DbContext 和作用域依赖

Question

我有一个简单的 DbContext 看起来像：

public class MyDbContext : DbContext
{
    private readonly IUserContext _userContext;

    public MyDbContext(IUserContext userContext) : base("DefaultConnectionString")
    {
        _userContext = userContext;

        Database.SetInitializer(new MigrateDatabaseToLatestVersion<MyDbContext, Configuration>());
    }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        // ... Here I need to creates some filters using the IUserContext dependency

        base.OnModelCreating(modelBuilder);
    }

}

此 DbContext 使用 Func<T> 工厂接线，using the guidelines in the Simple Injector documentation：container.RegisterFuncFactory<DbContext, MyDbContext>(Lifestyle.Scoped);

public static void RegisterFuncFactory<TService, TImpl>(
    this Container container, Lifestyle lifestyle = null)
    where TService : class
    where TImpl : class, TService
{
    lifestyle = lifestyle ?? Lifestyle.Transient;
    var producer = lifestyle.CreateProducer<TService, TImpl>(container);
    container.RegisterSingleton<Func<TService>>(producer.GetInstance);
}

但显然，DbContext 无法实现这种简单的情况，因为这条消息：

The target context 'MyDbContext' is not constructible. Add a default constructor or provide an implementation of IDbContextFactory.

我不太喜欢 IDbContextFactory 的想法，所以我能想到的唯一解决方案是删除对 MyDbContext 的依赖，将其设置为属性，修改RegisterFuncFactory方法，手动初始化上下文：

internal static void RegisterFuncFactory<TService, TImpl>(this Container container, Func<TImpl> instanceProducer, Lifestyle lifestyle = null) where TService : class where TImpl : class, TService
{
    lifestyle = lifestyle ?? Lifestyle.Transient;
    var producer = lifestyle.CreateProducer<TService>(instanceProducer, container);
    container.Register<Func<TService>>(() => producer.GetInstance, Lifestyle.Singleton);
}

container.RegisterFuncFactory<DbContext, MyDbContext>(() => new MyDbContext
{
    UserContext = container.GetInstance<IUserContext>()
}, Lifestyle.Scoped);

虽然不优雅但它可以工作，但是有另一种 "better" 方法来做我需要的吗？我喜欢明确地依赖于上下文，但似乎不可能。

更新

错误来自：

'System.Data.Entity.Migrations.Infrastructure.MigrationsException' occurred in EntityFramework.dll but was not handled in user code

在此代码中，查询方法的 return 语句位于此处：

internal sealed class EntityFrameworkRepository<TEntity> : IEntityWriter<TEntity>, IEntityReader<TEntity> where TEntity : Entity
{
    private readonly Func<DbContext> _contextProvider;

    public EntityFrameworkRepository(Func<DbContext> contextProvider)
    {
        _contextProvider = contextProvider;
    }

    public IQueryable<TEntity> Query()
    {
        var context = _contextProvider();
        return context.Set<TEntity>().AsNoTracking();
    }

    // Methods removed for brevity

}

Answer 1

添加第二个（默认）构造函数。这样 EF 迁移可以在命令行运行时使用此构造函数，而您可以让您的应用程序使用第二个构造函数。

当您添加第二个构造函数时，您在 DbContext 上失去了 Simple Injector 的自动装配功能，但这应该不是问题；你可以简单地连接你的上下文如下：

IUserContext userContext = new AspNetUserContext();

container.RegisterSingleton<IUserContext>(userContext);

var contextProducer = Lifestyle.Scoped.CreateProducer<DbContext>(
    () => new MyDbContext(userContext),
    container);

container.RegisterSingleton<Func<DbContext>>(contextProducer.GetInstance);

Answer 2

这个答案只是为了向更多用户展示我最终得到的结果。 @Steven 的答案是正确的。

为了能够在支持迁移的同时将依赖项注入 DbContext，我们必须使用两个构造函数。一份用于迁移，一份用于应用程序。

public class MyDbContext : DbContext
{
    private readonly IUserContext _userContext;

    // For migrations
    public MyDbContext() : base("DefaultConnectionString")
    {
        Database.SetInitializer(new MigrateDatabaseToLatestVersion<MyDbContext, Configuration>());
    }

    // For applications
    public MyDbContext(IUserContext userContext) : base("DefaultConnectionString")
    {
        _userContext = userContext; 
    }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        // ... Code removed for brevity

        base.OnModelCreating(modelBuilder);
    }
}

然后将其连接到组合根中，例如：

public static void RegisterEntityFramework<TContext>(this Container container, Func<TContext> context) where TContext : DbContext
{
    if (container == null) throw new ArgumentNullException(nameof(container));

    var contextProducer = Lifestyle.Scoped.CreateProducer<DbContext>(context, container);
    container.RegisterSingleton<Func<DbContext>>(() => contextProducer.GetInstance);
}

var userContext = new AspNetHttpUserContext();
var container = new Container();
container.Options.DefaultScopedLifestyle = new WebApiRequestLifestyle();
container.RegisterSingleton<IUserContext>(userContext);
container.RegisterEntityFramework(() => new WayFinderDbContext(userContext));
container.Verify();

DbContext 和作用域依赖

DbContext and scoped dependency

c#

entity-framework

simple-injector

entity-framework-6