学习篇二：code first （demo）

谁践踏了优雅 2021-06-24 16:11 376阅读 0赞

## 第一步：建数据库，建表（不建表，app.config里连接写好也可） ##

![2789632-ccf10832fea0298f.png][]

## 第二步：新建项目 ##

![2789632-627dc991623a0bff.png][]

在 - 工具 - 库程序包管理器 - 程序包管理器控制台 这里 默认项目， 在PM>后 输入  
Install-Package EntityFramework -Version 6.0.0  
Install-Package EntityFramework.zh-Hans -Version 6.0.0  
Install-Package MySql.Data.Entity.EF6（这一步我没install进去）故在此处添加引用。  
这时候在providers 里 添加一个mysql.data.MysqlClint节点， 这个步骤很重要。  
<provider invariantName="MySql.Data.MySqlClient" type="MySql.Data.MySqlClient.MySqlProviderServices, MySql.Data.Entity.EF6"></provider>

## 第三步：添加实体 ##

![2789632-e41b82776a6b9097.png][]

![2789632-399714f3a4549ca1.png][]

![2789632-7401c8939212ed4b.png][]

解决办法：  
1.在数据库执行：  
set global optimizer\_switch='derived\_merge=off';  
set optimizer\_switch='derived\_merge=off';

select @@optimizer\_switch;  
select @@GLOBAL.optimizer\_switch;

use <<database name>>;  
set global optimizer\_switch='derived\_merge=OFF';

![2789632-c72d447c50bace86.png][]

## 第四步：删除多余文件，修改app.config ##

![2789632-d2954e5b32306676.png][]

app.config

<?xml version="1.0" encoding="utf-8"?>
    <configuration>
      <configSections>
        
        <section name="entityFramework" type="System.Data.Entity.Internal.ConfigFile.EntityFrameworkSection, EntityFramework, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
      </configSections>
      <startup>
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
      </startup>
      <entityFramework>
        <defaultConnectionFactory type="System.Data.Entity.Infrastructure.LocalDbConnectionFactory, EntityFramework">
          <parameters>
            <parameter value="v13.0" />
          </parameters>
        </defaultConnectionFactory>
        <providers>
          <provider invariantName="System.Data.SqlClient" type="System.Data.Entity.SqlServer.SqlProviderServices, EntityFramework.SqlServer" />
          <provider invariantName="MySql.Data.MySqlClient" type="MySql.Data.MySqlClient.MySqlProviderServices, MySql.Data.Entity.EF6"></provider>
        </providers>
      </entityFramework>
      <connectionStrings>
        <add name="testEntities" connectionString="Data Source=localhost;port=3306;Initial Catalog=testcodefirstEntities;user id=root;password=0301;" providerName="MySql.Data.MySqlClient"/>
      </connectionStrings>
    </configuration>

## 第五步：新建表类，和联系上下问类 ##

testtable.cs

using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;
    using System.Threading.Tasks;
    
    namespace TestOne
    {
        public class testtable
        {
            public int Id { get; set; }
            public string Name { get; set; }
        }
    }

Mycontext.cs

using System;
    using System.Collections.Generic;
    using System.Data.Entity;
    using System.Linq;
    using System.Text;
    using System.Threading.Tasks;
    
    namespace TestOne
    {
        public class MyContext : DbContext
        {
            public MyContext()
                : base("name=testEntities")
            {
            }
            public DbSet<testtable> testtables { get; set; }
        }
    }

## 第六步：测试 ##

program.cs

using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;
    using System.Threading.Tasks;
    
    namespace TestOne
    {
        class Program
        {
            static void Main(string[] args)
            {
                var context = new MyContext();
                var test = context.testtables.Where(x => x.Id > 0).ToList();
                //context.testtables.Add(new testtable { Name = "55" });
                //context.SaveChanges();
                int count = test.Count();
                Console.Write(count);
                Console.ReadLine();
    
            }
        }
    }

执行：

![2789632-6f3273479c45133d.png][]

[2789632-ccf10832fea0298f.png]: /images/20210615/e9e0241f90084775a16934a6a757b5ef.png
[2789632-627dc991623a0bff.png]: /images/20210615/58f1e12c20b6456c9e3520d71d93899f.png
[2789632-e41b82776a6b9097.png]: /images/20210615/dd87967bafef4621ba582a536de97367.png
[2789632-399714f3a4549ca1.png]: /images/20210615/83bea13dcbc645beba7863254926aa26.png
[2789632-7401c8939212ed4b.png]: /images/20210615/c95a3a2144014f328c6672483e51a1a0.png
[2789632-c72d447c50bace86.png]: /images/20210615/852c857a028e4609b06ff3bd81805d1f.png
[2789632-d2954e5b32306676.png]: /images/20210615/225748093b924b2bb491d3b47559142e.png
[2789632-6f3273479c45133d.png]: /images/20210615/d372859478a640cf9c0bf6ff3e1eb457.png