How to: Create a Nested Group (C# Programming Guide)
The following example shows how to create nested groups in a LINQ query expression. Each group that is created according to student year or grade level is then further subdivided into groups based on the individuals' names.
Example
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
*/
Note that three nested foreach loops are required to iterate over the inner elements of a nested group.
Compiling the Code
This example contains references to objects that are defined in the sample application in How to: Query a Collection of Objects (C# Programming Guide). To compile and run this method, paste it into the StudentClass class in that application and add a call to it from the Main method.
When you adapt this method to your own application, remember that LINQ requires version 3.5 of the .NET Framework, and the project must contain a reference to System.Core.dll and a using directive for System.Linq. LINQ to SQL, LINQ to XML and LINQ to DataSet types require additional usings and references. For more information, see How to: Create a LINQ Project.