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") %  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")[] 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("\t" + 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("\t Year= {0}", yg.Year);
                         foreach (var mg in yg.MonthGroups)
                         {
                             Console.WriteLine("\t\t Month= {0}", mg.Month);
                             foreach (var order in mg.Orders)
                             {
                                 Console.WriteLine("\t\t\t OrderID= {0} ", order.Field<int>("OrderID"));
                                 Console.WriteLine("\t\t\t 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("\t" + 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("\t" + w);
                     }
                 }
             }

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