條件數學

Question

我有以下選擇：

q.Select(c => new SomeObject 
    { 
     Invoices = c.SomeList.Sum(sl => sl.SomeValue), 
     Payments = c.OtherList.Sum(ol => ol.OtherValue), 
     Balance = ??? // Should be Payments - Invoices 
    }); 

我想計算，並與對象的其餘部分一起返回平衡，但無法弄清楚如何做到這裏面的select語句。謝謝！

Answer 1

無需匿名類型也多次迭代。只需將您的Select表達式轉換爲代碼塊，然後使用局部變量。

q.Select(c => 
       { 
        var invoices = c.SomeList.Sum(sl => sl.SomeValue); 
        var payments = c.OtherList.Sum(ol => ol.OtherValue); 
        // do stuff 
        return new SomeObject 
        { 
         Invoices = invoices, 
         Payments = payments, 
         Balance = payments - invoices 
        }; 
       }); 

由於@juharr建議，你也可以讓Balance只讀屬性，返回Payments - Invoices。如果Balance不需要設置，這是建議的方法。

Answer 2

您可以引入一個匿名類型來保存中間計算，然後從這種類型的創建SomeObject：

q.Select(c => new 
       { Invoices = c.SomeList.Sum(sl => sl.SomeValue), 
       Payments = c.OtherList.Sum(ol => ol.OtherValue) 
       } 
).Select(x => new SomeObject 
       { Invoices = x.Invoices, 
       Payments = x.Payments, 
       Balance = x.Payments - x.Invoices 
       }); 

Answer 3

無法裏面你需要一個第二選擇做選擇

using System; 
using System.Collections.Generic; 
using System.Linq; 
using System.Text; 
using System.IO; 

namespace ConsoleApplication1 
{ 
    class Program 
    { 
     static void Main(string[] args) 
     { 
      List<SomeObject> q = new List<SomeObject>(); 

      q.Select(c => new 
      { 
       Invoices = c.SomeList.Sum(), 
       Payments = c.OtherList.Sum(), 

      }).Select(x => x.Payments - x.Invoices); 

     } 
    } 
    public class SomeObject 
    { 
     public int[] SomeList { get; set; } 
     public int[] OtherList { get; set; } 
    } 
}​ 

Answer 4

您還可以使用let條款：

var query= from e in q 
      let invoices= e.SomeList.Sum(sl => sl.SomeValue); 
      let payments = e.OtherList.Sum(ol => ol.OtherValue); 
      select new SomeObject 
         { 
         Invoices = invoices, 
         Payments = payments, 
         Balance = payments - invoices 
         };