EF6 Create Different DataContext on runtime(运行时改变连接字符串)

引言

在使用EF时，有时我们需要在程序运行过程中动态更改EF的连接字符串，但不幸的时EF是否对

ConfigurationManager.RefreshSection("xxx")

这行代码的影响不大，我没去深究，还请大侠指点。（本人猜测，EF内部实现没有订阅RefreshSection的事件去做相应的更换连接字符串的处理），如果确实想用该方法通过修改配置文件来实现动态更换连接字符串，那最保守的做法再加上个重启应用程序的方法。（确保EF能重新读到新的配置信息。）

这个时候就需要我们在DataContext新建时使用指定DBConnection来解决该问题，Demo如下：

APP.CONFIG (Uses EF 6)

<?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.SqlConnectionFactory, EntityFramework">
  <parameters>
    <parameter value="Data Source=localhost; Integrated Security=True; MultipleActiveResultSets=True" />
  </parameters>
</defaultConnectionFactory>
 </entityFramework>
</configuration>

the code to make as small as possible for Demo:

using System;
using System.Data.Common;
using System.Data.Entity;
using System.Data.Entity.Infrastructure;
using System.Data.Entity.Migrations;

namespace Ef6Test {
    public class Program {
    public static void Main(string[] args) {
        Database.SetInitializer(new MigrateDatabaseToLatestVersion<Ef6Ctx, Ef6MigConf>());
        WhichDb.DbName = "HACKDB1";
        var sqlConn = GetSqlConn4DBName(WhichDb.DbName);
        var context = new Ef6Ctx(sqlConn);
        context.Database.Initialize(true);
        AddJunk(context);
        //sqlConn.Close();  //?? whatever other considerations, dispose of context etc...

        Database.SetInitializer(new MigrateDatabaseToLatestVersion<Ef6Ctx, Ef6MigConf>()); // yes its default again reset this !!!!
        WhichDb.DbName = "HACKDB2";
        var sqlConn2 = GetSqlConn4DBName(WhichDb.DbName);
        var context2 = new Ef6Ctx(sqlConn2);
        context2.Database.Initialize(true);
        AddJunk(context2);
    }
    public static class WhichDb { // used during migration to know which connection to build
        public static string DbName { get; set; }
    }
    private static void AddJunk(DbContext context) {
        var poco = new pocotest();
        poco.f1 = DateTime.Now.ToString();
      //  poco.f2 = "Did somebody step on a duck?";  //comment in for second run
        context.Set<pocotest>().Add(poco);
        context.SaveChanges();
    }
    public static DbConnection GetSqlConn4DBName(string dbName) {
        var sqlConnFact =
            new SqlConnectionFactory(
                "Data Source=localhost; Integrated Security=True; MultipleActiveResultSets=True");
        var sqlConn = sqlConnFact.CreateConnection(dbName);
        return sqlConn;
    }
}
public class MigrationsContextFactory : IDbContextFactory<Ef6Ctx> {
    public Ef6Ctx Create() {
        var sqlConn = Program.GetSqlConn4DBName(Program.WhichDb.DbName); // NASTY but it works
        return new Ef6Ctx(sqlConn);
    }
}
public class Ef6MigConf : DbMigrationsConfiguration<Ef6Ctx> {
    public Ef6MigConf() {
        AutomaticMigrationsEnabled = true;
        AutomaticMigrationDataLossAllowed = true;
    }
}
public class pocotest {
    public int Id { get; set; }
    public string f1 { get; set; }
 //   public string f2 { get; set; } // comment in for second run
}
public class Ef6Ctx : DbContext {
    public DbSet<pocotest> poco1s { get; set; }
    public Ef6Ctx(DbConnection dbConn) : base(dbConn, true) { }
}
}

参考

https://stackoverflow.com/questions/18272708/how-to-force-dbcontext-refresh-connection-string-from-config

https://stackoverflow.com/questions/15504465/entityframework-code-first-custom-connection-string-and-migrations/16133150#16133150

https://github.com/mono/entityframework/blob/master/src/EntityFramework/Infrastructure/SqlConnectionFactory.cs