Question

这是我的SQL查询：

select  AU.Last_Name       
From t_Batch B
left join (select distinct customer_id, subbatchtypeid
from t_allocationconfig where isactive='Y')
AA on AA.customer_id = B.Customer_id and
AA.SubBatchtypeid = B.SubBatchtypeid
left join t_batchallocation BA on BA.Batchid = B.Batchid 
left join VW_users AU on AU.User_ID = BA.User_id;

我想将此SQL转换为Linq但我收到错误：

User_ID obecjt引用不是对象的实例

我该如何解决这个问题？

 IList<Batch> result = new List<Batch>();
 var batch = _batchservice.GetBatch().Where(r => r.ISACTIVE == "Y");
 var batchallocation = _batchallocationservice.GetBatchAllocation();
   var allocationconfig=_allocationconfigservice.GetAllocationConfig().Where(r=>r.ISACTIVE=="Y");
  result = (from  B in batch                          
                  join ac in (
                                  from cl in allocationconfig  where cl.ISACTIVE=="Y"
                                  select new {cl.CUSTOMER_ID,cl.SUBBATCHTYPEID}
                          ) on new  {B.CUSTOMER_ID,B.SUBBATCHTYPEID} 
                          equals new {ac.CUSTOMER_ID,ac.SUBBATCHTYPEID}  into aac 
                          from v in aac.DefaultIfEmpty(null)

                          join BA in batchallocation on B.BATCHID equals BA.BATCHID into x 
                          from y in x.DefaultIfEmpty()  

                          join AU in users on  y.USER_ID equals AU.USER_ID  into g 
                          from h in g.DefaultIfEmpty() 

                          select new Batch 
                          {
                           AllocatedUserName=h.LAST_NAME                                
                          }).ToList();

Answer 1

我想问题是x.DefaultIfEmpty返回一个空值，然后在连接访问users表（y.USER_ID）时导致错误。

DefaultIfEmpty子句有一个超载，它接受默认类型，例如

x.DefaultIfEmpty<BatchAllocation>(new BatchAllocation () { USER_ID = "-1", .... })

您可以尝试返回虚拟批处理分配默认值，其USER_ID属性为-1（假设没有用户ID可以为-1）这将返回h作为null，然后您可以过滤掉空值通过主查询中的位置。（其中h！= null＆amp;＆amp; h.LAST_NAME！= null）

以下示例 -

class Program
{
    static void Main(string[] args)
    {

        LeftOuterJoinExample();

        Console.ReadLine();

    }

    class Person
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
    }

    class Pet
    {
        public string Name { get; set; }
        public Person Owner { get; set; }
    }

    public static void LeftOuterJoinExample()
    {
        Person magnus = new Person { FirstName = "Magnus", LastName = "Hedlund" };
        Person terry = new Person { FirstName = "Terry", LastName = "Adams" };
        Person charlotte = new Person { FirstName = "Charlotte", LastName = "Weiss" };
        Person arlene = new Person { FirstName = "Arlene", LastName = "Huff" };

        Pet barley = new Pet { Name = "Barley", Owner = terry };
        Pet boots = new Pet { Name = "Boots", Owner = terry };
        Pet whiskers = new Pet { Name = "Whiskers", Owner = charlotte };
        Pet bluemoon = new Pet { Name = "Blue Moon", Owner = terry };
        Pet daisy = new Pet { Name = "Daisy", Owner = magnus };

        // Create two lists.
        List<Person> people = new List<Person> { magnus, terry, charlotte, arlene };
        List<Pet> pets = new List<Pet> { barley, boots, whiskers, bluemoon, daisy };

        var query = from person in people
                    join pet in pets on person equals pet.Owner into gj
                    from subpet in gj.DefaultIfEmpty<Pet>(new Pet() { Name = "", Owner = null })
                    where subpet != null && subpet.Name != String.Empty
                    select new { person.FirstName, PetName = (subpet == null ? String.Empty : subpet.Name) };

        foreach (var v in query)
        {
            Console.WriteLine("{0,-15}{1}", v.FirstName + ":", v.PetName);
        }
    }
}

Answer 2

在使用之前，您需要检查y是否为空

如何解决obejct引用不是linq左连接查询中的对象实例？

2 个答案: