Program.cs

string original = "{\"name\":\"John\", \"age\":30, \"car\":null}";
string base64 = Base64Encode(original);
string decoded = Base64Decode(base64);
Console.WriteLine($"Original: {original}");
Console.WriteLine($"Base64: {base64}");
Console.WriteLine($"Decoded: {decoded}");

string Base64Encode(string plainText)
{
  var plainTextBytes = System.Text.Encoding.UTF8.GetBytes(plainText);
  return System.Convert.ToBase64String(plainTextBytes);
}

string Base64Decode(string base64EncodedData)
{
  var base64EncodedBytes = System.Convert.FromBase64String(base64EncodedData);
  return System.Text.Encoding.UTF8.GetString(base64EncodedBytes);
}