Delphi Code Refactoring

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.

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📋 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;