Clean Code Principles

DRY - Don't Repeat Yourself

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

Code Duplication

// BAD: Duplicated validation logic
public class UserService
{
    public void CreateUser(string email, string name)
    {
        if (string.IsNullOrWhiteSpace(email) || !email.Contains("@"))
            throw new ArgumentException("Invalid email");
        // ...
    }