如何在C#中将指定Linq查询转换为Lambda表达式？

转换后的Lambda代码 #

var domainModel = _ctx.Hospital
    .Join(
        // 先对SickRoom按HospitalId分组并计算聚合值
        _ctx.SickRoom
            .GroupBy(x => x.HospitalId)
            .Select(group => new
            {
                HospitalId = group.Key,
                NumberOfFloors = group.Max(item => item.FloorId),
                NumberOfRooms = group.Count()
            })
            .OrderBy(agg => agg.HospitalId),
        // 左表（Hospital）的连接键
        hospital => hospital.HospitalId,
        // 右表（聚合后的SickRoom）的连接键
        roomAggregate => roomAggregate.HospitalId,
        // 投影最终结果
        (hospital, roomAggregate) => new
        {
            HospitalId = hospital.HospitalId,
            City = hospital.City,
            HospitalImage = hospital.HospitalImage,
            NumberOfFloors = roomAggregate.NumberOfFloors,
            NumberOfApartments = roomAggregate.NumberOfRooms
        }
    )
    .ToList();

转换说明 #

1. 原Linq中的双from语法属于隐式内连接，对应Lambda中的Join方法，逻辑和你提供的SQL内连接完全一致
2. 先对SickRoom执行分组聚合：按HospitalId分组，计算最大楼层数（Max(FloorId)）和房间总数（Count()），和原Linq的子查询逻辑完全匹配
3. 通过HospitalId将Hospital表和聚合后的SickRoom结果集连接，最后投影出需要的字段
4. 保留了原Linq中的OrderBy排序和ToList()转换为集合的操作

内容的提问来源于stack exchange，提问作者Anarch