Linq:Grouping Operators

　　　　　　　[Category("Grouping Operators")]
            [Description("This sample uses group by to partition a list of numbers by " +
                        "their remainder when divided by 5.")]
            public void DataSetLinq40()
            {

                var numbers = testDS.Tables["Numbers"].AsEnumerable();//testDS DataSet
                foreach (var n in numbers)
                {
                    Console.Write(n.Field<int>("number")+",");
                }
                Console.WriteLine();
                var numberGroups =
                    from n in numbers
                    group n by n.Field<int>("number") % 5 into g
                    select new { Remainder = g.Key, Numbers = g };

                foreach (var g in numberGroups)
                {
                    Console.WriteLine("Numbers with a remainder of {0} when divided by 5:", g.Remainder);
                    foreach (var n in g.Numbers)
                    {
                        Console.WriteLine(n.Field<int>("number"));
                    }
                }
                Console.ReadLine();
            }

运行结果（按照数字求余5的值分组）：

            [Category("Grouping Operators")]
            [Description("This sample uses group by to partition a list of words by " +
                         "their first letter.")]
            public void DataSetLinq41()
            {

                var words4 = testDS.Tables["Words4"].AsEnumerable();
                foreach (var g in words4)
                {
                    Console.Write(g.Field<string>("word")+",");
                }
                Console.WriteLine();
                var wordGroups =
                    from w in words4
                    group w by w.Field<string>("word")[0] into g
                    select new { FirstLetter = g.Key, Words = g };

                foreach (var g in wordGroups)
                {
                    Console.WriteLine("Words that start with the letter '{0}':", g.FirstLetter);
                    foreach (var w in g.Words)
                    {
                        Console.WriteLine(w.Field<string>("word"));
                    }
                }
                Console.ReadLine();
            }

运行结果(按照首字母分组)：

            [Category("Grouping Operators")]
            [Description("This sample uses group by to partition a list of products by category.")]
            public void DataSetLinq42()
            {

                var products = testDS.Tables["Products"].AsEnumerable();

                var productGroups =
                    from p in products
                    group p by p.Field<string>("Category") into g
                    select new { Category = g.Key, Products = g };

                foreach (var g in productGroups)
                {
                    Console.WriteLine("Category: {0}", g.Category);
                    foreach (var w in g.Products)
                    {
                        Console.WriteLine("	" + w.Field<string>("ProductName"));
                    }
                }
                Console.ReadLine();
            }

运行结果(按照产品类别分组):

            [Category("Grouping Operators")]
            [Description("This sample uses group by to partition a list of each customer's orders, " +
                         "first by year, and then by month.")]
            public void DataSetLinq43()
            {

                var customers = testDS.Tables["Customers"].AsEnumerable();

                var customerOrderGroups =
                    from c in customers
                    select
                        new
                        {
                            CompanyName = c.Field<string>("CompanyName"),
                            YearGroups =
                                from o in c.GetChildRows("CustomersOrders")
                                group o by o.Field<DateTime>("OrderDate").Year into yg
                                select
                                    new
                                    {
                                        Year = yg.Key,
                                        MonthGroups =
                                            from o in yg
                                            group o by o.Field<DateTime>("OrderDate").Month into mg
                                            select new { Month = mg.Key, Orders = mg }
                                    }
                        };

                foreach (var cog in customerOrderGroups)
                {
                    Console.WriteLine("CompanyName= {0}", cog.CompanyName);
                    foreach (var yg in cog.YearGroups)
                    {
                        Console.WriteLine("	 Year= {0}", yg.Year);
                        foreach (var mg in yg.MonthGroups)
                        {
                            Console.WriteLine("		 Month= {0}", mg.Month);
                            foreach (var order in mg.Orders)
                            {
                                Console.WriteLine("			 OrderID= {0} ", order.Field<int>("OrderID"));
                                Console.WriteLine("			 OrderDate= {0} ", order.Field<DateTime>("OrderDate"));
                            }
                        }
                    }
                }
                Console.ReadLine();
            }

DataTable:

运行结果（对每个客户的订单按年再按月分组）：

[Category("Grouping Operators")]
            [Description("This sample uses GroupBy to partition trimmed elements of an array using " +
                         "a custom comparer that matches words that are anagrams of each other.")]
            public void DataSetLinq44()
            {

                var anagrams = testDS.Tables["Anagrams"].AsEnumerable();

                var orderGroups = anagrams.GroupBy(w => w.Field<string>("anagram").Trim(), new AnagramEqualityComparer());

                foreach (var g in orderGroups)
                {
                    Console.WriteLine("Key: {0}", g.Key);
                    foreach (var w in g)
                    {
                        Console.WriteLine("	" + w.Field<string>("anagram"));
                    }
                }
            }

            [Category("Grouping Operators")]
            [Description("This sample uses GroupBy to partition trimmed elements of an array using " +
                         "a custom comparer that matches words that are anagrams of each other, " +
                         "and then converts the results to uppercase.")]
            public void DataSetLinq45()
            {

                var anagrams = testDS.Tables["Anagrams"].AsEnumerable();

                var orderGroups = anagrams.GroupBy(
                    w => w.Field<string>("anagram").Trim(),
                    a => a.Field<string>("anagram").ToUpper(),
                    new AnagramEqualityComparer()
                    );

                foreach (var g in orderGroups)
                {
                    Console.WriteLine("Key: {0}", g.Key);
                    foreach (var w in g)
                    {
                        Console.WriteLine("	" + w);
                    }
                }
            }

内容源自：http://code.msdn.microsoft.com/101-LINQ-Samples-3fb9811b