Question

我有以下代码

return (EseshEntities.Current.Users.Select(u => new { Comunity = u.Apartment.Building.District.City })).ToList();

如果建筑物是空的那么我们得到了excpetion cos无法访问null的属性吧？但是在实体中有任何选项，只有null的excpetion返回，所以如果属性不存在然后返回null？

Answer 1

这是怎么回事？

return (EseshEntities.Current.Users.Select(u => new { Comunity = u.Apartment.Building ==null?null:u.Apartment.Building.District.City })).ToList();

Answer 2

您可以尝试这样的事情：

var list = (from u in EseshEntities.Current.Users
            where u.Apartment.Building != null
            select new { Comunity = u.Apartment.Building.District.City }).ToList();
return list;

或者你可以试试这个：

return (EseshEntities.Current.Users.Select(u => new { Comunity = u.Apartment.ToInstance().Building.ToInstance().District.ToInstance().City })).ToList(); 

public static T ToInstance<T>(this T self)
{
    T mySelf = default(T);

    if (self != null)
        mySelf = self;
    else
        mySelf = Activator.CreateInstance<T>();

    return mySelf;
}

如果singleResult抛出异常，则强制实体返回null

2 个答案: