c# - 没有无参数 DbContext 和 DbContextFactory 构造函数的添加迁移

Question

我的应用程序在 DbContext 处没有无参数构造函数实现，我不喜欢为 IDbContextFactory<> 提供无参数构造函数实现。

原因是我想控制 DbContext 指向的位置。这就是为什么我所有的构造函数都会请求 ConnectionStringProvider。

public class MyDbContext : DbContext
{
    internal MyDbContext(IConnectionStringProvider provider) : base(provider.ConnectionString) {}
}

和

public class MyContextFactory : IDbContextFactory<MyDbContext>
{
    private readonly IConnectionStringProvider _provider;
    public MyContextFactory(IConnectionStringProvider provider)
    {
        _provider = provider;
    }
    public MyDbContext Create()
    {
        return new MyDbContext(_provider.ConnectionString);
    }
}

我绝对不想添加默认构造函数!我已经这样做了，但由于错误的 App.config 中的错误连接字符串或假定默认连接字符串(如 DbContext 的默认构造函数)，它在生产中崩溃了做。我想在

• 调试/发布(并且只注入(inject)一个不同的 IConnectionStringProvider )
• 调用 Add-Migration脚本
• 正在运行 DbMigrator.GetPendingMigrations()

目前我收到了其中一些消息:

The context factory type 'Test.MyContextFactory' does not have a public parameterless constructor. Either add a public parameterless constructor, create an IDbContextFactory implementation in the context assembly, or register a context factory using DbConfiguration.

---更新---

这可能是 How do I inject a connection string into an instance of IDbContextFactory<T>? 的副本但它没有解决办法。我解释一下原因:

• 我总是使用 Add-Migration带有连接字符串，那么我如何提供 DbContext或 IDbContextFactory<>那会消耗它吗？而不是无参数构造函数？

  Add-Migration MyMigration -ConnectionStringName "MyConnectionString"

• 同样的问题:我使用 DbMigrator.GetPendingMigrations()这也要求无参数 DbContext或 IDbContextFactory<>实现。

据我了解，EntityFramework 违反了封装 by implying default constructors和原因 temporal coupling这不是故障安全的。所以请提出一个没有无参数构造函数的解决方案。

Answer 1

I always use Add-Migration with connection string, so how can I provide a DbContext or IDbContextFactory<> that consumes it? Instead of parameterless constructors?

在花了一些时间对 Entity Framework 进行逆向工程后，结果证明答案是:你做不到!

这是运行 Add-Migration 时发生的情况(没有默认构造函数):

System.Data.Entity.Migrations.Infrastructure.MigrationsException: The target context 'Namespace.MyContext' is not constructible. Add a default constructor or provide an implementation of IDbContextFactory.
   at System.Data.Entity.Migrations.DbMigrator..ctor(DbMigrationsConfiguration configuration, DbContext usersContext, DatabaseExistenceState existenceState, Boolean calledByCreateDatabase)
   at System.Data.Entity.Migrations.DbMigrator..ctor(DbMigrationsConfiguration configuration)
   at System.Data.Entity.Migrations.Design.MigrationScaffolder..ctor(DbMigrationsConfiguration migrationsConfiguration)
   at System.Data.Entity.Migrations.Design.ToolingFacade.ScaffoldRunner.RunCore()
   at System.Data.Entity.Migrations.Design.ToolingFacade.BaseRunner.Run()

让我们看看 DbMigrator构造函数。当从 Add-Migration 运行时命令，usersContext为空，configuration.TargetDatabase是不 null 并且包含从命令行参数传递的信息，例如-ConnectionStringName , -ConnectionString和 -ConnectionProviderName .所以new DbContextInfo(configuration.ContextType, configuration.TargetDatabase)被称为。

internal DbMigrator(DbMigrationsConfiguration configuration, DbContext usersContext, DatabaseExistenceState existenceState, bool calledByCreateDatabase) : base(null)
{
    Check.NotNull(configuration, "configuration");
    Check.NotNull(configuration.ContextType, "configuration.ContextType");
    _configuration = configuration;
    _calledByCreateDatabase = calledByCreateDatabase;
    _existenceState = existenceState;
    if (usersContext != null)
    {
        _usersContextInfo = new DbContextInfo(usersContext);
    }
    else
    {
        _usersContextInfo = ((configuration.TargetDatabase == null) ?
            new DbContextInfo(configuration.ContextType) :
            new DbContextInfo(configuration.ContextType, configuration.TargetDatabase));
        if (!_usersContextInfo.IsConstructible)
        {
            throw Error.ContextNotConstructible(configuration.ContextType);
        }
    }
    // ...
}

对于 DbMigrator不要扔，DbContextInfo实例必须是可构造的。现在，让我们看看 DbContextInfo构造函数。对于 DbContextInfo是可构造的，两者都是CreateActivator()和 CreateInstance()不得返回 null。

private DbContextInfo(Type contextType, DbProviderInfo modelProviderInfo, AppConfig config, DbConnectionInfo connectionInfo, Func<IDbDependencyResolver> resolver = null)
{
    _resolver = (resolver ?? ((Func<IDbDependencyResolver>)(() => DbConfiguration.DependencyResolver)));
    _contextType = contextType;
    _modelProviderInfo = modelProviderInfo;
    _appConfig = config;
    _connectionInfo = connectionInfo;
    _activator = CreateActivator();
    if (_activator != null)
    {
        DbContext dbContext = CreateInstance();
        if (dbContext != null)
        {
            _isConstructible = true;
            using (dbContext)
            {
                _connectionString = DbInterception.Dispatch.Connection.GetConnectionString(dbContext.InternalContext.Connection, new DbInterceptionContext().WithDbContext(dbContext));
                _connectionStringName = dbContext.InternalContext.ConnectionStringName;
                _connectionProviderName = dbContext.InternalContext.ProviderName;
                _connectionStringOrigin = dbContext.InternalContext.ConnectionStringOrigin;
            }
        }
    }
    public virtual bool IsConstructible => _isConstructible;
}

CreateActivator基本上搜索 DbContext 类型或 IDbContextFactory<MyContext> 的无参数构造函数执行并返回 Func<MyContext> .那么CreateInstance调用那个激活器。不幸的是，DbConnectionInfo connectionInfo DbContextInfo 的参数激活器不使用构造函数，但仅在创建上下文实例后才应用(为简洁起见，删除了不相关的代码):

public virtual DbContext CreateInstance()
{
    dbContext = _activator == null ? null : _activator();
    dbContext.InternalContext.ApplyContextInfo(this);
    return dbContext;
}

然后，在ApplyContextInfo里面，奇迹发生了:连接信息(来自 _connectionInfo)在新创建的上下文中被覆盖。

因此，鉴于您必须有一个无参数的构造函数，我的解决方案与您的类似，但进行了一些更积极的检查。

1. 默认构造函数仅在调试配置中编译时添加。
2. 如果未从 Add-Migration 中调用，默认构造函数将抛出命令。

这是我的上下文:

public class MyContext : DbContext
{
    static MyContext()
    {
        System.Data.Entity.Database.SetInitializer(new MigrateDatabaseToLatestVersion<MyContext, MyContextConfiguration>(useSuppliedContext: true));
    }

#if DEBUG
    public MyContext()
    {
        var stackTrace = new System.Diagnostics.StackTrace();
        var isMigration = stackTrace.GetFrames()?.Any(e => e.GetMethod().DeclaringType?.Namespace == typeof(System.Data.Entity.Migrations.Design.ToolingFacade).Namespace) ?? false;
        if (!isMigration)
            throw new InvalidOperationException($"The {GetType().Name} default constructor must be used exclusively for running Add-Migration in the Package Manager Console.");
    }
#endif
    // ...
}

那我终于可以跑了

Add-Migration -Verbose -ConnectionString "Server=myServer;Database=myDatabase;Integrated Security=SSPI" -ConnectionProviderName "System.Data.SqlClient"

对于运行迁移，我还没有找到使用 DbMigrator 的解决方案明确地，所以我使用 MigrateDatabaseToLatestVersion数据库初始化器 useSuppliedContext: true如 How do I inject a connection string into an instance of IDbContextFactory? 中所述.