C# Back to Basics - Recursion and Recursive Methods

In this article, we are going to talk about Recursion and Recursive Methods in C#. Furthermore, we will learn how to use recursive methods and how they differ from regular methods.

This article is part of the series

For the complete navigation of this series check out: C# Back to Basics.

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If you want to download the source code for our examples, you can do that from here Recursive Methods in C# Source Code.

So what is recursion?

Recursion is a concept in which method calls itself. Every recursive method needs to be terminated, therefore, we need to write a condition in which we check is the termination condition satisfied. If we don’t do that, a recursive method will end up calling itself endlessly.

Example 1: Create an application which calculates the sum of all the numbers from n to m recursively:

class Program
{
    public static int CalculateSumRecursively(int n, int m)
    {
        int sum = n;

        if(n < m)
        {
            n++;
            return sum += CalculateSumRecursively(n, m);
        }

        return sum;
   }

    static void Main(string[] args)
    {
        Console.WriteLine("Enter number n: ");
        int n = Convert.ToInt32(Console.ReadLine());

        Console.WriteLine("Enter number m: ");
        int m = Convert.ToInt32(Console.ReadLine());

        int sum = CalculateSumRecursively(n, m);

        Console.WriteLine(sum);

        Console.ReadKey();
    }
}

Code Explanation

The method CalculateSumRecursively is our recursive method that calculates the sum of the numbers from n to m. The first thing we do is to set our sum to the value of n. Then, we check if the value of n is less than the value of m. If it is, we increase the value of n by 1 and add to our sum a result of the same method but with the increased n. If it is not, we just return the value of the sum variable.

The C# will reserve memory storage for every recursive method so that the values from the previous method are not overridden.

So let’s see our example through the diagram:

Additional Example

Let’s practice some more with the Example2: Create an application which prints out how many times the number can be divided by 2 evenly:

class Program
{
    public static int CountDivisions(double number)
    {
        int count = 0;

        if(number > 0 && number % 2 == 0)
        {
            count++;
            number /= 2;

            return count += CountDivisions(number);
        }

        return count;
    }

    static void Main(string[] args)
    {
        Console.WriteLine("Enter your number: ");
        double number = Convert.ToDouble(Console.ReadLine());

        int count = CountDivisions(number);
        Console.WriteLine($"Total number of divisions: {count}");

        Console.ReadKey();
    }
}