c# - Entity Framework 的导航属性为空

Question

我首先使用带有代码的 Entity Framework 。我的关系属性不断中断。

我有对象 Element:

public class Element : IElement
{
    // ... some event handlers (removed)

    [Key]
    public Guid ID { get; set; } = Guid.NewGuid();

    public string Name { get; set; }

    // navigation properties
    public virtual ElementType ElementType { get; private set; }

    public virtual NotifiableCollection<Property> Properties { get; private set; } = new NotifiableCollection<Property>();

    // Parameterless constructor for serialization
    private Element() { }

    public Element(ElementType elementType) : base()
    {
        // loop through and create Properties for each Property Type
        ElementType = elementType;
        if (ElementType?.PropertyTypes != null)
        {
            ElementType.PropertyTypes.ToList().ForEach((property) =>
            {
                Properties.Add(new Property(property));
            });
        }
    }
}

和元素类型:

public class ElementType : IElementType
{
    // ... some event handlers (removed)

    [Key]
    public Guid ID { get; set; } = Guid.NewGuid();

    public string Name { get; set; }

    // navigation properties
    public virtual NotifiableCollection<PropertyType> PropertyTypes { get; set; } = new NotifiableCollection<PropertyType>();

    public virtual NotifiableCollection<Element> Elements { get; set; } = new NotifiableCollection<Element>();

    public ElementType()
    {
        // ensure our Element's get updates
        PropertyTypes.CollectionChanged += (e, a) =>
        {
            //update the database to send out renewal to interested entities
            if (a.ChangeType == ChangeType.Added)
            {
                foreach (Element element in Elements)
                {
                    element.Properties.Add(new Property(a.Item));
                }
            }
        };
    }
}

当我第一次创建这些对象时它工作正常(因为我明确设置了导航属性然后保存):

但是，当我关闭所有内容并从数据库中获取这些内容时:

导航属性未解析。表定义很好地设置了外键关系:

CREATE TABLE [dbo].[Elements] (
    [ID]             UNIQUEIDENTIFIER NOT NULL,
    [Name]           NVARCHAR (MAX)   NULL,
    [ElementType_ID] UNIQUEIDENTIFIER NULL,
    CONSTRAINT [PK_dbo.Elements] PRIMARY KEY CLUSTERED ([ID] ASC),
    CONSTRAINT [FK_dbo.Elements_dbo.ElementTypes_ElementType_ID] FOREIGN KEY ([ElementType_ID]) REFERENCES [dbo].[ElementTypes] ([ID])
);


GO
CREATE NONCLUSTERED INDEX [IX_ElementType_ID]
    ON [dbo].[Elements]([ElementType_ID] ASC);

我可以看到数据都是正确的:

ID                                    Name                             ElementType_ID
ff186746-62cb-4246-9c64-f2d007b23ac0  Aircon Test 27/03/2017 12:54:03  57d93ac1-ad3b-4718-a593-80639cc24907

与 ElementType 表中的 ID 匹配。

我的存储库中有这个集合:

context.Configuration.ProxyCreationEnabled = true;
context.Configuration.LazyLoadingEnabled = true;

并且在我尝试解析此属性时上下文仍然处于事件状态。

一切正常，但我在使用 EF 时多次遇到这个问题，我的导航属性只是随机中断。我不记得接触过与此元素关联的任何代码，只是运行它，但现在它不起作用。谁能帮忙？

编辑:这是存储库代码:

public sealed class Repository : IRepository
{
    public event ObjectMaterializedEventHandler ObjectMaterialized;

    public Repository() {
        (context as IObjectContextAdapter).ObjectContext.ObjectMaterialized += ObjectContext_ObjectMaterialized; ;
        context.Configuration.ProxyCreationEnabled = true;
        context.Configuration.LazyLoadingEnabled = true;
    }

    // I do this to wire in some events later
    private void ObjectContext_ObjectMaterialized(object sender, ObjectMaterializedEventArgs e)
    {
        ObjectMaterialized?.Invoke(this, e);
    }

    private DataContext context = new DataContext(false);

    public IEnumerable<T> GetAll<T>() where T : class
    {
        return context.Set<T>().ToList() as IEnumerable<T>;
    }

    public T GetItem<T>(Guid id) where T : class
    {
        return context.Set<T>().Find(id) as T;
    }
    ...
}

上下文像这样存储它们:

public class DataContext : DbContext
{
    ...
    public DbSet<Element> Elements { get; set; }
    public DbSet<ElementType> ElementTypes { get; set; }
}

我认为这与访问有关。我正在使用 context.Set().Find(id) 作为 T 访问元素，但它失败了。但是，如果我浏览 ElementTypes，找到它的实体列表，那么它工作正常。

Answer 1

在 Ivan 的帮助下在评论中找到了答案。

问题在于拥有私有(private)构造函数:

// Parameterless constructor for serialization
private Element() { }

proxies 的要求之一是公共(public)或 protected 构造函数:

For either of these proxies to be created: A custom data class must be declared with public access.

• A custom data class must not be sealed (NotInheritable in Visual Basic)

• A custom data class must not be abstract (MustInherit in Visual Basic).

• A custom data class must have a public or protected constructor that does not have parameters. Use a protected constructor without parameters if you want the CreateObject method to be used to create a proxy for the POCO entity. Calling the CreateObject method does not guarantee the creation of the proxy: the POCO class must follow the other requirements that are described in this topic.

• The class cannot implement the IEntityWithChangeTracker or IEntityWithRelationships interfaces because the proxy classes implement these interfaces.

• The ProxyCreationEnabled option must be set to true.

For lazy loading proxies: Each navigation property must be declared as public, virtual (Overridable in Visual Basic), and not sealed (NotOverridable in Visual Basic) get accessor.