Add C# implementation of Ceasar Cipher

cryptography/caesar_cipher/csharp/caesar_cipher.cs

@@ -0,0 +1,35 @@
+using System;
+using System.Linq;
+
+namespace CaesarCipher
+{
+    class Program
+    {
+        static char Encrypt(char ch, int shift)
+        {
+            if (!char.IsLetter(ch)) return ch; 
+            char offset = char.IsUpper(ch) ? 'A' : 'a';
+            return (char)((ch + shift - offset) % 26 + offset);
+        }
+
+        static string Encrypt(string input, int shift)
+        {
+            return String.Join(String.Empty, input.Select(ch => Encrypt(ch, shift)));
+        }
+
+        static string Decrypt(string input, int shift)
+        {
+            return Encrypt(input, 26 - shift);
+        }
+
+        static void Main()
+        {
+            string original = "Hey al-go-rithms! Hello from C#";
+            string encrypted = Encrypt(original, 5);
+            string decrypted = Decrypt(encrypted, 5);
+            Console.WriteLine($"Original: {original}");
+            Console.WriteLine($"Encrypted: {encrypted}");
+            Console.WriteLine($"Decrypted: {decrypted}");
+        }
+    }
+}