Linq空对象必须有一个值.. net 6和EF Core中的错误

Question

我有这样的代码:

IQueryable<WinnerClassExtend> dataList = 
from Class in _context.AllClass
join Winner in _context.AllWinners on Class.ClassId equals Winner.ClassId
let Extend = new { Winner, Class }
group Extend by Class.ClassId into Group
select new WinnerClassExtend
{
Class = Group.Select(x => x.Class).FirstOrDefault(),
NumberOfWinnerinClass = Group.Select(x => x.Winner).Count()
};
var count = dataList.Count();

我试着得到IQueryable的计数就像我在。net框架和实体框架中所做的那样，但它似乎不工作…我得到一个错误

可空对象必须有一个值

我不知道有什么问题，我也试着用这个，

IQueryable<WinnerClassExtend> dataList =
_context.AllClass
.Join(_context.AllWinners , Class => Class.ClassId, Winner => Winner.ClassId, (Class, Winner) => new { Class, Winner })
.GroupBy(x => new { x.Class.ClassId })
.Select(x => new WinnerClassExtend
{
Class = x.Select(i => i.Class).FirstOrDefault(),
NumberOfWinnerinClass = x.Select(i => i.Winner).Count()
});
var count = dataList.Count();

返回相同的。

我读过这个错误的一些问题，但它似乎不像同样的问题，有人能帮助吗?

Answer 1

尝试以下查询:

var dataList = 
from Class in _context.AllClass
select new WinnerClassExtend
{
Class = Class,
NumberOfWinnerinClass = _context.AllWinners.Where(w => w.ClassId == Class.ClassId).Count()
};
var count = dataList.Count();

或者另一个

var grouped = 
from Winner in _context.AllWinners
group Winner by Class.ClassId into Group
select new 
{
ClassId = Group.Key,
NumberOfWinnerinClass = Group.Count()
};
var dataList = 
from Class in _context.AllClass
join g in grouped on ClassId.ClassId equals g.ClassId
select new WinnerClassExtend
{
Class = Class,
NumberOfWinnerinClass = g.NumberOfWinnerinClass
};
var count = dataList.Count();

在两种情况下比较执行计划。

Answer 2

我觉得你需要一个左外连接:

var dataList =
from c in _context.AllClass
join w in _context.AllWinners on c.ClassId equals w.ClassId into winners
select new WinnerClassExtend
{
Class = c,
NumberOfWinnerinClass = winners.Count()
};
var count = dataList.Count();