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: Create a Nested Group (C# Programming Guide)

How to: Group Files by Extension (LINQ)

How to: Group Query Results (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)

Reference

group clause (C# Reference)

Group By Clause (Visual Basic)

System.Linq

Concepts

Standard Query Operators Overview