C# 无法创建类型的常量值 此上下文中仅支持原始类型或枚举类型

Unable to create a constant value of type Only primitive types or enumeration types are supported in this context

提问by user2515186

I am getting this error for the query below

我在下面的查询中收到此错误

Unable to create a constant value of type API.Models.PersonProtocol. Only primitive types or enumeration types are supported in this context

无法创建类型为 的常量值API.Models.PersonProtocol。此上下文中仅支持原始类型或枚举类型

ppCombinedbelow is an IEnumerableobject of PersonProtocolType, which is constructed by concat of 2 PersonProtocollists.

ppCombined下面是 的一个IEnumerable对象PersonProtocolType，它由 2 个PersonProtocol列表的 concat 构造而成。

Why is this failing? Can't we use LINQ JOINclause inside of SELECTof a JOIN?

为什么这会失败？我们不能JOIN在SELECTa 中使用 LINQ子句JOIN吗？

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
                 })
        });

采纳答案by Slauma

This cannot work because ppCombinedis a collection of objects in memory and you cannot join a set of data in the database with another set of data that is in memory. You can try instead to extract the filtered items personProtocolof the ppCombinedcollection in memory afteryou have retrieved the other properties from the database:

这ppCombined是行不通的，因为它是内存中的对象集合，您无法将数据库中的一组数据与内存中的另一组数据连接起来。你可以尝试，而不是提取经过滤项personProtocol中的ppCombined集合中的内存后，您检索从数据库中其他属性：

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()
        });

回答by Roelant

Don't know if anyone searches for this. I had the same problem. A select on the query and then doing the where (or join) and using the select variable solved the problem for me. (problem was in the collection "Reintegraties" for me)

不知道有没有人搜索这个。我有同样的问题。对查询进行选择，然后执行 where（或连接）并使用 select 变量为我解决了问题。（问题出在我的“重新整合”系列中）

query.Select(zv => new
            {
                zv,
                rId = zv.this.Reintegraties.FirstOrDefault().Id
            })
            .Where(x => !db.Taken.Any(t => t.HoortBijEntiteitId == x.rId
                                             && t.HoortBijEntiteitType == EntiteitType.Reintegratie
                                             && t.Type == TaakType))
            .Select(x => x.zv);

hope this helps anyone.

希望这可以帮助任何人。

回答by Colin

In my case, I was able to resolve the issue by doing the following:

就我而言，我能够通过执行以下操作来解决该问题：

I changed my code from this:

我从这个改变了我的代码：

var r2 = db.Instances.Where(x => x.Player1 == inputViewModel.InstanceList.FirstOrDefault().Player2 && x.Player2 == inputViewModel.InstanceList.FirstOrDefault().Player1).ToList();

To this:

对此：

var p1 = inputViewModel.InstanceList.FirstOrDefault().Player1;
var p2 = inputViewModel.InstanceList.FirstOrDefault().Player2;
var r1 = db.Instances.Where(x => x.Player1 == p1 && x.Player2 == p2).ToList();

回答by khaled saleh

Just add AsEnumerable() andToList() , so it looks like this

只需添加 AsEnumerable() 和ToList() ，所以它看起来像这样

db.Favorites
    .Where(x => x.userId == userId)
    .Join(db.Person, x => x.personId, y => y.personId, (x, y).ToList().AsEnumerable()

ToList().AsEnumerable()

回答by James Perih

It's worth adding, since the OP's code sample doesn't provide enough context to prove otherwise, but I received this error as well on the following code:

值得补充的是，因为 OP 的代码示例没有提供足够的上下文来证明否则，但我在以下代码中也收到了此错误：

public RetailSale GetByRefersToRetailSaleId(Int32 refersToRetailSaleId)
{
    return GetQueryable()
        .FirstOrDefault(x => x.RefersToRetailSaleId.Equals(refersToRetailSaleId));
}

Apparently, I cannot use Int32.Equalsin this context to compare an Int32 with a primitive int; I had to (safely) change to this:

显然，我不能Int32.Equals在这种情况下使用将 Int32 与原始 int 进行比较；我不得不（安全地）更改为：

public RetailSale GetByRefersToRetailSaleId(Int32 refersToRetailSaleId)
{
    return GetQueryable()
      .FirstOrDefault(x => x.RefersToRetailSaleId == refersToRetailSaleId);
}