使用Lambda按多列分组

122

如何使用Lambda对多个列进行分组？

我看到了使用linq对实体执行此操作的示例，但我正在寻找lambda形式。

c# .net entity-framework lambda

— 瑙尔
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252

var query = source.GroupBy(x => new { x.Column1, x.Column2 });

— 阿杜奇
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这实际可行吗？我认为对您分组的每个对象进行的相等性测试将失败，因为它们是对象而不是结构。

— 雅各布

@Aducci：谢谢。您能举例说明如何获得组项目的IEnumerable吗？

— 瑙尔，

6

@Jacob-匿名类型是具有适当重写GetHashCode和Equals方法的不可变类。他们正是针对这种用例而设计的。

— 谜团

5

@Naor- GroupBy返回一个IEnumerable<IGrouping<TKey, TSource>>本质上为IEnumerable<IEnumerable<TSource>>且具有Key内部可枚举属性的an 。这是否有助于您获得组项目的“ IEnumerable”？

— Enigmativity

如果我的'source'变量是Dictionary Collection，则无法使用。有什么建议吗？

— Joao Paulo

6

如果你的桌子是这样的

rowId     col1    col2    col3    col4
 1          a       e       12       2
 2          b       f       42       5
 3          a       e       32       2
 4          b       f       44       5


var grouped = myTable.AsEnumerable().GroupBy(r=> new {pp1 =  r.Field<int>("col1"), pp2 = r.Field<int>("col2")});

— 詹卡·马布卜（Jhankar Mahbub）
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6

重要的是要注意，AsEnumerable在将表分组之前会将整个表带入内存。这在某些桌子上绝对很重要。看到这个答案的更深入的了解：stackoverflow.com/questions/17968469/...

— 布兰登巴克利

4

除了上面的aduchis答案之外，如果您随后需要根据这些按键分组，则可以定义一个类来包装许多键。

return customers.GroupBy(a => new CustomerGroupingKey(a.Country, a.Gender))
                .Where(a => a.Key.Country == "Ireland" && a.Key.Gender == "M")
                .SelectMany(a => a)
                .ToList();

其中CustomerGroupingKey使用组密钥：

    private class CustomerGroupingKey
    {
        public CustomerGroupingKey(string country, string gender)
        {
            Country = country;
            Gender = gender;
        }

        public string Country { get; }

        public string Gender { get; }
    }

— 戴维·麦克莱尼
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1

可能会节省一些时间：最好将默认构造与对象初始化一起使用。上面的示例代码中的方法不会像EF Core这样的ORM很好地对待。

— 康斯坦丁

2

     class Element
        {
            public string Company;        
            public string TypeOfInvestment;
            public decimal Worth;
        }

   class Program
    {
        static void Main(string[] args)
        {
         List<Element> elements = new List<Element>()
            {
                new Element { Company = "JPMORGAN CHASE",TypeOfInvestment = "Stocks", Worth = 96983 },
                new Element { Company = "AMER TOWER CORP",TypeOfInvestment = "Securities", Worth = 17141 },
                new Element { Company = "ORACLE CORP",TypeOfInvestment = "Assets", Worth = 59372 },
                new Element { Company = "PEPSICO INC",TypeOfInvestment = "Assets", Worth = 26516 },
                new Element { Company = "PROCTER & GAMBL",TypeOfInvestment = "Stocks", Worth = 387050 },
                new Element { Company = "QUASLCOMM INC",TypeOfInvestment = "Bonds", Worth = 196811 },
                new Element { Company = "UTD TECHS CORP",TypeOfInvestment = "Bonds", Worth = 257429 },
                new Element { Company = "WELLS FARGO-NEW",TypeOfInvestment = "Bank Account", Worth = 106600 },
                new Element { Company = "FEDEX CORP",TypeOfInvestment = "Stocks", Worth = 103955 },
                new Element { Company = "CVS CAREMARK CP",TypeOfInvestment = "Securities", Worth = 171048 },
            };

            //Group by on multiple column in LINQ (Query Method)
            var query = from e in elements
                        group e by new{e.TypeOfInvestment,e.Company} into eg
                        select new {eg.Key.TypeOfInvestment, eg.Key.Company, Points = eg.Sum(rl => rl.Worth)};



            foreach (var item in query)
            {
                Console.WriteLine(item.TypeOfInvestment.PadRight(20) + " " + item.Points.ToString());
            }


            //Group by on multiple column in LINQ (Lambda Method)
            var CompanyDetails =elements.GroupBy(s => new { s.Company, s.TypeOfInvestment})
                               .Select(g =>
                                            new
                                            {
                                                company = g.Key.Company,
                                                TypeOfInvestment = g.Key.TypeOfInvestment,            
                                                Balance = g.Sum(x => Math.Round(Convert.ToDecimal(x.Worth), 2)),
                                            }
                                      );
            foreach (var item in CompanyDetails)
            {
                Console.WriteLine(item.TypeOfInvestment.PadRight(20) + " " + item.Balance.ToString());
            }
            Console.ReadLine();

        }
    }

— 桑迪普·特里帕蒂（Sandeep Tripathi）
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1

我想出了一个定义类的组合，例如David的答案，但不要求将Where类与之配合使用。看起来像：

var resultsGroupings = resultsRecords.GroupBy(r => new { r.IdObj1, r.IdObj2, r.IdObj3})
                                    .Select(r => new ResultGrouping {
                                        IdObj1= r.Key.IdObj1,
                                        IdObj2= r.Key.IdObj2,
                                        IdObj3= r.Key.IdObj3,
                                        Results = r.ToArray(),
                                        Count = r.Count()
                                    });



private class ResultGrouping
        {
            public short IdObj1{ get; set; }
            public short IdObj2{ get; set; }
            public int IdObj3{ get; set; }

            public ResultCsvImport[] Results { get; set; }
            public int Count { get; set; }
        }

resultRecords我要分组的初始列表在哪里，以及它的List<ResultCsvImport>。请注意，这里的想法是，我将IdObj1和IdObj2和IdObj3按3列分组

— 杰夫·莫雷蒂
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