如何：创建嵌套组（C# 编程指南） · MSDN C# 编程指南 & 参考手册 2015

# 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）](https://msdn.microsoft.com/zh-cn/library/bb907066.aspx)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](https://msdn.microsoft.com/zh-cn/library/bb546156.aspx)。 ## 请参阅 [LINQ 查询表达式（C# 编程指南）](https://msdn.microsoft.com/zh-cn/library/bb397676.aspx)