GroupBy. This method transforms a collection into groups. Each group has a key. With this method from the System.Linq namespace, you can apply grouping to many collections.

Some usage notes. The GroupBy method can be used in away similar to a Dictionary of Lists. As with other LINQ methods, GroupBy has possible performance drawbacks.

GroupJoin

group

Example. We call GroupBy with the argument being a lambda expression. Each element is identified as "a" in the lambda expression in the example program.

using System;
using System.Linq;

class Program
{
    static void Main()
    {
        // Input array.
        int[] array = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };

        // Group elements by IsEven.
        var result = array.GroupBy(a => IsEven(a));

        // Loop over groups.
        foreach (var group in result)
        {
            // Display key for group.
            Console.WriteLine("IsEven = {0}:", group.Key);

            // Display values in group.
            foreach (var value in group)
            {
                Console.Write("{0} ", value);
            }

            // End line.
            Console.WriteLine();
        }
    }

    static bool IsEven(int value)
    {
        return value % 2 == 0;
    }
}
IsEven = False:
1 3 5 7 9
IsEven = True:
2 4 6 8

Discussion. Using GroupBy() is fine for certain parts of programs. However, if you are generating a collection that will be repeatedly used, it would be better to use ToDictionary.

ToDictionary

Collection notes. If each group contains multiple elements as is typical with GroupBy, you could use a List as the value of the Dictionary instance. Thus you would have a Dictionary of Lists.

Dictionary

List

A summary. We looked at the GroupBy extension method in the C# programming language. This is effective in dividing up a collection of elements.