LINQ查詢與聚合在哪裏條款

Question

我有一個包含開始/結束日期的對象列表。如果當前記錄存在，我試圖找到當前記錄，或者如果失敗了，最近過期的記錄。

• 目前被定義爲在過去，無論是空的最後一天（開放式）或日期和結束日期在未來的一個非空的開始日期。
• 最近過期可能被定義爲沒有最近結束日期的當前時間段。從理論上講，每個對象表示的時間跨度不應該重疊，所以我認爲最近的結束日期對於最近的過期已經足夠了。

我試圖按照this answer到類似的問題，但我似乎無法找到正確的條目時有列表是未來名花有主項（即有開始/結束日期，其中兩者都在未來）。

我的嘗試如下，其中a是開始日期，b是結束日期，c是調試時使用的指示符，d用於定義列表項類型。我只對感興趣的類型2.

[TestMethod] 
    public void TestRetrievalOfListElements() 
    { 

     var baseDate = DateTime.Now; 

     var list = new[] 
     { 
      new { a = new DateTime?(baseDate.AddDays(-90)), 
        b = new DateTime?(baseDate.AddDays(-60)), c = "LongExpired", d=1 }, 
      new { a = new DateTime?(baseDate.AddDays(-190)), 
        b = new DateTime?(baseDate.AddDays(-160)), c = "LongestExpired", d=1 }, 
      new { a = new DateTime?(baseDate.AddDays(-59)), 
        b = new DateTime?(baseDate.AddDays(+20)), c = "Current", d=1 }, 
      new { a = new DateTime?(baseDate.AddDays(-159)), 
        b = new DateTime?(baseDate.AddDays(-91)), c = "LongerExpired", d=1 }, 
      new { a = new DateTime?(baseDate.AddDays(-90)), 
        b = new DateTime?(baseDate.AddDays(-60)), c = "LongExpired", d=2 }, 
      new { a = new DateTime?(baseDate.AddDays(-190)), 
        b = new DateTime?(baseDate.AddDays(-160)), c = "LongestExpired", d=2 }, 
      new { a = new DateTime?(baseDate.AddDays(-59)), 
        b = new DateTime?(baseDate.AddDays(+20)), c = "Current", d=2 }, 
      new { a = new DateTime?(baseDate.AddDays(-159)), 
        b = new DateTime?(baseDate.AddDays(-91)), c = "LongerExpired", d=2 }, 
      new { a = new DateTime?(baseDate.AddDays(+21)), 
        b = new DateTime?(baseDate.AddDays(+60)), c = "Future", d=2 }, 

     }.ToList(); 

     // The following isn't really right either as it doesn't take into account 
     // whether d is of type 1 or 2 either. Not sure how to combine aggregates and 
     // other conditions ie MIN and of type 2 

     var oldestEntry = (from x in list 
          where x.a == list.Min(d => d.a) && 
          x.d == 2 
          select x).FirstOrDefault(); 

     Assert.IsTrue(oldestEntry.a == baseDate.AddDays(-190), "Expected oldest date to be 190 days earlier than today"); 

     var latestEntryThatIsntInTheFuture = 
          (from x in list 
          from y in list.Where(z => z.d == 2 && z.a <= baseDate) 
          where x.a == list.Max(d => d.a) && 
          x.d == 2 
          select x).FirstOrDefault(); 

     Assert.IsTrue(latestEntryThatIsntInTheFuture.a == baseDate.AddDays(-59), "Expected latest date that isn't in the future to be 59 days earlier than today"); 



    } 

Answer 1

這可能是你在找什麼：

// Find the current which is defined as a non null start date that is in the past 
// and either a null end date (open ended) or and end date in the future. 
var now = DateTime.Now; 
var current = list 
    .Where(x => x.a.HasValue && x.a.Value < now && (!x.b.HasValue || x.b.Value > now)) 
    .FirstOrDefault(); 
if (current == null) 
{ 
    //Then find most recently expired 
    current = list.Where(x => x.b.HasValue) 
        .OrderByDescending(x => x.b) 
        .FirstOrDefault(); 
} 

所以你並不需要在查詢聚集在我看來。