如何：对查询结果进行分组（C# 编程指南）
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。
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();
}
}
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()。
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
*/
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()。
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
*/
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。
//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()。
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
*/
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()。
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
*/
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()。
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 如何：创建 LINQ 项目。