chore(CSharp): added algorithm to determine factorial (#437)

algorithms/CSharp/README.md

@@ -16,3 +16,6 @@ For running the `.cs` file please using [.Net Finddle](https://dotnetfiddle.net/
 
 ## Maths
 - [Abundant Number](src/Maths/abundant-number.cs)
+
+## Recusrsion
+- [Factorial](src/Recursion/factorial.cs)

algorithms/CSharp/src/Recursion/factorial.cs

@@ -0,0 +1,27 @@
+using System;
+
+namespace Algorithms.Recursion
+{
+    public class Factorial
+    {
+        public static int FactorialUsingRecursion(int number)
+        {
+            if(number == 0 || number == 1)
+            {
+                return 1;
+            }
+
+            return number * FactorialUsingRecursion(number - 1);
+        }
+
+        public static void Main()
+        {
+            Console.WriteLine($"Factorial of 7 is: {FactorialUsingRecursion(17)}");
+        }
+    }
+}
+
+/*
+ * n! = n * (n - 1) * (n - 2) * .... * 3 * 2 * 1
+ * we have used int here but for larger number, int will not be enough
+ */

algorithms/CSharp/test/Recursion/factorial.cs

@@ -0,0 +1,18 @@
+using NUnit.Framework;
+
+namespace Algorithms.Tests.Recursion
+{
+    [TestFixture]
+    public class FactorialTest
+    {
+        [TestCase(0, 1)]
+        [TestCase(7, 5040)]
+        [TestCase(3, 6)]
+        [TestCase(1, 1)]
+        public void TestFactorial_ShouldGetExpectedResult(int number, int expected)
+        {
+            int result = Algorithms.Recursion.Factorial.FactorialUsingRecursion(number);
+            Assert.AreEqual(expected, result);
+        }
+    }
+}