A group operation puts data into groups such that the elements in each group share a common attribute.

Methods

Method	Description	Query expression
`GroupBy`	Groups elements that share a common attribute. Groups are represented by an IGrouping<TKey,TElement>.	group … by
`ToLookup`	Inserts elements into a Lookup<TKey, TElement> based on a key-selector function. A Lookup is a one-to-many dictionary.	N/A

Examples

Query Expression

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)
    if (group.Key == 0) // These are the even numbers
        group.ForEach(i => Console.WriteLine(i));
    else // These are the odd numbers
        …

Method

Assuming:

class Pet
{
    public string Name { get; set; }
    public double Age { get; set; }
}

Given a List<Pet> petList:

var query = petsList.GroupBy(
    pet => Math.Floor(pet.Age), // key-selector: group pet.Age values by the floor of the age
    pet => pet.Age, // element-selector: map each source element to an element in an IGrouping
    (baseAge, ages) => new // result-selector: project an anonymous type from each group
    {
        Key = baseAge,
        Count = ages.Count(),
        Min = ages.Min(),
        Max = ages.Max()
    });