Example extract and override factory method

ExtractAndOverrideFactoryMethod.cs

class FileTransactionsRepository : ITransactionsRepository
{
     // more code...
}

class Account 
{
    private ITransactionsRepository _transactionsRepository;

    public Account()
    {
        var filePath = Config.GetTransactionsFileLocation(); // <- side effect in a constructor!! x(
        var xmlPath = Config.GetTransactionsXmlLocation(); // <- side effect in a constructor!! x(
        var persister = new FileXmlPersister(xmlPath);
        this._transactionsRepository = new FileTransactionsRepository(filePath, persister);
        // more code...
    }
    
    // more code...
}

//////////////////////////////////////////////

// After extracting the factory method, 
// you can apply Subclass & Override Method to override the factory method

class Account 
{
    private ITransactionsRepository _transactionsRepository;

    public Account()
    {
        // extracted a factory method
        this._transactionsRepository = CreateTransactionsRepository();
        // more code...
    }

    // factory method
    private ITransactionsRepository CreateTransactionRepository()
    {
        var filePath = Config.GetTransactionsFileLocation(); // <- side effect in a constructor!! x(
        var xmlPath = Config.GetTransactionsXmlLocation(); // <- side effect in a constructor!! x(
        var persister = new FileXmlPersister(xmlPath);
        this._transactionsRepository = new FileTransactionsRepository(filePath, persister);
    }
    
    // more code...
}

//////////////////////////////////////////////

// After applying Subclass & Override Factory Method

class Account 
{
    private ITransactionsRepository _transactionsRepository;

    public Account()
    {
        this._transactionsRepository = CreateTransactionsRepository();
        // more code...
    }

    // made protected for testing :(
    protected virtual ITransactionsRepository CreateTransactionRepository()
    {
        var filePath = Config.GetTransactionsFileLocation(); // <- side effect in a constructor!! x(
        var xmlPath = Config.GetTransactionsXmlLocation(); // <- side effect in a constructor!! x(
        var persister = new FileXmlPersister(xmlPath);
        this._transactionsRepository = new FileTransactionsRepository(filePath, persister);
    }
    
    // more code...
}


//////////////////////////////////////////////

// In some test we'll override the factory method 
// to return a test double of ITransactionsRepository

class ForTestingAccount : Account
{
    
    protected override ITransactionsRepository CreateTransactionRepository()
    {
        return new FakeTransactionRepository();
    }
}