C# 如何使用LINQ对通用列表中的值求和？

我有这门课：

public class ItemsForMonthYear
{
    public String ItemDescription { get; set; }
    public String monthYr { get; set; }
    public int TotalPackages { get; set; }
    public Decimal TotalPurchases { get; set; }
    public Decimal AveragePrice { get; set; }
    public Double PercentOfTotal { get; set; }
}

…我在其中存储数据。我有一个通用列表，用于存储其中的N个：

List<ItemsForMonthYear> itemsForMonthYearList;

…但所需的下一个单元格val是基于给定月份的所有TotalPurchases值计算得出的。因此，我需要计算具有相同月值的所有项目的“总百分比”值。在SQL中，我可以执行以下操作：

SELECT SUM(TotalPurchases) FROM ItemsForMonthYear WHERE monthYr = 'Apr 14'

percentOfTotalCell.Value2 = GetPercentForPurchaseForMonthYearAsStr(ifmy.TotalPurchases, curMonth);

private String GetPercentForPurchaseForMonthYearAsStr(decimal totPurchasesForItemMonthYear, string totPurchasesForMonthYear)
{    
    percentOfTotal = // What LINQ magic goes here?
    percentOfTotalStr = percentOfTotal.ToString("P", CultureInfo.InvariantCulture);
    return percentOfTotalStr;
}

…然后将结果除以每个ItemDescription的TotalPurchases值

瞧，如果“4月14日”一个月的所有采购总额为100万美元，而“可可泡芙”项目的采购总额为10万美元，那么我可以计算出“可可泡芙”的总采购百分比为10%

我可以“蛮力”并循环浏览整个通用列表：

Decimal allTotalPurchases;
foreach (ItemsForMonthYear ifmy in itemsForMonthYearList)
{
    if (ifmy.monthYr.Equals("Apr 14") 
    {
        allTotalPurchases = allTotalPurchases + ifmy.TotalPurchases;
    }
}

…但我认为有一种更优雅/性能更友好的方法可以使用LINQ实现这一点。我想这样做：

SELECT SUM(TotalPurchases) FROM ItemsForMonthYear WHERE monthYr = 'Apr 14'

percentOfTotalCell.Value2 = GetPercentForPurchaseForMonthYearAsStr(ifmy.TotalPurchases, curMonth);

private String GetPercentForPurchaseForMonthYearAsStr(decimal totPurchasesForItemMonthYear, string totPurchasesForMonthYear)
{    
    percentOfTotal = // What LINQ magic goes here?
    percentOfTotalStr = percentOfTotal.ToString("P", CultureInfo.InvariantCulture);
    return percentOfTotalStr;
}

…其中，arg“totPurchasesForMonthYear”将是上述示例中的十万美元，arg“totPurchasesForMonthYear”将是“Apr 14”等值

---

有人知道LINQ的聪明诀窍吗？

你可以这样做来计算一个月的总购买量：

decimal totalPurchases = itemsForMonthYearList.Where(item => item.monthYr == "Apr 14")
                                              .Sum(item => item.TotalPurchases);

然后，您可以通过执行以下操作来计算给定项目的百分比：

decimal percentage = (itemsForMonthYearList.Single(item => item.ItemDescription == "Cocoa Puffs")
                                           .TotalPurchases / totalPurchases) * 100;

所需的下一个单元格值是根据给定月份的所有TotalPurchases值计算得出的。因此，我需要计算具有相同月值的所有项目的“总百分比”值

首先，您需要获得具有相同的

monthYr

：

var allMonthYr = itemsForMonthYearList.Where(x => x.monthYr == ifmy.monthYr);

然后是一个简单的求和：

var totalPurchasesOfMonthYr = allMonthYr.Sum(x => x.TotalPurchases);
var percentOfTotal = ifmy.TotalPurchases / totalPurchasesOfMonthYr;

现在，这被证明是非常低效的——因为您不必要地重复了很多次这个列表。最好分组，然后求和一次：

var groupedItemsForMonthYr = itemsForMonthYearList.ToLookup(x => x.monthYr);
var totalsForMonthYr = groupedItemsForMonthYr.ToDictionary(
    x => x.Key, 
    x => x.Sum(x => x.TotalPurchases)
);

var percentOfTotal = ifmy.TotalPurchases / totalsForMonthYr[ifmy.monthYr];

---

马克·布莱克特（Mark Brackett）自诩为“极为低效”的第一次拍摄的这段代码运行良好：

private String GetPercentForPurchaseForMonthYearAsStr(decimal totPurchasesForItemMonthYear, string monthToCalculate)
{
    var allRecordsForMonthYr = itemsForMonthYearList.Where(x => x.monthYr == monthToCalculate);
    var totalPurchasesOfMonthYr = allRecordsForMonthYr.Sum(x => x.TotalPurchases);
    var percentOfTotal = totPurchasesForItemMonthYear / totalPurchasesOfMonthYr;
    return percentOfTotal.ToString("P", CultureInfo.CurrentCulture);
}

---

由于分组代码不能为我编译，我对上面的代码很满意。

这是Mark Brackett的答案，我认为是适当的修复：

// This is code that goes outside your loop
var groupedItemsForMonthYr=itemsForMonthYearList.GroupBy(x=>x.monthYr);
var totalsForMonthYr=groupedItemsForMonthYr.ToDictionary(
    x => x.Key, 
    x => x.Sum(y => y.TotalPurchases)
);

// This is the code that goes inside your loop
var percentOfTotal = ifmy.TotalPurchases / totalsForMonthYr[ifmy.monthYr];

下面是我的答案，给出了一个

列表

，并返回了一个

列表

，在项目中正确填写了百分比：

var items=itemsForMonthYearList.GroupBy(x=>x.monthYr)
  .Select(x=>new { total=x.Sum(y=>y.TotalPurchases), i=x })
  .Select(x=>x.i.Select(y=>new ItemsForMonthYear {
        ItemDescription=y.ItemDescription,
        monthYr=y.monthYr,
        TotalPackages=y.TotalPackages,
        TotalPurchases=y.TotalPurchases,
        AveragePrice=y.AveragePrice,
        PercentOfTotal=(double)y.TotalPurchases/(double)x.total
      }))
  .SelectMany(x=>x);

以下是我的测试代码：

void Main()
{
    var itemsForMonthYearList=new[]{
        new ItemsForMonthYear { ItemDescription="A",monthYr="Aug 2014",TotalPackages=1,TotalPurchases=1,AveragePrice=1},
        new ItemsForMonthYear { ItemDescription="A",monthYr="Sep 2014",TotalPackages=1,TotalPurchases=1,AveragePrice=1},
        new ItemsForMonthYear { ItemDescription="A",monthYr="Sep 2014",TotalPackages=1,TotalPurchases=1,AveragePrice=1},
        new ItemsForMonthYear { ItemDescription="A",monthYr="Oct 2014",TotalPackages=1,TotalPurchases=1,AveragePrice=1},
        new ItemsForMonthYear { ItemDescription="A",monthYr="Oct 2014",TotalPackages=1,TotalPurchases=1,AveragePrice=1},
        new ItemsForMonthYear { ItemDescription="A",monthYr="Oct 2014",TotalPackages=1,TotalPurchases=1,AveragePrice=1},
    };

    var items=itemsForMonthYearList.GroupBy(x=>x.monthYr)
      .Select(x=>new { total=x.Sum(y=>y.TotalPurchases), i=x })
      .Select(x=>x.i.Select(y=>new ItemsForMonthYear {
            ItemDescription=y.ItemDescription,
            monthYr=y.monthYr,
            TotalPackages=y.TotalPackages,
            TotalPurchases=y.TotalPurchases,
            AveragePrice=y.AveragePrice,
            PercentOfTotal=(double)y.TotalPurchases/(double)x.total
          }))
      .SelectMany(x=>x);
      items.Dump();
}

public class ItemsForMonthYear
{
    public String ItemDescription { get; set; }
    public String monthYr { get; set; }
    public int TotalPackages { get; set; }
    public Decimal TotalPurchases { get; set; }
    public Decimal AveragePrice { get; set; }
    public Double PercentOfTotal { get; set; }
}

输出：

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分组代码看起来很有希望，但我发现了三个错误：（1）无法将lambda表达式转换为类型“System.Collections.Generic.IEqualityComparer”，因为它不是委托类型（2）无法在此范围内声明名为“x”的局部变量，因为它将赋予“x”不同的含义，已在“父或当前”范围中用于表示其他内容（3）运算符“/”不能应用于“decimal”和“System.Linq.IGrouping”类型的操作数请参阅我的答案，了解我认为需要对上述代码进行哪些修复才能使其正确运行。