GroupBy在lambda表達式中

Question

from x in myCollection 
    group x by x.Id into y 
    select new { 
     Id = y.Key, 
     Quantity = y.Sum(x => x.Quantity) 
    }; 

您會如何將上述內容寫入lambda表達式？我卡在group into部分。

Answer 1

查詢延續（SELECT ... INTO和組...成，但不加入...進入）等價於剛分手了查詢表達式。所以我喜歡把你的例子爲：

var tmp = from x in myCollection 
      group x by x.Id; 
var result = from y in tmp 
      select new { 
       Id = y.Key, 
       Quantity = y.Sum(x => x.Quantity) 
      }; 

改變那些進入點符號：

var tmp = myCollection.GroupBy(x => x.Id); 
var result = tmp.Select(y => new { 
       Id = y.Key, 
       Quantity = y.Sum(x => x.Quantity) 
      }); 

然後，你可以將它們合併回：

var tmp = myCollection.GroupBy(x => x.Id) 
         .Select(y => new { 
           Id = y.Key, 
           Quantity = y.Sum(x => x.Quantity) 
           }); 

一旦你的工作是什麼C＃編譯器使用查詢表達式，其餘部分相對簡單:)

Answer 2

myCollection 
    .GroupBy(x => x.Id) 
    .Select(x => 
     new 
     { 
      Id = x.Key, 
      Quantity = x.Sum(y => x.Quantity 
     }); 

Answer 3

myCollection.GroupBy(x => x.Id) 
      .Select(y => new { 
           Id = y.Key, 
           Quantity = y.Sum(x => x.Quantity) 
          }); 

Answer 4

 var mostFrequent = 
      lstIn.Where(i => !string.IsNullOrEmpty(i)) 
       .GroupBy(s => s) 
       .OrderByDescending(g => g.Count()) 
       .Select(s => s.Key) 
       .FirstOrDefault(); 

Answer 5

因此，對於這裏的大多數答案，每個人似乎都在處理從組的計數中獲取Id的簡單對象，以及作爲group.Key的Key本身。

雖然這可能是這個主要用途。沒有真正滿足我的需求。

對於我自己的情況，我基本上想通過某些對象屬性進行分組，然後從該組中獲取特定對象。這是一個示例代碼。

using System; 
using System.Collections.Generic; 
using System.Linq; 

public class Program 
{ 
    public static void Main() 
    { 
     Console.WriteLine("Hello World"); 
     var response = new List<ResponseClass>(); 
      var listOfStudents = new List<Student>(); 
      // Insert some objects into listOfStudents object. 
      listOfStudents.GroupBy(g => g.Class).ToList() 
       .ForEach(g => response.Add(g.OrderByDescending(s => s.CreatedOn).Select(a => 
       new ResponseClass 
       { 
        SName = a.StudentName, 
        SAge = a.Age, 
        SClass = a.Class, 
        SCreatedOn = a.CreatedOn, 
        RandomProperty = Guid.NewGuid().ToString() 
       }) 
       .First())); 

       Console.WriteLine("This compiles and should work just fine"); 
    } 
    class Student 
    { 
     public string StudentName { get; set; } 
     public int Age { get; set; } 
     public string Class { get; set; } 
     public DateTime CreatedOn { get; set; } 
    } 

    class ResponseClass 
    { 
     public string SName { get; set; } 
     public int SAge { get; set; } 
     public string SClass { get; set; } 
     public DateTime SCreatedOn { get; set; } 
     public string RandomProperty { get; set; } 
    } 
} 

如果你寧願使用foreach循環（我喜歡的λ，因爲我覺得它更容易閱讀），但是如果你這樣做，你能做到這一點，像這樣。

希望這可以幫助別人。 :)