Explain Single Responsibility Principle with a code example

SRP: a class should have one reason to change.

Violation — UserService doing too much:

public class UserService {
    public void Register(string email, string password) {
        // validate
        if (!email.Contains("@")) throw new ArgumentException();
        // hash password
        var hash = BCrypt.HashPassword(password);
        // persist
        db.Users.Add(new User { Email = email, Hash = hash });
        db.SaveChanges();
        // send welcome email
        smtp.Send(email, "Welcome", "Hi there");
        // log
        File.AppendAllText("audit.log", $"{DateTime.Now}: {email}\n");
    }
}

Five reasons to change: validation rules, hashing algo, persistence, email content, audit format. Each touches the same file.

Refactored — each class owns one concern:

public class UserRegistration(
    IEmailValidator validator,
    IPasswordHasher hasher,
    IUserRepository users,
    IWelcomeEmailSender mailer,
    IAuditLog audit)
{
    public async Task RegisterAsync(string email, string password) {
        validator.Validate(email);
        var hash = hasher.Hash(password);
        await users.AddAsync(new User { Email = email, Hash = hash });
        await mailer.SendAsync(email);
        await audit.LogAsync(AuditEvent.UserRegistered, email);
    }
}

UserRegistration orchestrates. Each collaborator has one job. Changing the email content doesn't touch the registration class.

Useful test: if two different stakeholders (design, finance, security) edit the same class, you've violated SRP.