diff --git a/archive/c/c-sharp/Base64EncodeDecode.cs b/archive/c/c-sharp/Base64EncodeDecode.cs
new file mode 100644
index 000000000..60b05cdd6
--- /dev/null
+++ b/archive/c/c-sharp/Base64EncodeDecode.cs
@@ -0,0 +1,67 @@
+using System.Text;
+
+public class Base64EncodeDecode
+{
+    public static void Usage()
+    {
+        Console.WriteLine("Usage: please provide a mode and a string to encode/decode");
+        Environment.Exit(1);
+    }
+
+
+    private static bool IsValidBase64(string input)
+    {
+        if (string.IsNullOrWhiteSpace(input) || input.Length % 4 != 0)
+            return false;
+
+        foreach (char c in input)
+        {
+            if (!"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=".Contains(c))
+                return false;
+        }
+
+        int padCount = input.EndsWith("==") ? 2 :
+                       input.EndsWith('=') ? 1 : 0;
+
+        int firstPadIndex = input.IndexOf('=');
+        return firstPadIndex == -1 || firstPadIndex >= input.Length - padCount;
+    }
+
+    public static int Main(string[] args)
+    {
+        if (args.Length != 2)
+        {
+            Usage();
+            return 1;
+        }
+
+        string mode = args[0].ToLowerInvariant();
+        string value = args[1];
+
+        if (string.IsNullOrWhiteSpace(mode) || string.IsNullOrWhiteSpace(value))
+        {
+            Usage();
+            return 1;
+        }
+
+        try
+        {
+            string result = mode switch
+            {
+                "encode" => Convert.ToBase64String(Encoding.UTF8.GetBytes(value)),
+                "decode" => IsValidBase64(value)
+                            ? Encoding.UTF8.GetString(Convert.FromBase64String(value))
+                            : throw new ArgumentException("Input is not valid Base64."),
+                _ => throw new ArgumentException("Unknown mode. Use 'encode' or 'decode'.")
+            };
+
+            Console.WriteLine(result);
+            return 0;
+        }
+        catch
+        {
+            Usage();
+            return 1;
+        }
+    }
+}