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How to: Group a Group (C# Programming Guide)

Updated: September 2008

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.

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.

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.

Date

History

Reason

September 2008

Added explanation to text and added output for code example.

Customer feedback.

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