c# - 在通用列表上模拟 `rollup` 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; }
}

具有以下数据。

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，因为这就是我们要对其进行汇总的内容。我如何做到这一点？或者我可以有一个参数，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不是“自然”的同一类型。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，然后是总体总计。

即使您可以编写一个简洁的查询来创建此信息，调用者也必须从结果中区分每种总数。我想知道将您想要的结果作为单个 IEnumerable<XXX> 查看是否有意义首先;创建一个像这样的不错的 OO 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办法:

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);