組總和的LINQ to SQL

Question

我想總結基於studentId

List<Timeliness> studentData = (from redt in RoundEndDateTable.AsEnumerable() 
               join st in Activity on redt.Field<string>("strActivityName") equals st.ActivityName 
               orderby (DateTime.Parse(redt.ItemArray[1].ToString()) - DateTime.Parse(st.RewardType.ToString())) descending 
               select new Timeliness 
               { 

                DayDifference = int.Parse((DateTime.Parse(redt.ItemArray[1].ToString()) - DateTime.Parse(st.RewardType)).ToString().Split('.')[0]), 
                StudentId = st.AssociateId.ToString() 
               }).ToList(); 

的DayDifference我嘗試下面的代碼

var groupedCustomerList = studentData 
    .GroupBy(u => u.StudentId) 
    .Select(grp => grp.ToList()) 
    .ToList(); 

不知道在哪裏可以在上面的代碼

添加總和

在第一個查詢中，我獲取學生ID重複

例如：

[0] - studentid:123 DayDifference:16 
[1] - studentid:123 DayDifference:8 
[2] - studentid:121 DayDifference:16 

我需要

[0] - studentid:123 DayDifference:24 
[2] - studentid:121 DayDifference:16 

Answer 1

這應該創建一個匿名類型的集合，包含每個 「密鑰」（StudentId），以及 「DayDifference」 值的每個StudentId總和：

var result = 
    studentData.GroupBy(u => u.StudentId) 
       .Select(grp => new { Id = grp.Key, TotalDiff = grp.Sum(x => x.DayDifference) }) 
       .ToList();