c# - 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);
    }

}

这DbContext使用 Func<T> 接线工厂，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 不可能实现这种简单的情况因为这个消息:

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, 所以我能想出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方法并手动初始化上下文:

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，但是否有另一种“更好”的方式来做我需要的事情？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

在这段代码中，查询方法的返回语句在这里:

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 迁移可以在从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 的自动连接功能，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);