Don't Repeat Yourself, Kyiv 2010

dont_repeat_yourself.cs

public class UserManager {
  public bool IsLoggedIn { get; private set; }

  public void LogIn(string userName, string password) {
    if (IsLoggedIn) {
      throw new InvalidOperationException("The user is already logged in");
    }

    if (userName == _userName && password == _password) {
      IsLoggedIn = true;
    } else {
      throw new ArgumentException("Invalid username or password");
    }
  }

  public void ChangePassword(string userName, string oldPassword, string newPassword) {
    if (IsLoggedIn) {
      throw new InvalidOperationException("The user is already logged in");
    }

    if (userName == _userName && oldPassword == _password) {
      _password = newPassword;
    } else {
      throw new ArgumentException("Invalid username or password");
    }
  }
}

less_repetition.cs

public class UserManager {
  public bool IsLoggedIn { get; private set; }

  public void LogIn(string userName, string password) {
    EnsureUserNotLoggedIn();

    VerifyCredentials(userName, password);
    IsLoggedIn = true;
  }

  public void ChangePassword(string userName, string oldPassword, string newPassword) {
    EnsureUserNotLoggedIn();

    VerifyCredentials(userName, oldPassword);
    _password = newPassword;
  }

  private void EnsureUserNotLoggedIn() {
    if (IsLoggedIn) {
      throw new InvalidOperationException("The user is already logged in");
    }
  }

  private void VerifyCredentials(string userName, string password) {
    if (userName != _userName || password != _password) {
      throw new ArgumentException("Invalid username or password");
    }
  }
}

Note how the same logic to check that a user is not already logged in, compare usernames and passwords, and throw certain exceptions is repeated in two places. We can clean this up to make the code more readable and less duplicative.