String decryptor relying on the stack. Made for teaching

See articles above : https://docs.microsoft.com/en-us/dotnet/api/system.collections.stack?view=netcore-3.1 https://www.tutorialsteacher.com/csharp/csharp-stack

It's more reliable than checking series of instructions as argument can be initialized before the call

Absolutely not, you can calculate the int of a switch or calculating math operation for example