更新：2007 年 11 月

分组是 LINQ 最强大的功能之一。下面的示例演示如何以各种方式对数据进行分组：

  • 按照单个属性。

  • 按照字符串属性的首字母。

  • 按照计算出的数值范围。

  • 按照布尔谓词或其他表达式。

  • 按照复合键。

此外，最后两个查询将它们的结果投影到一个新的匿名类型中，该类型仅包含学生的名字和姓氏。有关更多信息，请参见 group 子句（C# 参考）。

示例

本主题中的所有示例都使用下列帮助器类和数据源。

C# 复制代码
public class StudentClass
{
    #region data
    protected enum GradeLevel { FirstYear = 1, SecondYear, ThirdYear, FourthYear };
    protected class Student
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public int ID { get; set; }
        public GradeLevel Year;
        public List<int> ExamScores;
    }

    protected static List<Student> students = new List<Student>
    {
        new Student {FirstName = "Terry", LastName = "Adams", ID = 120, Year = GradeLevel.SecondYear, ExamScores = new List<int>{ 99, 82, 81, 79}},
        new Student {FirstName = "Fadi", LastName = "Fakhouri", ID = 116, Year = GradeLevel.ThirdYear,ExamScores = new List<int>{ 99, 86, 90, 94}},
        new Student {FirstName = "Hanying", LastName = "Feng", ID = 117, Year = GradeLevel.FirstYear, ExamScores = new List<int>{ 93, 92, 80, 87}},
        new Student {FirstName = "Cesar", LastName = "Garcia", ID = 114, Year = GradeLevel.FourthYear,ExamScores = new List<int>{ 97, 89, 85, 82}},
        new Student {FirstName = "Debra", LastName = "Garcia", ID = 115, Year = GradeLevel.ThirdYear, ExamScores = new List<int>{ 35, 72, 91, 70}},
        new Student {FirstName = "Hugo", LastName = "Garcia", ID = 118, Year = GradeLevel.SecondYear, ExamScores = new List<int>{ 92, 90, 83, 78}},
        new Student {FirstName = "Sven", LastName = "Mortensen", ID = 113, Year = GradeLevel.FirstYear, ExamScores = new List<int>{ 88, 94, 65, 91}},
        new Student {FirstName = "Claire", LastName = "O'Donnell", ID = 112, Year = GradeLevel.FourthYear, ExamScores = new List<int>{ 75, 84, 91, 39}},
        new Student {FirstName = "Svetlana", LastName = "Omelchenko", ID = 111, Year = GradeLevel.SecondYear, ExamScores = new List<int>{ 97, 92, 81, 60}},
        new Student {FirstName = "Lance", LastName = "Tucker", ID = 119, Year = GradeLevel.ThirdYear, ExamScores = new List<int>{ 68, 79, 88, 92}},
        new Student {FirstName = "Michael", LastName = "Tucker", ID = 122, Year = GradeLevel.FirstYear, ExamScores = new List<int>{ 94, 92, 91, 91}},
        new Student {FirstName = "Eugene", LastName = "Zabokritski", ID = 121, Year = GradeLevel.FourthYear, ExamScores = new List<int>{ 96, 85, 91, 60}}
    };
    #endregion

    //Helper method
    protected static int GetPercentile(Student s)
    {
        double avg = s.ExamScores.Average();
        return avg > 0 ? (int)avg / 10 : 0;
    }



    public void QueryHighScores(int exam, int score)
    {
        var highScores = from student in students
                         where student.ExamScores[exam] > score
                         select new {Name = student.FirstName, Score = student.ExamScores[exam]};

        foreach (var item in highScores)
        {
            Console.WriteLine("{0,-15}{1}", item.Name, item.Score);
        }
    }
}

public class Program
{
    public static void Main()
    {
        StudentClass sc = new StudentClass();
        sc.QueryHighScores(1, 90);

        // Keep the console window open in debug mode
        Console.WriteLine("Press any key to exit");
        Console.ReadKey();
    }
}

下面的示例演示如何通过使用元素的单个属性作为组键对源元素进行分组。在这种情况下，该键为 string。还可以使用子字符串作为键。分组操作将对该类型使用默认的相等比较器。

C# 复制代码
private static void GroupBySingleProperty()
{
    Console.WriteLine("Group by a single property in an object");

    // queryLastNames is an IEnumerable<IGrouping<string, DataClass.Student>>
    // var is easier to type.
    var queryLastNames =
        from student in students
        group student by student.LastName into newGroup
        orderby newGroup.Key
        select newGroup;

    foreach (var nameGroup in queryLastNames)
    {
        Console.WriteLine("Key: {0}", nameGroup.Key);
        foreach (var student in nameGroup)
        {
            Console.WriteLine("\t{0}, {1}", student.LastName, student.FirstName);
        }
    }
}
/* Output:
  Group by a single property in an object
  Key: Feng
          Feng, Hanying
  Key: Garcia
          Garcia, Hugo
          Garcia, Cesar
          Garcia, Debra
  Key: Mortensen
          Mortensen, Sven
  Key: O'Donnell
          O'Donnell, Claire
  Key: Omelchenko
          Omelchenko, Svetlana
  Key: Tucker
          Tucker, Michael
          Tucker, Lance
 */

下面的示例演示如何通过使用除对象属性以外的某个项作为组键对源元素进行分组。

C# 复制代码
private static void GroupBySubstring()
{            
    Console.WriteLine("\r\nGroup by something other than a property of the object:");

    var queryFirstLetters =
        from student in students
        group student by student.LastName[0];

    foreach (var studentGroup in queryFirstLetters)
    {
        Console.WriteLine("Key: {0}", studentGroup.Key);
        // Nested foreach is required to access group items
        foreach (var student in studentGroup)
        {
            Console.WriteLine("\t{0}, {1}", student.LastName, student.FirstName);
        }
    }           
}
/* Output:
        Group by first character:
        Key: O
                Omelchenko, Svetlana
                O'Donnell, Claire
        Key: G
                Garcia, Hugo
                Garcia, Cesar
                Garcia, Debra
        Key: M
                Mortensen, Sven
        Key: T
                Tucker, Michael
                Tucker, Lance
        Key: F
                Feng, Hanying
     */

下面的示例演示如何通过使用某个数值范围作为组键对源元素进行分组。然后，查询将结果投影到一个匿名类型中，该类型仅包含学生的名字和姓氏以及该学生所属的百分点范围。使用匿名类型的原因是没有必要使用完整的 Student 对象来显示结果。GetPercentile 是一个帮助器函数，它根据学生的平均得分计算百分点：

C# 复制代码
static int GetPercentile(Student s)
{
   double avg = s.Scores.Average();
   return avg > 0 ? (int)avg / 10 : 0;
}
C# 复制代码
private static void GroupByRange()
{            
    Console.WriteLine("\r\nGroup by numeric range and project into a new anonymous type:");

    var queryNumericRange =
        from student in students
        let percentile = GetPercentile(student)
        group new { student.FirstName, student.LastName } by percentile into percentGroup
        orderby percentGroup.Key
        select percentGroup;

    // Nested foreach required to iterate over groups and group items.
    foreach (var studentGroup in queryNumericRange)
    {
        Console.WriteLine("Key: {0}", (studentGroup.Key * 10));
        foreach (var item in studentGroup)
        {
            Console.WriteLine("\t{0}, {1}", item.LastName, item.FirstName);
        }
    }            
}
/* Output:
     Group by numeric range and project into a new anonymous type:
     Key: 60
             Garcia, Debra
     Key: 70
             Omelchenko, Svetlana
             O'Donnell, Claire
     Key: 80
             Garcia, Hugo
             Mortensen, Sven
             Garcia, Cesar
             Feng, Hanying
             Tucker, Lance
     Key: 90
             Tucker, Michael
     */

下面的示例演示如何通过使用布尔比较表达式对源元素进行分组。与上述示例一样，结果被投影到一个匿名类型中，因为不需要完整的源元素。请注意，在执行查询时，该匿名类型中的属性将变成 Key 成员上的属性，并且可以通过名称进行访问。

C# 复制代码
private static void GroupByBoolean()
{            
    Console.WriteLine("\r\nGroup by a boolean into two groups with string keys");
    Console.WriteLine("\"True\" and \"False\" and project into a new anonymous type:");
    var queryGroupByAverages = from student in students
                               group new { student.FirstName, student.LastName }
                                    by student.ExamScores.Average() > 75 into studentGroup
                               select studentGroup;

    foreach (var studentGroup in queryGroupByAverages)
    {
        Console.WriteLine("Key: {0}", studentGroup.Key);
        foreach (var student in studentGroup)
            Console.WriteLine("\t{0} {1}", student.FirstName, student.LastName);
    }            
}
/* Output:
         Group by a boolean into two groups with string keys
         "True" and "False" and project into a new anonymous type:
         Key: True
                 Svetlana Omelchenko
                 Hugo Garcia
                 Sven Mortensen
                 Michael Tucker
                 Cesar Garcia
                 Hanying Feng
                 Lance Tucker
         Key: False
                 Claire O'Donnell
                 Debra Garcia
*/

下面的示例演示如何使用匿名类型来封装包含多个值的键。在这种情况下，第二个键值是一个布尔值，它指定该学生在第一次考试中的得分是否超过了 85 分。可以按照该键中的任何属性对多组值进行排序。

C# 复制代码
private static void GroupByCompositeKey()
{

    var queryHighScoreGroups =
        from student in students
        group student by new { FirstLetter = student.LastName[0], Score = student.ExamScores[0] > 85 } into studentGroup
        orderby studentGroup.Key.FirstLetter
        select studentGroup;

    Console.WriteLine("\r\nGroup and order by a compound key:");
    foreach (var scoreGroup in queryHighScoreGroups)
    {
        string s = scoreGroup.Key.Score == true ? "more than" : "less than";
        Console.WriteLine("Name starts with {0} who scored {1} 85", scoreGroup.Key.FirstLetter, s);
        foreach (var item in scoreGroup)
        {
            Console.WriteLine("\t{0} {1}", item.FirstName, item.LastName);
        }
    }
}
/* Output:
          Group and order by a compound key:
          Name starts with F who scored more than 85
                  Hanying Feng
          Name starts with G who scored more than 85
                  Hugo Garcia
                  Cesar Garcia
          Name starts with G who scored less than 85
                  Debra Garcia
          Name starts with M who scored more than 85
                  Sven Mortensen
          Name starts with O who scored more than 85
                  Svetlana Omelchenko
          Name starts with O who scored less than 85
                  Claire O'Donnell
          Name starts with T who scored more than 85
                  Michael Tucker
          Name starts with T who scored less than 85
                  Lance Tucker
       */

编译代码

此示例包含对如何：查询对象集合（C# 编程指南）内示例应用程序中定义的对象的引用。若要编译和运行此方法，请将它粘贴到该应用程序的 StudentClass 类中，并且在 Main 方法中添加对它的调用。

在改写此方法以适合您自己的应用程序时，请记住 LINQ 需要 .NET Framework 3.5 版，并且项目必须包含一个对 System.Core.dll 的引用和一条针对 System.Linq 的 using 指令。LINQ to SQL、LINQ to XML 和 LINQ to DataSet 类型需要其他 using 指令和引用。有关更多信息，请参见 如何：创建 LINQ 项目。

请参见