如何：创建嵌套组（C# 编程指南）
下面的示例演示如何在 LINQ 查询表达式中创建嵌套组。对根据学生年级创建的每个组，将根据每个人的姓名进一步划分为小组。
public void QueryNestedGroups()
{
var queryNestedGroups =
from student in students
group student by student.Year into newGroup1
from newGroup2 in
(from student in newGroup1
group student by student.LastName)
group newGroup2 by newGroup1.Key;
// Three nested foreach loops are required to iterate
// over all elements of a grouped group. Hover the mouse
// cursor over the iteration variables to see their actual type.
foreach (var outerGroup in queryNestedGroups)
{
Console.WriteLine("DataClass.Student Level = {0}", outerGroup.Key);
foreach (var innerGroup in outerGroup)
{
Console.WriteLine("\tNames that begin with: {0}", innerGroup.Key);
foreach (var innerGroupElement in innerGroup)
{
Console.WriteLine("\t\t{0} {1}", innerGroupElement.LastName, innerGroupElement.FirstName);
}
}
}
}
/*
Output:
DataClass.Student Level = SecondYear
Names that begin with: Adams
Adams Terry
Names that begin with: Garcia
Garcia Hugo
Names that begin with: Omelchenko
Omelchenko Svetlana
DataClass.Student Level = ThirdYear
Names that begin with: Fakhouri
Fakhouri Fadi
Names that begin with: Garcia
Garcia Debra
Names that begin with: Tucker
Tucker Lance
DataClass.Student Level = FirstYear
Names that begin with: Feng
Feng Hanying
Names that begin with: Mortensen
Mortensen Sven
Names that begin with: Tucker
Tucker Michael
DataClass.Student Level = FourthYear
Names that begin with: Garcia
Garcia Cesar
Names that begin with: O'Donnell
O'Donnell Claire
Names that begin with: Zabokritski
Zabokritski Eugene
*/
请注意，需要使用三个嵌套的 foreach 循环来循环访问嵌套组的内部元素。
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 How to: Create a LINQ Project。