Added CSharp Fibonacci Checker program (#913)

algorithms/CSharp/README.md

@@ -40,6 +40,7 @@ To run the `.cs` file, kindly use [.Net Finddle](https://dotnetfiddle.net/)
 ## Maths
 
 - [Abundant Number](src/Maths/abundant-number.cs)
+- [Fibonacci Checker](src/Maths/fibonacci-checker.cs)
 - [Naismith's Rule](src/Maths/naismith-rule.cs)
 
 ## Queues

algorithms/CSharp/src/Maths/fibonacci-checker.cs

@@ -0,0 +1,32 @@
+/* Fibonacci Checker - A program to check whether a number is in the Fibonacci sequence.
+This sequence is defined by: Fn = Fn-1 + Fn-2 (to find Fn, you add the previous two numbers. The program calculates the Fibonacci sequence up to and including the given number. */
+using System;
+
+namespace Algorithms.Maths
+{
+    public class Fibonacci
+    {
+        public static bool Checker(int num)
+        {
+            if (num == 2 || num == 3)
+            {
+                return true;
+            }
+            var num1 = 0;
+            var num2 = 1;
+            var num3 = 1;
+
+            for (var i = 0; i <= num; i++)
+            {
+                if (num1 == num)
+                {
+                    return true;
+                }
+                num1 = num2;
+                num2 = num3;
+                num3 = num1 + num2;
+            }
+            return false;
+        }
+    }
+}

algorithms/CSharp/test/Maths/fibonacci-checker-tests.cs

@@ -0,0 +1,20 @@
+using NUnit.Framework;
+
+namespace Algorithms.Tests.Maths
+{
+    [TestFixture]
+    internal class Checker
+    {
+        [TestCase(1, true)]
+        [TestCase(3, true)]
+        [TestCase(4, false)]
+        [TestCase(34, true)]
+        [TestCase(70, false)]
+        [TestCase(110, false)]
+        public void FibonacciChecker_ShouldGetExpectedResult(int num, bool expected)
+        {
+            var result = Algorithms.Maths.Fibonacci.Checker(num);
+            Assert.AreEqual(expected, result);
+        }
+    }
+}