Question

我有以下课程：

public class customRequest
{
    public string REQ_STATUS { get; set; }
    public DateTime REQ_RVE_DATnTIM { get; set; }
    public string EMP_FNAME { get; set; }
    public string EVE_CAT_NAME { get; set; }
    public string EVE_NAME { get; set; }
}

以及SQL内部联接查询，它在SQL中完美运行：

SELECT req.REQ_STATUS,req.REQ_RVE_DATnTIM, emp.EMP_FNAME, cat.EVE_CAT_NAME,eve.EVE_NAME from REQUESTS req inner join Employees emp on req.REQ_EMP_ID = emp.ID inner join EVENTCATEGORIES cat on req.REQ_EVE_CAT_ID = cat.ID inner join EVENTS eve on req.REQ_EVE_NAM_ID = eve.ID order by req.REQ_STATUS

我想要的是将它转换为我的linq到sql查询。下面是我试过的Linq to Sql查询：

slist = from req in db.REQUESTS
                    join emp in db.Employees on req.REQ_EMP_ID equals emp.ID
                    join cat in db.EVENTCATEGORIES on req.REQ_EVE_CAT_ID equals cat.ID
                    join eve in db.EVENTS on req.REQ_EVE_NAM_ID equals eve.ID
                    select new { req.REQ_STATUS, req.REQ_RVE_DATnTIM, emp.EMP_FNAME, cat.EVE_CAT_NAME, eve.EVE_NAME };
        return slist.ToList();

但它在上次加入时显示错误：

错误1无法将类型'System.Linq.IQueryable'隐式转换为'System.Collections.Generic.List'。存在显式转换（您是否缺少演员表？）H：\ 64bit \ Project Finalizing \ Blue_Pumpkin \ Blue_Pumpkin \ Admin \ Requests.aspx.cs 32 25 Blue_Pumpkin

Answer 1

我假设您希望从查询中返回customRequest类。您需要将select语句显式转换为customRequest ..

slist = (from req in db.REQUESTS
                    join emp in db.Employees on req.REQ_EMP_ID equals emp.ID
                    join cat in db.EVENTCATEGORIES on req.REQ_EVE_CAT_ID equals cat.ID
                    join eve in db.EVENTS on req.REQ_EVE_NAM_ID equals eve.ID
                    select new customRequest {
                       REQ_STATUS = req.REQ_STATUS, req.REQ_RVE_DATnTIM, 
                       EMP_FNAME = emp.EMP_FNAME, 
                       EVE_CAT_NAME = cat.EVE_CAT_NAME, 
                       EVE_NAME = eve.EVE_NAME }).ToList();
            return slist;

如果恰好需要返回一个列表..你需要将返回类型声明为列表变量并从那里使用ToList()，如果你要返回单行结果集然后你可以使用First()或FirstOrDefault()

Answer 2

您可以将ToList（）与您的linq查询一起使用：

var slist = (from req in db.REQUESTS
                join emp in db.Employees on req.REQ_EMP_ID equals emp.ID
                join cat in db.EVENTCATEGORIES on req.REQ_EVE_CAT_ID equals cat.ID
                join eve in db.EVENTS on req.REQ_EVE_NAM_ID equals eve.ID
                select new select new customRequest {
                   REQ_STATUS = req.REQ_STATUS,
                   REQ_RVE_DATnTIM= req.REQ_RVE_DATnTIM, 
                   EMP_FNAME = emp.EMP_FNAME, 
                   EVE_CAT_NAME = cat.EVE_CAT_NAME, 
                   EVE_NAME = eve.EVE_NAME }){
                   REQ_STATUS = req.REQ_STATUS, req.REQ_RVE_DATnTIM, 
                   EMP_FNAME = emp.EMP_FNAME, 
                   EVE_CAT_NAME = cat.EVE_CAT_NAME, 
                   EVE_NAME = eve.EVE_NAME }).ToList();

如何将sql内部连接查询转换为linq到sql查询并转换为列表

2 个答案: