Can we protect our codes with ienumerator against decompiling?

I saw that Enumerators and coroutines can not be decompiled using standard decompiler like ilspy. is it right?
And suppose we have a key string. A hacker must not read it. If I write private string key=“329210”; he can see it. Is it appropriate I serialize or write it in the inspector instead of the code?

plz answer. thank you. or guide how can get the answer