无法创建类型的常量值在此上下文中仅支持基本类型或枚举类型

我在下面的查询中收到此错误

无法创建类型的常量值API.Models.PersonProtocol。此上下文中仅支持基本类型或枚举类型

ppCombined下面是一个IEnumerable的对象PersonProtocolType，由 2 连接而成PersonProtocol lists.

为什么会失败？我们不能使用 LINQJOIN里面的子句SELECT of a JOIN?

var persons = db.Favorites
    .Where(x => x.userId == userId)
    .Join(db.Person, x => x.personId, y => y.personId, (x, y) =>
        new PersonDTO
        {
            personId = y.personId,
            addressId = y.addressId,                   
            favoriteId = x.favoriteId,
            personProtocol = (ICollection<PersonProtocol>) ppCombined
                .Where(a => a.personId == x.personId)
                .Select( b => new PersonProtocol()
                 {
                     personProtocolId = b.personProtocolId,
                     activateDt = b.activateDt,
                     personId = b.personId
                 })
        });

这行不通，因为ppCombined是内存中对象的集合，您不能将数据库中的一组数据与内存中的另一组数据连接起来。您可以尝试提取过滤后的项目personProtocol of the ppCombined内存中的集合after您已从数据库中检索了其他属性：

var persons = db.Favorites
    .Where(f => f.userId == userId)
    .Join(db.Person, f => f.personId, p => p.personId, (f, p) =>
        new // anonymous object
        {
            personId = p.personId,
            addressId = p.addressId,   
            favoriteId = f.favoriteId,
        })
    .AsEnumerable() // database query ends here, the rest is a query in memory
    .Select(x =>
        new PersonDTO
        {
            personId = x.personId,
            addressId = x.addressId,   
            favoriteId = x.favoriteId,
            personProtocol = ppCombined
                .Where(p => p.personId == x.personId)
                .Select(p => new PersonProtocol
                {
                    personProtocolId = p.personProtocolId,
                    activateDt = p.activateDt,
                    personId = p.personId
                })
                .ToList()
        });