如何：对查询结果进行分组

 

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：  

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#
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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, used in GroupByRange.
    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();
    }
}

示例

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，键是 string，即学生的姓氏。  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。  

将下面的方法粘贴到 StudentClass 类中。  将 Main 方法中的调用语句更改为 sc.GroupBySingleProperty()。  

C#
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public void GroupBySingleProperty()
{
    Console.WriteLine("Group by a single property in an object:");

    // Variable queryLastNames is an IEnumerable<IGrouping<string, 
    // DataClass.Student>>. 
    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: Adams
            Adams, Terry
    Key: Fakhouri
            Fakhouri, Fadi
    Key: Feng
            Feng, Hanying
    Key: Garcia
            Garcia, Cesar
            Garcia, Debra
            Garcia, Hugo
    Key: Mortensen
            Mortensen, Sven
    Key: O'Donnell
            O'Donnell, Claire
    Key: Omelchenko
            Omelchenko, Svetlana
    Key: Tucker
            Tucker, Lance
            Tucker, Michael
    Key: Zabokritski
            Zabokritski, Eugene
*/

示例

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。  在此示例中，键是学生姓氏的第一个字母。  

将下面的方法粘贴到 StudentClass 类中。  将 Main 方法中的调用语句更改为 sc.GroupBySubstring()。  

C#
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public 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 something other than a property of the object:
    Key: A
            Adams, Terry
    Key: F
            Fakhouri, Fadi
            Feng, Hanying
    Key: G
            Garcia, Cesar
            Garcia, Debra
            Garcia, Hugo
    Key: M
            Mortensen, Sven
    Key: O
            O'Donnell, Claire
            Omelchenko, Svetlana
    Key: T
            Tucker, Lance
            Tucker, Michael
    Key: Z
            Zabokritski, Eugene
*/

示例

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 helper 函数，它根据学生的平均分数计算百分比。  该方法返回 0 到 10 之间的整数。  

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

将下面的方法粘贴到 StudentClass 类中。  将 Main 方法中的调用语句更改为 sc.GroupByRange()。  

C#
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public 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
            O'Donnell, Claire
    Key: 80
            Adams, Terry
            Feng, Hanying
            Garcia, Cesar
            Garcia, Hugo
            Mortensen, Sven
            Omelchenko, Svetlana
            Tucker, Lance
            Zabokritski, Eugene
    Key: 90
            Fakhouri, Fadi
            Tucker, Michael
*/

示例

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。  在此示例中，布尔表达式会测试学生的平均考试分数是否超过 75。  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。  

将下面的方法粘贴到 StudentClass 类中。  将 Main 方法中的调用语句更改为 sc.GroupByBoolean()。  

C#
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public 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
            Terry Adams
            Fadi Fakhouri
            Hanying Feng
            Cesar Garcia
            Hugo Garcia
            Sven Mortensen
            Svetlana Omelchenko
            Lance Tucker
            Michael Tucker
            Eugene Zabokritski
    Key: False
            Debra Garcia
            Claire O'Donnell
*/

示例

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。  在此示例中，第一个键值是学生姓氏的第一个字母。  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。  可以按照该键中的任何属性对多组值进行排序。  

将下面的方法粘贴到 StudentClass 类中。  将 Main 方法中的调用语句更改为 sc.GroupByCompositeKey()。  

C#
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public 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 A who scored more than 85
            Terry Adams
    Name starts with F who scored more than 85
            Fadi Fakhouri
            Hanying Feng
    Name starts with G who scored more than 85
            Cesar Garcia
            Hugo 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 less than 85
            Claire O'Donnell
    Name starts with O who scored more than 85
            Svetlana Omelchenko
    Name starts with T who scored less than 85
            Lance Tucker
    Name starts with T who scored more than 85
            Michael Tucker
    Name starts with Z who scored more than 85
            Eugene Zabokritski
*/

编译代码

将您想要测试的每种方法都复制并粘贴到 StudentClass 类中。  将该方法的调用语句添加到 Main 方法中并按 F5。  

在改写这些方法以适合您自己的应用程序时，请记住 LINQ 需要 .NET Framework 3.5 或 4 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 How to: Create a LINQ Project。