guid.NewGuid(), Name = "DropCreateDatabaseIfModelChanges" });

            base.Seed(context);
        }
    }
}