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Generic Merge Sort in C# .NET

Niels Swimberghe
Belgian Full Stack Developer solving problems and delivering value to customers using .NET technologies for back-end systems, and modern JavaScript technologies for the front-end.
Originally published at swimburger.net on ・2 min read

Early this year, I decided to brush up on my algorithms and data structure knowledge. I took these great two courses (1, 2) on PluralSight by Robert Horvick.

To practice what I learned in this course, I decided to create generic versions of the different algorithms and data structures.

What do I mean by generic versions? These types of courses always use integers or strings to demonstrate the algorithms. Instead of using those primitive data types, I'm reimplementing the algorithms and data structures using C#'s generic type parameters.

Here's a console application with a generic method MergeSort to perform a merge sort on an enumerable:

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        var randomNumbers = new int[] { 5, 4, 5, 7, 6, 9, 4, 1, 1, 3, 4, 50, 56, 41 };

        var sortedNumbers = MergeSort(randomNumbers.AsEnumerable());
        Console.WriteLine("Merge Sort:");
        PrintList(sortedNumbers);
        Console.ReadKey();
        Console.Clear();
    }

    private static IEnumerable<T> MergeSort<T>(IEnumerable<T> list) where T : IComparable
    {
        T[] items = list.ToArray();
        return InternalMergeSort(items);
    }

    private static T[] InternalMergeSort<T>(T[] items) where T : IComparable
    {
        int listLength = items.Length;

        if (listLength == 1)
        {
            return items;
        }

        int median = listLength / 2;

        T[] left = new T[median];
        T[] right = new T[listLength - median];
        Array.Copy(items, left, left.Length);
        Array.Copy(items, median, right, 0, right.Length);

        InternalMergeSort(left);
        InternalMergeSort(right);

        return Merge(items, left, right);
    }

    private static T[] Merge<T>(T[] items, T[] left, T[] right) where T : IComparable
    {
        int leftIndex = 0;
        int rightIndex = 0;

        int leftLength = left.Length;
        int rightLength = right.Length;
        int totalItems = leftLength + rightLength;

        for (int targetIndex = 0; targetIndex < totalItems; targetIndex++)
        {
            if(leftIndex >= leftLength)
            {
                items[targetIndex] = right[rightIndex];
                rightIndex++;
            }
            else if(rightIndex >= right.Length)
            {
                items[targetIndex] = left[leftIndex];
                leftIndex++;
            }
            else if(left[leftIndex].CompareTo(right[rightIndex]) < 0)
            {
                items[targetIndex] = left[leftIndex];
                leftIndex++;
            }
            else
            {
                items[targetIndex] = right[rightIndex];
                rightIndex++;
            }
        }

        return items;
    }

    private static void PrintList<T>(IEnumerable<T> list)
    {
        foreach (var item in list)
        {
            Console.WriteLine(item);
        }
    }
}

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By using a generic type parameter with the constraint that the type has to implement the IComparable interface, you can perform the merge sort algorithm without knowing the exact type you are working with.

If you want to understand the logic behind the merge sort algorithm, I recommend checking out the courses mentioned earlier. There's also a lot of other great resources out there online!

Disclaimer: This code works, but is only developed for the sake of practice. Use at your own risk or just use a sorting library. If you see some room for improvement, there most likely is, I'm all ears~

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