Linq将sql选入一个新类

Question

我的问题是,当我有2个查询后,第一个不填充CampaignID属性,而第二个查询.这是我的代码;

查询1 ;

var query = from c in _context.MCTargets
                    where c.TargetDateFrom==d1 && c.TargetDateTo<=d2
                    group c by c.MarketingCampaignID into g
                    select new MSReport{
                        CampaignID = g.Key, // CampaignID is not populated here.
                               StartDate = d1,
                               EndDate = d2
                    };

查询2 ;

var query2 = from c in _context.MCTargets
                    where c.TargetDateFrom == d1 && c.TargetDateTo <= d2
                    group c by c.MarketingCampaignID into g
                    select new 
                    {
                        CampaignID = g.Key,
                        StartDate = d1,
                        EndDate = d2
                    };

MSReport.cs

public class MSReport
{
    public DateTime StartDate { get; set; }
    public DateTime EndDate { get; set; }
    public int CampaignID { get; set; }

    public MSReport()
    {
        // CampaignID = 0 here  
        // doing something with CampaignID here like setting some calculated properties.    
    }  
 }

提前谢谢,抱歉我的解释不好.

Answer 1

使用对象初始值设定项语法时,初始化程序中指定的值将在执行对象的构造函数后设置.如果需要将要填充的值用于构造函数,则必须添加构造函数的形式,该值将值作为参数并填充字段或属性本身.

在你的班上:

public MSReport(int campaignID, DateTime startDate, DateTime endDate)
{
       CampaignID = campaignID;
       StartDate = startDate;
       EndDate = endDate;

       // doing something with CampaignID here like setting some calculated properties.
}     

在您的查询中:

new MSReport(g.Key, d1, d2)

这适用于Linq to SQL和Linq to Objects.对于Linq to Entities,必须采取不同的方法.

您可以使用匿名对象执行查询,然后运行第二个查询将其转换为您想要的对象:

var query = from c in _context.MCTargets
                  where c.TargetDateFrom==d1 && c.TargetDateTo<=d2
                  group c by c.MarketingCampaignID into g
                  select new {
                      CampaignID = g.Key,
                      StartDate = d1,
                      EndDate = d2
                  };

IEnumerable<MSReport> queryMSReports = from item in query.AsEnumerable()
                                       select new MSReport(item.CampaignID, item.StartDate, item.EndDate);

这会将对象从Linq断开连接到实体,并允许您使用具有参数的构造函数创建所需的对象.有关详细信息,请参阅MSDN上的LINQ to Entites'无参数构造函数'错误论坛帖子.

您的另一个选择是使用您的MSReport类和对象初始化程序语法进行查询,然后在您的类上有一个您必须稍后调用的Calculate方法.

Answer 2

也许默认构造函数在参数初始化之前运行？尝试使用您的参数在 MSReport 中添加构造函数并进行调试。

    public class MSReport
    {
        public DateTime StartDate { get; set; }
        public DateTime EndDate { get; set; }
        public int CampaignID { get; set; }

        public MSReport(int campaginId, ....)
        {
// use and initialize camaginId here
        }  
     }

并做：

select new MSReport(g.Key) {
                               StartDate = d1,
                               EndDate = d2
                    }