Example extract and override factory method

ExtractAndOverrideFactoryMethod.cs

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

class Account 
{
    private ITransactionsRepository _transactionsRepository;

    public Account()
    {
        var reader = new ModelReader(Config.TransactionsFileLocation);
        var persister = new XmlStore(Config.TransactionsFileLocation);
        this._transactionsRepository = new FileTransactionsRepository(reader, persister);
        // more code...
    }
    
    // more code...
}

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

// After extracting the factory method, 
// you can apply 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 reader = new ModelReader(Config.TransactionFile);
        var persister = new XmlStore(Config.TransactionFile);
        return new FileTransactionsRepository(reader, 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 reader = new ModelReader(Config.TransactionFile);
        var persister = new XmlStore(Config.TransactionFile);
        return new FileTransactionsRepository(reader, persister);
    }
    
    // more code...
}


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


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