為什么Equals沒有按預期工作

Question

在閱讀了大約40-50個問題和答案（我已經嘗試了很多事情）之后，盡管所有答案都略有偏離，但我仍然無法理解這是怎么回事：

IEnumerable<string> textSegs = from cd in cds 
      where cd.Artist.Equals("Dream Theater") 
      select cd.Artist;

foreach(string s in textSegs)
   Console.Write("\nTrack: " + s);

//This outputs:  'Track: Dream Theater'

現在，關於另一部分：

IEnumerable<string> textSegs = from seg in myXMLDoc.Descendants("name")
   where ((string)seg).Equals("Dream Theater") 
   select (string)seg;
//This puts: exactly what I need

然后我想這將達到神奇的效果：

IEnumerable<string> textSegs = from seg in myXMLDoc.Descendants("name")
     where ((string)seg).Equals(from cd in cds 
                                where cd.Artist.Equals("Dream Theater") 
                                select cd.Artist)
     select (string)seg;

//This outputs: Everything that is inside the XMLDoc (no filter applied)

至於該代碼所采用的格式。恐怕它必須像這樣（賦值）。 我嘗試將子查詢轉換為字符串，但它告訴我：

Cannot convert type 'IEnumerable<string>' to 'string'

任何幫助表示贊賞！

Answer 1

聽起來像你正試圖這樣做：

IEnumerable<string> textSegs = 
     from seg in myXMLDoc.Descendants("name")
     where ((string)seg).Equals(
         (from cd in cds 
          where cd.Artist.Equals("Dream Theater") 
          select cd.Artist).First())
     select (string)seg;

或者這個，這更容易閱讀：

IEnumerable<string> textSegs = 
     from seg in myXMLDoc.Descendants("name")
     let artist = 
         (from cd in cds 
          where cd.Artist.Equals("Dream Theater") 
          select cd.Artist).First()
     where ((string)seg).Equals(artist)
     select (string)seg;

Answer 2

您本質上需要詢問一組數據是否包含另一組數據：

var artistQuery = from cd in cds 
                  where cd.Artist.Equals("Dream Theater") 
                  select cd.Artist;

IEnumerable<string> textSegs = from seg in myXMLDoc.Descendants("name")
                               where artistQuery.Contains((string) seg)
                               select (string)seg;

我已經分解了上面的每個查詢以顯示步驟。 你也可以把它寫成一個聲明：

IEnumerable<string> textSegs = from seg in myXMLDoc.Descendants("name")
                               where (from cd in cds 
                                      where cd.Artist.Equals("Dream Theater") 
                                      select cd.Artist).Contains((string) seg)
                               select (string)seg;

Answer 3

嘗試加入，我想不出更干凈的方法：

from seg in myXMLDoc.Descendants("name")
join cd in cds
    on (string)seg equals cd.Artist 
where cd.Artist.Equals("Dream Theater")
select (string)seg;

還沒有編譯，所以可能有一個或兩個錯誤，但可以肯定地在這些地方:)

Answer 4

Equals右側的“from cd”將返回符合條件的所有結果，而不僅僅是一個。