应用程序中验证的最佳位置在哪里？经验法则？

I am making a C# app for a class project. I want to ensure a string has one of three values. Normally, in a web app, I would do validation with javascript on the client side. However, this is currently a console app. I know that I should do the validation early, but what are some good rules of thumb for validation?I am making a C# app for a class project. I wan