Grouping Data

Grouping refers to the operation of putting data into groups so that the elements in each group share a common attribute.

The following illustration shows the results of grouping a sequence of characters. The key for each group is the character.

The standard query operator methods that group data elements are listed in the following section.

Methods

Method Name	Description	C# Query Expression Syntax	Visual Basic Query Expression Syntax	More Information
GroupBy	Groups elements that share a common attribute. Each group is represented by an IGrouping<TKey, TElement> object.	group … by -or- group … by … into …	Group … By … Into …	Enumerable.GroupBy Queryable.GroupBy
ToLookup	Inserts elements into a Lookup<TKey, TElement> (a one-to-many dictionary) based on a key selector function.	Not applicable.	Not applicable.	Enumerable.ToLookup

Query Expression Syntax Example

The following code example uses the group by clause in C# or the Group By clause in Visual Basic to group integers in a list according to whether they are even or odd.

Dim numbers As New System.Collections.Generic.List(Of Integer) _
    (New Integer() {35, 44, 200, 84, 3987, 4, 199, 329, 446, 208})

Dim query = From number In numbers _
            Group By Remainder = (number Mod 2) Into Group

Dim sb As New System.Text.StringBuilder()
For Each group In query
    sb.AppendLine(If(group.Remainder = 0, vbCrLf & "Even numbers:", vbCrLf & "Odd numbers:"))
    For Each num In group.Group
        sb.AppendLine(num)
    Next 
Next 

' Display the results.
MsgBox(sb.ToString())

' This code produces the following output: 

' Odd numbers: 
' 35 
' 3987 
' 199 
' 329 

' Even numbers: 
' 44 
' 200 
' 84 
' 4 
' 446 
' 208

List<int> numbers = new List<int>() { 35, 44, 200, 84, 3987, 4, 199, 329, 446, 208 };

IEnumerable<IGrouping<int, int>> query = from number in numbers
                                         group number by number % 2;

foreach (var group in query)
{
    Console.WriteLine(group.Key == 0 ? "\nEven numbers:" : "\nOdd numbers:");
    foreach (int i in group)
        Console.WriteLine(i);
}

/* This code produces the following output:

    Odd numbers:
    35
    3987
    199
    329

    Even numbers:
    44
    200
    84
    4
    446
    208
*/

See Also

Tasks

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

How to: Group Files by Extension (LINQ)

How to: Group Results in Various Ways (C# Programming Guide)

How to: Perform a Subquery on a Grouping Operation (C# Programming Guide)

How to: Split a File Into Many Files by Using Groups (LINQ)

Concepts

Standard Query Operators Overview

Reference

group clause (C# Reference)

Group By Clause (Visual Basic)

System.Linq