nop_implementation_example.cs

class BankAccount {
  public void Spend(double amount) { ... }
}

/* ---- */
class OneClient {
  ...
    bankAccount.Spend(amount);
  ...
}
/* ---- */
class OtherClient {
    ...
    bankAccount.Spend(amount);
    ...
}

  /* new changes => log expenses that exceed 1000 euros in a text file */

// Refactor step 1
class BankAccount {
  // ...
  
  public BankAccount(...) { // used by clients in production code
    // ...
  }

  public void Spend(double amount) { // used by clients in production code
    // ...
   
    LogExpense(amount);
  }
  
  private void LogExpense(double amount) { 
     
  }
}

// Refactor step 2
class BankAccount {
  //...
  
  public BankAccount(...) { // used by clients in production code
    // ...
    _expenseLogger = new NoExpenseLogger();
  }

  public void Spend(double amount) { // used by clients in production code
    // ...
    
    LogExpense(amount);
  }
  
  private void LogExpense(double amount) { 
     _expenseLogger.Log(amount);
  }
}

class NoExpenseLogger {
  public void Log(double amount) { }
}

// Refactor step 3
class BankAccount {
  // ...
  
  public BankAccount(...) { // used by clients in production code
    // ...
    _expenseLogger =  new NoExpenseLogger();
  }
  
  public BankAccount(..., ExpenseLogger expenseLogger) { // not used yet by clients in production code, used in tests
    // ...
    _expenseLogger =  expenseLogger;
  }

  public void Spend(double amount) { // used by clients in production code
    // ...
    
    LogExpense(amount);
  }
  
  private void LogExpense(double amount) {
     _expenseLogger.Log(amount);
  } 
}

class NoExpenseLogger : ExpenseLogger {
  public void Log(double amount) {  }
}

interface ExpenseLogger {
  void Log(double amount) 
}

class BigExpenseLogger : ExpenseLogger { // used only in tests
  /// ...
  
  BigExpenseLogger(Logger logger, double threshold) {
    _logger = logger;
    _threshold = threshold;
  }
  
  public void Log(amount) {  
    if(amount > _threshold) {  
       _logger.Log("Big expense!! " + amount);
    }
  }
}

// Final state: new feature working
class BankAccount {
  /// ...
  
  public BankAccount(..., ExpenseLogger expenseLogger) { // used by clients in production code
    // ...
    _expenseLogger =  expenseLogger;
  }

  public void Spend(double amount) { // used by clients in production code
    // ...
    
    _expenseLogger.Log(amount);
  }
}

class BigExpenseLogger : ExpenseLogger { // used by clients in production
  /// ...
  
  BigExpenseLogger(Logger logger, double threshold) {
    _logger = logger;
    _threshold = threshold;
  }
  
  public void Log(amount) {  
    if(amount > _threshold) {  
       _logger.Log("Big expense!! " + amount);
    }
  }
}

interface ExpenseLogger {
  void Log(double amount) 
}