Entity Framework系列之Code First

第一步：安装Entity Framework6.0

使用管理NuGet程序包界面安装EntityFramework

第二步：新建实体类

using System.ComponentModel.DataAnnotations;

namespace EFDemo4
{
    public class User
    {
        [Key]
        public int Id { get; set; }
        public string Name { get; set; }
    }
}

更新数据结构

当我们的数据结构变化了后，就会出现数据模型和数据库不一致的情况。此时运行则会出现如下异常：

未经处理的异常: System.InvalidOperationException: The model backing the 'BloggingContext' context has changed since the database was created. Consider using Code First Migrations to update the database (http://go.microsoft.com/fwlink/?LinkId=238269).

解决方案：

1、Code First Migrations（数据迁移）

2、在程序启动前增加一行代码：System.Data.Entity.Database.SetInitializer(new DropCreateDatabaseIfModelChanges<TestDBContext>()); 要注意的事，该方法将重建整个数据库，因此原来的数据将会丢失。

using System;
using System.Data.Entity;
using System.Windows.Forms;

namespace EFDemo4
{
    static class Program
    {
        /// <summary>
        /// 应用程序的主入口点。
        /// </summary>
        [STAThread]
        static void Main()
        {
#if DEBUG
            System.Data.Entity.Database.SetInitializer(new DropCreateDatabaseIfModelChanges<TestDBContext>());
#endif
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new Form1());
        }
    }
}

第三步：新建实体上下文类

using System.Data.Entity;

namespace EFDemo4
{
    public class TestDBContext: DbContext
    {
        public TestDBContext()
            : base("name=TestDB3")
        {

        }
        public DbSet<User> Users { get; set; }
    }
}

App.config文件中的数据库连接字符串

<connectionStrings>
    <add name="TestDB3" providerName="System.Data.SqlClient" connectionString="Data Source=.CMDB;Initial Catalog=TestDB3;Integrated Security=SSPI;" />
  </connectionStrings>

第四步：增删改查

在执行数据库操作之后，数据库对象会自动创建

using System;
using System.Data.Entity;
using System.Linq;
using System.Windows.Forms;

namespace EFDemo4
{
    public partial class Form1 : Form
    {
        TestDBContext entity = new TestDBContext();
        public Form1()
        {
            InitializeComponent();
        }

        /// <summary>
        /// 新增
        /// </summary>
        private void Add()
        {
            User model = new User()
            {
                Name = DateTime.Now.ToString("yyyy-MM-dd hh:mm:ss")
            };
            entity.Users.Add(model);
            entity.SaveChanges();
            Query();
        }

        /// <summary>
        /// 删除
        /// </summary>
        private void Delete()
        {
            if (listBox1.SelectedItem == null)
            {
                return;
            }
            int id = Convert.ToInt32(listBox1.SelectedItem.ToString().Split('-')[0]);
            User model = entity.Users.Where(a => a.Id == id).FirstOrDefault();
            if (model != null)
            {
                entity.Entry(model).State = EntityState.Deleted;
                entity.SaveChanges();
                Query();
            }
        }

        /// <summary>
        /// 修改
        /// </summary>
        private void Edit()
        {
            if (listBox1.SelectedItem == null)
            {
                return;
            }
            int id = Convert.ToInt32(listBox1.SelectedItem.ToString().Split('-')[0]);
            User model = entity.Users.Where(a => a.Id == id).FirstOrDefault();
            if (model != null)
            {
                model.Name = DateTime.Now.ToString("yyyy-MM-dd hh:mm:ss");
                entity.Entry(model).State = EntityState.Modified;
                entity.SaveChanges();
                Query();
            }
        }

        /// <summary>
        /// 查询
        /// </summary>
        private void Query()
        {
            listBox1.Items.Clear();
            var expr = from p in entity.Users select p;
            foreach (var item in expr)
            {
                listBox1.Items.Add(string.Format("{0}-{1}", item.Id, item.Name));
            }
        }

        private void toolStripButton1_Click(object sender, EventArgs e)
        {
            Add();
        }

        private void toolStripButton4_Click(object sender, EventArgs e)
        {
            Edit();
        }

        private void toolStripButton3_Click(object sender, EventArgs e)
        {
            Delete();
        }

        private void toolStripButton2_Click(object sender, EventArgs e)
        {
            Query();
        }
    }
}