为什么不能在NHibernate中将泛型与CreateCriteria一起使用?

问题描述

在我的基本仓库中，我有下面的代码可以正常工作:

In my Base repo I have this code which works fine:

abstract class BaseRepo <T> : IRepo <T>
{
    private ISession _session;

    public Entity GetById<Entity>(int Id)
    {
        return _session.Get<Entity>(Id);
    }

    // other methods
}

我想添加另一种方法来返回对象(实体)的所有行.我想做类似的事情:

I want to add another method to return all rows for an object (entity). I want to do something like:

    public IList<Entity> GetAll<Entity>()
    {
        return _session.CreateCriteria<Entity>().List<Entity>;
    }

但我收到一条错误消息:

but I get an error saying:

The type 'Entity' must be a reference type in order to use it as parameter 'T' in the generic type or method 'NHibernate.ISession.CreateCriteria<T>()'

以下是我的DAL设计供参考:我应该使用泛型来简化我的DAL?

Here's my DAL design for reference: Should I use generics to simplify my DAL?

推荐答案

CreateCriteria 方法要求您使用引用类型-在DAL方法上添加约束:

CreateCriteria method requires you to use reference types - add constraint on your DAL method:

public IList<Entity> GetAll<Entity>()
     where Entity : class
{
    return _session.CreateCriteria<Entity>().List<Entity>();
}

这自然意味着您传递给此方法的任何 Entity 类型都必须是引用类型.

This naturally implies that any Entity type you pass to this method must be a reference type.

我还建议仅命名通用类型参数 TEntity - Entity 有点令人困惑(因为这是实体基类的完美称呼).

I also suggest naming your generic type parameter TEntity - Entity alone is a bit confusing (as it's perfectly fine name for say, entity base class).

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