在通用列表上模拟“汇总”函数

Question

例如，我有一个Element 类型的通用列表。

public class Element
{
    public string Country { get; set; }
    public string City { get; set; }
    public int Population { get; set; }
}

有以下数据。

var elements = new List<Element>
   {
       new Element { Country = "Country A", City = "Barrie", Population = 12 },
       new Element { Country = "Country A", City = "Barrie2", Population = 12 },
       new Element { Country = "Country A", City = "Barrie2", Population = 12 },
       new Element { Country = "Country A", City = "Barrie", Population = 12 },
       new Element { Country = "Country D", City = "Essex", Population = 12 },
       new Element { Country = "Country A", City = "Barrie", Population = 12 },
       new Element { Country = "Country A", City = "Barrie", Population = 12 },
       new Element { Country = "Country D", City = "Essex", Population = 12 },
       new Element { Country = "Country A", City = "Barrie", Population = 12 },
       new Element { Country = "Country A", City = "Barrie", Population = 12 }
   };

基本上，我想要按国家和城市分组的人口总数。

类似的东西。

Country A | Barrie  | `running total for Barrie`
Country A | Barrie2 | `running total for Barrie2`
          |         | `total for Country A`
Country D | Essex   | `running total for Essex`
          |         | `total for Country D`
          |         | `total for everything`

我在任何地方都找不到扩展（我说扩展是因为我计划多次使用汇总），所以我想我会试一试。所以我从这个简单的查询开始。

var groupedElements = elements
    .GroupBy(x => new { x.Country, x.City })
    .Select(x => new { Country = x.Key, City = x.Select(xx => xx.City), Population = x.Sum(xx => xx.Population) })
    .ToList();

此查询按预期工作，所以我认为我在正确的轨道上。接下来我想我必须弄清楚groupedElements 中的哪个属性是聚合的，因为这就是我们要做汇总的内容。我该如何做到这一点？或者也许我可以有一个参数让我指定我希望在哪个列上执行聚合函数。

Answer 1

我认为这并不像您想象的那么容易。首先，您想要的结果序列的项目“自然”不是同一类型。您需要每个国家/地区的运行总数，每个国家/地区的分组总数，然后是总体总数。

即使您可以编写一个简洁的查询来创建此信息，调用者也必须将每种总计与结果区分开来。我想知道首先将您想要的结果视为单个IEnumerable<XXX> 是否有意义；像这样创建一个好的 OO 解决方案并从那里着手可能更有意义：

public interface IGeographicEnity
{
   string Name { get; }
   int Population { get; }
}

public class City : IGeographicEntity {...}

public class Country : IGeographicEntity
{
   public IList<City> Cities { get {...} }       
   public int Population { get { return Cities.Sum(c => c.Population); } }
   ...
}

public class World : IGeographicEntity
{
    public IList<Country> Countries { get {...} }       
    public int Population { get { return Countries.Sum(c => c.Population); }
    ...
}

如果你仍然想坚持你最初的想法，这里是我能想到的最好的：

public class PopulationTotal
{
    // You can use subclassing instead of an enum to represent this
    public enum Kind
    { RunningTotalWithinCountry, TotalForCountry, TotalOverall }

    public string GroupName { get; private set; }
    public int Value { get; private set; }
    public Kind Kind { get; private set; }

    public static IEnumerable<PopulationTotal> GetTotals(IEnumerable<Element> elements)
    {
        int overallTotal = 0;

        foreach (var elementsByCountry in elements.GroupBy(e => e.Country))
        {
            int runningTotalForCountry = 0;

            foreach (var element in elementsByCountry)
            {
                runningTotalForCountry += element.Population;
                yield return new PopulationTotal
                                    {
                                        GroupName = element.City,
                                        Kind = Kind.RunningTotalWithinCountry,
                                        Value = runningTotalForCountry
                                    };
            }

            overallTotal += runningTotalForCountry;

            yield return new PopulationTotal
                                {
                                    GroupName = elementsByCountry.Key,
                                    Kind = Kind.TotalForCountry,
                                    Value = runningTotalForCountry
                                };
        }

        yield return new PopulationTotal
                            {
                                GroupName = null,
                                Kind = Kind.TotalOverall,
                                Value = overallTotal
                            };
    }
}

用法：

var totals = PopulationTotal.GetTotals(elements);