模仿通用列表上的“rollup”功能

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 }
   };

本质上，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），所以我想我会尝试一下。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();

这个查询按预期工作，所以我认为我走在正确的轨道上。接下来，我想我必须弄清楚 groupedElements 中的哪个属性是聚合的，因为这就是我们要进行汇总的内容。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一列上执行聚合函数。

Answer 1

我认为这并不像您想象的那么容易做到。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/地区的运行总计、每个国家/地区的分组总计，以及总体总计。

即使您可以编写一个简洁的查询来创建此信息，调用者也必须区分结果中的每种总计。我想知道首先将您想要的结果视为单个 IEnumerable 是否有意义；创建一个像这样的好的面向对象解决方案并从那里开始可能更有意义：

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); }
    ...
}

如果您仍然想坚持原来的想法，这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);