程式隨筆
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【Linq】Multiple Column with OrderBy

Posted on Edited on Disqus:

用Linq的好處是強型別，讓你在寫程式的時後不會因為Key錯字，但也有些衍伸的問題導致程式會寫得很醜，以前最常碰到的例子就是，當排序可能依據【多欄位】升降冪，程式就會又臭又長。

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void Main()
{
 //要用來排序的欄位
 OrderByColumnEnum OrderByColumn = OrderByColumnEnum.Pin;
 //排序的方法
 OrderByEnum OrderBy = OrderByEnum.ASC;

 //準備資料
 var BuildingList = new List<Building>();
 for (int i = 0; i < 5; i++)
 {
  BuildingList.Add(new Building 
  {
   Age = 1 * (i+1),
   Pin = 1 * (i+1),
   CaseDate = DateTime.Now.AddDays(i +1)
  });
 }

 //排序欄位
 switch (OrderByColumn)
 {
  case OrderByColumnEnum.Pin:
   switch (OrderBy)
   {
    case OrderByEnum.ASC:
     //因為裡面用到的欄位不同,又是強型別,只能寫死...
     BuildingList = BuildingList.OrderBy(x => x.Pin).ToList();
     break;
    case OrderByEnum.DESC:
     BuildingList = BuildingList.OrderByDescending(x => x.Pin).ToList();
     break;
   }
   break;
  case OrderByColumnEnum.Age:
   switch (OrderBy)
   {
    case OrderByEnum.ASC:
     BuildingList = BuildingList.OrderBy(x => x.Age).ToList();
     break;
    case OrderByEnum.DESC:
     BuildingList = BuildingList.OrderByDescending(x =>x.Age).ToList();
     break;
   }
   break;
  case OrderByColumnEnum.CaseDate:
   switch (OrderBy)
   {
    case OrderByEnum.ASC:
     BuildingList = BuildingList.OrderBy(x => x.CaseDate).ToList();
     break;
    case OrderByEnum.DESC:
     BuildingList = BuildingList.OrderByDescending(x => x.CaseDate).ToList();
     break;
   }
   break;
 }
 BuildingList.Dump();
}

public class Building
{
 public int Pin { get; set; }
 public int Age { get; set; }
 public DateTime CaseDate { get; set; }

}

/// <summary>
/// 排序欄位
/// </summary>
public enum OrderByColumnEnum
{
 /// <summary>
 /// 成交日期
 /// </summary>
 CaseDate,
 /// <summary>
 /// 建坪
 /// </summary>
 Pin,
 /// <summary>
 /// 屋齡
 /// </summary>
 Age

}

/// <summary>
/// 排序方法
/// </summary>
public enum OrderByEnum
{
 ASC = 1,
 DESC = 2
}

因為每個欄位不同名稱,又有可能升降冪，所以寫的超級長，如果欄位達到5~6個時，幾乎已經無法維護，但其實只要搭配一點點反射可以把這段改得很漂亮的

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//要用來排序的欄位
 OrderByColumnEnum OrderByColumn = OrderByColumnEnum.Pin;
 //排序的方法
 OrderByEnum OrderBy = OrderByEnum.ASC;

 //準備資料
 var BuildingList = new List<Building>();
 for (int i = 0; i < 5; i++)
 {
  BuildingList.Add(new Building 
  {
   Age = 1 * (i+1),
   Pin = 1 * (i+1),
   CaseDate = DateTime.Now.AddDays(i +1)
  });
 }

 //排序欄位
 var param = OrderByColumnEnum.Pin.ToString();

 //透過PropertyInfo操作
 var propertyInfo = typeof(Building).GetProperty(param);
 switch (OrderBy)
 {
  case OrderByEnum.ASC:
   BuildingList = BuildingList.OrderBy(x => propertyInfo.GetValue(x, null)).ToList();
   break;
  case OrderByEnum.DESC:
   BuildingList = BuildingList.OrderByDescending(x => propertyInfo.GetValue(x, null)).ToList();
   break;
 }

 BuildingList.Dump();

這樣程式是不是變得乾淨許多,但這邊要特別注意的是，因為我的Enum跟Building的屬性剛好都可以對應起來，如果我的Enum名稱與Building的屬性不能對應，那要另外寫對應方式，例如寫到擴充的Enum Attribute，或是用最簡單的Switch Case解決。