流利的NHibernate正确映射基类的只读属性

Question

我想为某些实体创建基类，因为它们都共享Event列表属性。 我还想使Event列表成为只读属性 。

因此，我创建了一个基本的EventRelatedEntity类，然后在与事件有关的每个实体类上派生了它。

还请注意， EventRelatedEntity类没有NHibernate映射类，因为它没有链接到表。

请参见下面的代码。

基类：

public class EventRelatedEntity
{
    private readonly List<Event> events;

    public virtual IReadOnlyCollection<Event> Events { get; protected set; }

    public EventRelatedEntity()
    {
        events = new List<Event>();
        Events = events.AsReadOnly();
    }

    protected virtual void AddEvent<T>(T entity, string message)
    {
        if (events == null)
            events = new List<Event>();

        Event newEvent = new Event();

        if (typeof(T) == typeof(Company))
        {
            newEvent.CompanyId = (entity as Company).Id;
            // ...and do some other stuff...
        }
        else if (typeof(T) == typeof(Document))
        {
            newEvent.DocumentId = (entity as Document).Id;
            // ...and do some other stuff...
        }
        else if (typeof(T) == typeof(Typology))
        {
            newEvent.TypologyId = (entity as Typology).Id;
            // ...and do some other stuff...
        }

        newEvent.Message = message;

        events.Add(newEvent);
    }
}

实体类

public class Company : EventRelatedEntity
{
    [Key]
    public virtual int Id { get; protected set; }
    [Required]
    public virtual string Alias { get; set; }
    [Required]
    public virtual string CompanyName { get; set; }
    // ...and some other properties...

    #region Actions

    public virtual void AddEvent(string message)
    {
        base.AddEvent(this, message);
    }

    #endregion
}

public class Document : EventRelatedEntity
{
    [Key]
    public override int Id { get; protected set; }
    [Required]
    public virtual User User { get; protected set; }
    // ...and some other properties...

    #region Actions

    public virtual void AddEvent(string message)
    {
        base.AddEvent(this, message);
    }

    #endregion
}

// ...and some other classes...

实体的流利NHibernate映射类

public class CompanyMap : ClassMap<Company>
{
    public CompanyMap()
    {
        Table("Companies");
        LazyLoad();
        Id(x => x.Id).GeneratedBy.Identity().Column("Id");
        Map(x => x.Alias).Column("Alias").Not.Nullable();
        Map(x => x.CompanyName).Column("CompanyName").Not.Nullable();
        // ...and some other mappings...

        // Link with Events table
        HasMany(x => x.Events) // Events is declared in the base class (EventRelatedEntity)
            .KeyColumn("CompanyId")
            .Access.LowerCaseField()
            .Cascade.AllDeleteOrphan();
    }
}

public class DocumentMap : ClassMap<Document>
{   
    public DocumentMap()
    {
        Table("Documents");
        LazyLoad();
        Id(x => x.Id).GeneratedBy.Identity().Column("Id");
        References(x => x.User).Column("UserId");
        // ...and some other mappings...

        // Link with Events table
        HasMany(x => x.Events) // Events is declared in the base class (EventRelatedEntity)
            .KeyColumn("DocumentId")
            .Access.LowerCaseField()
            .Cascade.AllDeleteOrphan();
    }
}

// ...and some other mapping classes...

最后，我想避免直接访问List<>.Add()方法。 我想要一个只读集合。 将新Event添加到实体的事件列表的唯一方法必须是相应实体类的AddEvent方法。

例：

Document document = session.Get<Document>(1);
// ...the same for other derived classes...

// I WANT TO AVOID THIS!
document.Events.Add(new Event());
// WANTS TO BE THE ONLY PERMITTED WAY TO ADD NEW EVENTS
document.AddEvent("My new event message");

问题是当我这样做时：

Document document = session.Get<Document>(1);

我从NHibernate收到一个错误：

无法将类型为'NHibernate.Collection.Generic.PersistentGenericBag'1 [SolutionDOC_Interface.Entity.Event]'的对象强制转换为'System.Collections.Generic.List'1 [SolutionDOC_Interface.Entity.Event]'类型的对象。

我认为这与EventRelatedEntity类没有NHibernate映射的事实有关，但是我不能提供映射，因为它与DB中的表无关。 也许如果我在每个类（公司，文档等）中声明事件列表而没有使用继承，则NHibernate可以工作，但是这种方法会产生很多我想避免的代码重复。

更新2017/10/18

像@ryan建议的那样更改代码后，现在可以使用了。

修改后的代码：

public class EventRelatedEntity
{
    private readonly IList<Event> events;

    public virtual IReadOnlyCollection<Event> Events { get; protected set; }

    public EventRelatedEntity()
    {
        events = new List<Event>();
        Events = (events as List<Event>).AsReadOnly();
    }

    // ...
}

Answer 1

使用列表接口而不是具体的列表类，则NHibernate.Collection.Generic.PersistentGenericBag应该起作用。

使用IList<Event>而不是List<Event>因此EventRelatedEntity变为：

public class EventRelatedEntity
{
    private readonly IList<Event> events;

    // rest of implementation...
}