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Getting Started with LINQ in C#

GroupBy and OrderBy Methods

Learn how to group and order the elements of a collection using GroupBy and OrderBy methods.

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How to group elements of a collection

The GroupBy method groups the elements of a collection based on a grouping key. This method returns a collection of groups, or buckets, organized by that key.

For example, let’s group our movies by rating.

C#
using System;
using System.Collections.Generic;
using System.Linq;
namespace MovieCatalog
{
    internal class Program
    {
        public static void Main(string[] args)
        {
            var movies = new List<Movie>
            {
                new Movie("Titanic", 1998, 4.5f),
                new Movie("The Fifth Element", 1997, 4.6f),
                new Movie("Terminator 2", 1991, 4.7f),
                new Movie("Avatar", 2009, 5),
                new Movie("Platoon", 1986, 4),
                new Movie("My Neighbor Totoro", 1988, 5)
            };
            var groupedByRating = movies.GroupBy(movie => movie.Rating);
            foreach (var group in groupedByRating)
            {
                Console.WriteLine($"Rating: {group.Key}");
                foreach (var movie in group)
                {
                    Console.WriteLine($"{movie.Name}");
                }
                Console.WriteLine();
            }
        }
    }
    internal class Movie
    {
        public Movie(string name, int releaseYear, float rating)
        {
            Name = name;
            ReleaseYear = releaseYear;
            Rating = rating;
        }
        public string Name { get; set; }
        public int ReleaseYear { get; set; }
        public float Rating { get; set; }
    }
}

The GroupBy method receives as a parameter a delegate with a property to use as a key when grouping elements. In our previous example, we used the Rating and wrote movie => movie.Rating.

The result of GroupBy is a collection of groups or buckets. In our example, the type of the return collection was IEnumerable<IGrouping<float, Movie>>. That’s why we needed two foreach loops to print the movies in each group. One to print the ratings ...