encryption not decrypted in c# code example

Example: c# encrypt decrypt string

public static string Base64Decode(string base64EncodedData)
{
   var base64EncodedBytes = System.Convert.FromBase64String(base64EncodedData);
   return System.Text.Encoding.UTF8.GetString(base64EncodedBytes);
}
public static string Base64Encode(string plainText)
{
   var plainTextBytes = System.Text.Encoding.UTF8.GetBytes(plainText);
   return System.Convert.ToBase64String(plainTextBytes);
}