c＃linq獲取當前值和以前的值

Question

在下面的linq查詢中，我想添加一個屬性，獲取以前的Department以及當前的Department。 回報以下LINQ查詢：

EffectiveDate EmployeeID Department 
11/4/2012  10   0000 
1/14/2013  10   9121 
2/2/2016  10   9123 

我怎麼能顯示前Department旁邊當前Department這樣嗎？

EffectiveDate EmployeeID Department PreviousDepartment 
11/4/2012  10   0000  null 
1/14/2013  10   9121  0000 
2/2/2016  10   9123  9121 

下面是當前查詢

var users = from s in userTable 
      where s.EmployeeID == "10" 
      group new {s} by new { s.EmployeeID, s.Department} into g 
      select new 
      { 
       EffectiveDate = g.Max(m => m.s.EffectiveDate), 
       EmployeeID = g.Key.EmployeeID, 
       Department = g.Key.Department 
       //PreviousDepartment = ??? 
      }; 

感謝您的幫助

Answer 1

一種方式來做到這一點是存儲在內存中的數據，然後修改它像這樣：

var users = from s in userTable 
      where s.EmployeeID == "10" 
      group new {s} by new { s.EmployeeID, s.Department} into g 
      select new MyClass 
      { 
       EffectiveDate = g.Max(m => m.s.EffectiveDate), 
       EmployeeID = g.Key.EmployeeID, 
       Department = g.Key.Department 
       PreviousDepartment = null 
      }; 

var result = users.ToList(); 

for(int i = 1; i < result.Count; i++) 
{ 
    result[i].PreviousDepartment = result[i-1].Department; 
} 

請注意，該代碼正在生成MyClass的新實例，而不是匿名因爲匿名類型屬性是隻讀的。確保你創建了具有正確屬性的類。

Answer 2

如果使用交互式擴展（NuGet包IX-主），你可以做到以下幾點：

var users = 
    userTable 
    .Where(x => x.EmployeeID == "100") 
    .GroupBy(x => new { x.EmployeeID, x.Department }) 
    .Select(x => new 
     { 
      EffectiveDate = x.Max(m => m.EffectiveDate), 
      EmployeeID = x.Key.EmployeeID, 
      Department = x.Key.Department 
     }) 
    .OrderBy(x => x.EffectiveDate) 
    .AsEnumerable() 
    .Scan(
     new 
     { 
      EffectiveDate = default(DateTime), 
      EmployeeID = default(string), 
      Department = default(string), 
      PreviousDepartment = default(string) 
     }, 
     (a, x) => new 
     { 
      EffectiveDate = x.EffectiveDate, 
      EmployeeID = x.EmployeeID, 
      Department = x.Department, 
      PreviousDepartment = a.Department 
     }); 

的好處是，你堅持的聲明性編程風格。缺點是它比內存中的數據操作更詳細，可能更難讀取。哦，你可能需要安裝一個NuGet包。對我而言，這不是一個問題，因爲我不認爲我沒有添加Ix-Main就開始任何項目。

Ix-Main，如果沒有它，永遠不要離開家。

Answer 3

您可以使用LINQ中的Zip方法生成當前對和先前對。一個缺點是，它會枚舉輸入兩次（這可能是一個問題，如果它來自數據庫），但另一方面，它不會將整個輸入保存在內存中，並且它是懶惰的（即，您沒有消耗全部）。

例如，使用INT值作爲演示：

[TestMethod] 
public void CurrentAndPrevious() 
{ 
    var input = new int[] { 1, 2, 3, 4, 5 }; 

    var output = Enumerable.Repeat(0,1) // an initial zero value 
     .Concat(input)     // followed by the list 
              // zipped with the list 
     .Zip(input, (x, y) => new {current = y, previous = x}); 

    // a test that passes (using FluentAssertions syntax) 
    string.Join(",", output.Select(x => $"({x.current},{x.previous})")) 
     .Should().Be("(1,0),(2,1),(3,2),(4,3),(5,4)"); 
} 

Answer 4

甲方法語法實現它。

var result = userTable.Select((x, i) => { Department j = null ; if (i>0) j = userTable.ElementAt(i-1).Department; return new { x.EffectiveDate , x.EmployeeID , x.Department , j};});