Group by

In C# a query expression can return its results as a group. With the group-by operator, we separate the results of an expression into parts.

This syntax makes some query expressions in C# programs more useful. The group-by clause comes at the end of a query expression. No select clause is required.

Example

With group by, the first part indicates what item is to be grouped. And after "by", you specify the condition that results in groups.

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 = from element in array
                     orderby element
                     group element by IsEven(element);

        // Loop over groups.
        foreach (var group in result)
        {
            // Display key and its values.
            Console.WriteLine(group.Key);
            foreach (var value in group)
            {
                Console.WriteLine(value);
            }
        }
    }

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

With group, we must know how to access its results. First, we use foreach to enumerate the groups themselves. On each group, we access the Key property.

The group-by clause is an alternative to the select clause in query expressions. It allows you to divide the results in any number of categories.