C#：接口为ICollection<ILocalized>的ICollection

Question

我正在使用EntitFramework，它生成实体类：

我有这样的课程：

public class Car
{
    //...
    public String Brand { get; set; }
    //...

    public virtual ICollection<CarLocalized> CarLocalizeds { get; set; }
    //...
}

public class CarLocalized :ILocalized
{
    public int LangID { get; set; }

    public Lang Lang { get; set; }
}

public static class Helper {
    public static List<String> GetLangIDList(ICollection<ILocalized> list)
    {
        //I want all the ID of the lang where car is translated for:
        var somethin = list.Select(m => m.LCID_SpracheID.ToString()).ToList();
        return somethin;
    }
}

public class HomeController : Controller
{
    public ActionResult Translated()
    { 
        Car car = db.Cars.Find(2);
        List<String> transletedIDs = Helper.GetLangIDList(car.CarLocalizeds);

        return View(transletedIDs);
    }

}

但现在的问题是

List<String> transletedIDs = Helper.GetLangIDList(car.CarLocalizeds);

不起作用。为什么我不能将签名设置为ICollection并给它一个ICollection，其中CarLocalized实现了签名中所需的接口？

请帮帮我..。

THx

Answer 1

问题是ICollection<T>不是“协变”的。GetLangIDList方法似乎没有修改列表，它只是查询列表。在这种情况下，您可以使用IEnumerable<T>，它是“协变”。

public static List<String> GetLangIDList(IEnumerable<ILocalized> list)
{
    //I want all the ID of the lang where car is translated for:
    var somethin = list.Select(m => m.LCID_SpracheID.ToString()).ToList();
    return somethin;
}

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