Delphi Code Refactoring — Skill

Use this skill when the user requests refactoring, code review or removal of code smells in Object Pascal. Refactoring never changes observable behavior — it only improves the internal structure of the code.

When to Use

• User asks to "improve", "clean up" or "refactor" existing code
• When reviewing code and finding code smells
• When preparing legacy code to receive new functionality
• Before adding tests to untestable code
• When answering "why is this code difficult to understand?"

Fundamental Principle

"Refactoring is the art of changing the structure of code without changing its behavior." Always write (or check for) tests before refactoring.

📋 Catalog of Code Smells and Techniques

1. Extract Method — Long Method

Detect: Method with more than 20 lines or that needs comments to explain blocks.

Before:

procedure TInvoiceService.GenerateInvoice(AOrder: TOrder);
var
  LTax, LSubtotal, LTotal: Currency;
  LItem: TOrderItem;
  LLines: TStringList;
begin
  //Calculate subtotal
  LSubtotal := 0;
  for LItem in AOrder.Items do
    LSubtotal += LItem.UnitPrice * LItem.Quantity;

  //Calculates tax
  if AOrder.IsExempt then LTax := 0
  else LTax := LSubtotal * 0.12;

  LTotal := LSubtotal + LTax;

  //Generates report lines
  LLines := TStringList.Create;
  try
    LLines.Add('NOTA FISCAL');
    LLines.Add(Format('Cliente: %s', [AOrder.Customer.Name]));
    for LItem in AOrder.Items do
      LLines.Add(Format('  %s x%d = R$%.2f',
        [LItem.Product.Name, LItem.Quantity, LItem.UnitPrice * LItem.Quantity]));
    LLines.Add(Format('Total: R$%.2f', [LTotal]));
    LLines.SaveToFile(AOrder.InvoicePath);
  finally
    LLines.Free;
  end;
end;

After:

procedure TInvoiceService.GenerateInvoice(AOrder: TOrder);
var
  LSubtotal, LTax: Currency;
begin
  LSubtotal := CalculateSubtotal(AOrder);
  LTax      := CalculateTax(AOrder, LSubtotal);
  SaveInvoiceFile(AOrder, LSubtotal + LTax);
end;

function TInvoiceService.CalculateSubtotal(AOrder: TOrder): Currency;
var LItem: TOrderItem;
begin
  Result := 0;
  for LItem in AOrder.Items do
    Result := Result + (LItem.UnitPrice * LItem.Quantity);
end;

function TInvoiceService.CalculateTax(AOrder: TOrder; ASubtotal: Currency): Currency;
const
  STANDARD_TAX_RATE = 0.12;
begin
  if AOrder.IsExempt then Result := 0
  else Result := ASubtotal * STANDARD_TAX_RATE;
end;

procedure TInvoiceService.SaveInvoiceFile(AOrder: TOrder; ATotal: Currency);
var
  LLines: TStringList;
  LItem: TOrderItem;
begin
  LLines := TStringList.Create;
  try
    LLines.Add('NOTA FISCAL');
    LLines.Add(Format('Cliente: %s', [AOrder.Customer.Name]));
    for LItem in AOrder.Items do
      LLines.Add(Format('  %s x%d = R$%.2f',
        [LItem.Product.Name, LItem.Quantity, LItem.UnitPrice * LItem.Quantity]));
    LLines.Add(Format('Total: R$%.2f', [ATotal]));
    LLines.SaveToFile(AOrder.InvoicePath);
  finally
    LLines.Free;
  end;
end;

2. Extract Class — Class with Multiple Responsibilities

Detect: Class with fields of different nature, methods without cohesion.

Before:

TEmployee = class
private
  //Personal data
  FName: string;
  FBirthDate: TDate;
  FCpf: string;
  //Salary data
  FBaseSalary: Currency;
  FBonusPercentage: Double;
  FDepartmentId: Integer;
  //HR Data
  FHiredDate: TDate;
  FVacationDaysLeft: Integer;
  FPerformanceScore: Integer;
public
  function CalculateGrossSalary: Currency;
  function CalculateNetSalary: Currency;
  function CalculateVacationPay: Currency;
  function GetNextVacationDate: TDate;
  function GetYearsOfService: Integer;
  function IsEligibleForBonus: Boolean;
end;

After:

//Value Object — immutable
TEmployeePersonalData = record
  Name: string;
  BirthDate: TDate;
  Cpf: string;
end;

//Cohesive class: salary responsibility
TEmployeeSalary = class
private
  FBaseSalary: Currency;
  FBonusPercentage: Double;
public
  constructor Create(ABaseSalary: Currency; ABonusPercentage: Double);
  function CalculateGross: Currency;
  function CalculateNet: Currency;
  function IsEligibleForBonus: Boolean;
end;

//Cohesive class: HR responsibility
TEmployeeHrRecord = class
private
  FHiredDate: TDate;
  FVacationDaysLeft: Integer;
  FPerformanceScore: Integer;
public
  function GetYearsOfService: Integer;
  function GetNextVacationDate: TDate;
  function CalculateVacationPay(AGrossSalary: Currency): Currency;
end;

//Main entity — now just aggregates the parts
TEmployee = class
private
  FPersonalData: TEmployeePersonalData;
  FSalary: TEmployeeSalary;
  FHrRecord: TEmployeeHrRecord;
public
  constructor Create(AData: TEmployeePersonalData;
    ASalary: TEmployeeSalary; AHr: TEmployeeHrRecord);
  destructor Destroy; override;
  property PersonalData: TEmployeePersonalData read FPersonalData;
  property Salary: TEmployeeSalary read FSalary;
  property HrRecord: TEmployeeHrRecord read FHrRecord;
end;

3. Replace Nested Conditionals with Guard Clauses

Detect: More than 2 levels of if..then..begin..end nested.

Before:

function TBankService.Withdraw(AAccount: TAccount; AAmount: Currency): Boolean;
begin
  Result := False;
  if Assigned(AAccount) then
  begin
    if AAccount.IsActive then
    begin
      if AAmount > 0 then
      begin
        if AAccount.Balance >= AAmount then
        begin
          if not AAccount.IsBlocked then
          begin
            AAccount.Balance := AAccount.Balance - AAmount;
            FRepository.Save(AAccount);
            Result := True;
          end;
        end;
      end;
    end;
  end;
end;

After:

function TBankService.Withdraw(AAccount: TAccount; AAmount: Currency): Boolean;
begin
  if not Assigned(AAccount) then
    raise EArgumentNilException.Create('Conta não pode ser nula');
  if not AAccount.IsActive then
    raise EBusinessRuleException.Create('Conta inativa');
  if AAmount <= 0 then
    raise EValidationException.Create('Valor do saque deve ser positivo');
  if AAccount.Balance < AAmount then
    raise EInsufficientFundsException.CreateFmt(
      'Saldo insuficiente. Disponível: R$%.2f', [AAccount.Balance]);
  if AAccount.IsBlocked then
    raise EBusinessRuleException.Create('Conta bloqueada');

  AAccount.Balance := AAccount.Balance - AAmount;
  FRepository.Save(AAccount);
  Result := True;
end;

4. Replace Magic Numbers with Constants

Detect: Numeric or string literals without an explanatory name in the code.

Before:

if AProduct.Stock < 5 then NotifyLowStock(AProduct);
LInstallmentValue := AOrder.Total / 12;
if APassword.Length < 8 then raise ...;
if AUser.FailedLogins >= 3 then LockAccount(AUser);
LInterest := ADebt * 0.02;  //late payment interest

After:

const
  LOW_STOCK_THRESHOLD       = 5;
  MAX_INSTALLMENTS          = 12;
  MIN_PASSWORD_LENGTH       = 8;
  MAX_FAILED_LOGIN_ATTEMPTS = 3;
  MONTHLY_INTEREST_RATE     = 0.02;

if AProduct.Stock < LOW_STOCK_THRESHOLD then NotifyLowStock(AProduct);
LInstallmentValue := AOrder.Total / MAX_INSTALLMENTS;
if APassword.Length < MIN_PASSWORD_LENGTH then raise ...;
if AUser.FailedLogins >= MAX_FAILED_LOGIN_ATTEMPTS then LockAccount(AUser);
LInterest := ADebt * MONTHLY_INTEREST_RATE;

5. Replace Conditional with Polymorphism (Strategy/State)

Detect: case or if/else if string that checks the type or regime of an object.

Before:

function TShippingService.CalculateFreight(AOrder: TOrder): Currency;
begin
  case AOrder.ShippingMethod of
    smPAC:     Result := AOrder.Weight * 2.50 + 8.0;
    smSEDEX:   Result := AOrder.Weight * 4.80 + 15.0;
    smTransp:  Result := AOrder.Weight * 1.20;
    smRetirada:Result := 0;
  end;
end;

After:

//Interface Strategy
IFreightStrategy = interface
  ['{GUID}']
  function Calculate(AWeightKg: Double): Currency;
  function GetCarrierName: string;
end;

//Implementations: one per variation
TPACFreightStrategy = class(TInterfacedObject, IFreightStrategy)
  function Calculate(AWeightKg: Double): Currency;   //weight * 2.50 + 8.0
  function GetCarrierName: string;
end;

TSEDEXFreightStrategy = class(TInterfacedObject, IFreightStrategy)
  function Calculate(AWeightKg: Double): Currency;   //weight * 4.80 + 15.0
  function GetCarrierName: string;
end;

TTransportadoraFreightStrategy = class(TInterfacedObject, IFreightStrategy)
  function Calculate(AWeightKg: Double): Currency;   //weight * 1.20
  function GetCarrierName: string;
end;

TRetiradaFreightStrategy = class(TInterfacedObject, IFreightStrategy)
  function Calculate(AWeightKg: Double): Currency;   //always 0
  function GetCarrierName: string;
end;

//Context — no longer needs the case
function TShippingService.CalculateFreight(AOrder: TOrder;
  AStrategy: IFreightStrategy): Currency;
begin
  Result := AStrategy.Calculate(AOrder.WeightKg);
end;

6. Introduce Parameter Object

Detect: Method with > 4 parameters, especially if several are optional.

Before:

function TReportService.GenerateSalesReport(
  AStartDate: TDate;
  AEndDate: TDate;
  const AProductCategory: string;
  const ASalesRepId: string;
  AMinAmount: Currency;
  AGroupByMonth: Boolean;
  AIncludeReturns: Boolean): TReportResult;

After:

TSalesReportFilter = record
  StartDate: TDate;
  EndDate: TDate;
  ProductCategory: string;
  SalesRepId: string;
  MinAmount: Currency;
  GroupByMonth: Boolean;
  IncludeReturns: Boolean;
  //Constructor with defaults
  class function Default: TSalesReportFilter; static;
end;

class function TSalesReportFilter.Default: TSalesReportFilter;
begin
  Result.StartDate       := StartOfYear(Date);
  Result.EndDate         := Date;
  Result.ProductCategory := '';
  Result.SalesRepId      := '';
  Result.MinAmount       := 0;
  Result.GroupByMonth    := False;
  Result.IncludeReturns  := False;
end;

function TReportService.GenerateSalesReport(
  AFilter: TSalesReportFilter): TReportResult;

7. Remove with Statement

Detect: Any with Objeto do begin...end in the code.

Rule: Never use with. Prefer local variables or explicit qualification.

Before:

procedure TCustomerForm.LoadData;
begin
  with qryCustomers do
  begin
    Close;
    SQL.Text := 'SELECT * FROM customers WHERE id = :id';
    ParamByName('id').AsInteger := FCustomerId;
    Open;
    with FieldByName('address') do
    begin
      edtStreet.Text := AsString;
    end;
  end;
end;

After:

procedure TCustomerForm.LoadData;
var LAddressField: TField;
begin
  qryCustomers.Close;
  qryCustomers.SQL.Text := 'SELECT * FROM customers WHERE id = :id';
  qryCustomers.ParamByName('id').AsInteger := FCustomerId;
  qryCustomers.Open;

  LAddressField := qryCustomers.FieldByName('address');
  edtStreet.Text := LAddressField.AsString;
end;

8. Extract Interface / Invert Dependency

Detect: Service or class depending directly on concrete class (not interface).

Before:

TOrderService = class
private
  FEmailSender: TSmtpEmailSender;      //concrete class
  FRepository:  TFireDACOrderRepo;     //concrete class
public
  constructor Create;  //instance internally — impossible to test
end;

constructor TOrderService.Create;
begin
  FEmailSender := TSmtpEmailSender.Create('smtp.server.com', 587);
  FRepository  := TFireDACOrderRepo.Create(GetDatabaseConnection);
end;

After:

//1. Extract interfaces
IEmailSender = interface
  ['{GUID}']
  procedure Send(const ATo, ASubject, ABody: string);
end;

IOrderRepository = interface
  ['{GUID}']
  function FindById(AId: Integer): TOrder;
  procedure Save(AOrder: TOrder);
end;

//2. Inject into the constructor — testable and flexible
TOrderService = class
private
  FEmailSender: IEmailSender;
  FRepository:  IOrderRepository;
public
  constructor Create(ARepository: IOrderRepository; AEmailSender: IEmailSender);
end;

//3. Em testes, injete fakes
TFakeEmailSender = class(TInterfacedObject, IEmailSender)
  FSentMessages: TStringList;
  procedure Send(const ATo, ASubject, ABody: string);
end;

9. Rename — Self-Descriptive Names

Detect: Variables like x, tmp, data, flag; methods like Proc1, DoIt, Handle.

//❌ BEFORE
var x, tmp: Integer;
    s: string;
    flag: Boolean;
procedure Handle(d: TData);
function Calc(v: Double): Double;

//✅ AFTER
var LRetryCount, LMaxRetries: Integer;
    LCustomerFullName: string;
    LIsPaymentApproved: Boolean;
procedure ProcessCustomerOrder(AOrderData: TOrderData);
function CalculateShippingCost(AWeightKg: Double): Currency;

10. Inline Method — Unnecessarily Delegate Method

Detect: Method that just calls another method, without additional logic.

//❌ BEFORE — unnecessary delegation
function TOrder.GetTotal: Currency;
begin
  Result := CalculateOrderTotal;  //just delegate
end;

function TOrder.CalculateOrderTotal: Currency;
begin
  Result := FSubtotal + FTax - FDiscount;
end;

//✅ AFTER — just a method with a descriptive name
function TOrder.GetTotal: Currency;
begin
  Result := FSubtotal + FTax - FDiscount;
end;

🔄 Secure Refactoring Process

1. ENTENDER  → Leia e compreenda o código atual
2. TESTAR    → Escreva/execute testes que cobrem o comportamento atual
3. PEQUENO   → Faça uma refatoração por vez (não tudo de uma vez)
4. TESTAR    → Execute os testes novamente — devem continuar passando
5. COMMIT    → Commit atômico por refatoração (mensagem: "refactor: extract CalculateTax")
6. REPETIR   → Próxima refatoração

✅ Final Checklist

• [ ] Is there testing covering the behavior before refactoring?
• [ ] Does the observable behavior remain the same?
• [ ] No with was introduced?
• [ ] No nameless magic number?
• [ ] Resulting method has ≤ 20 lines?
• [ ] Name of the extracted method needs no comment?
• [ ] Dependencies are now via interface?
• [ ] Do guard clauses replace deep nesting?
• [ ] Were excess parameters grouped in DTO/Record?