---
name: delphi-pascal
description: Delphi and Pascal development patterns, Object Pascal best practices, VCL/FMX framework guidance, and legacy Pascal modernization. Use when building Windows desktop apps, maintaining legacy Delphi codebases, or migrating Pascal to modern platforms.
---

# Delphi & Pascal Development

> Patterns and practices for Object Pascal / Delphi development.

## When to Use This Skill

- Building Windows desktop applications with VCL or FireMonkey (FMX)
- Maintaining or modernizing legacy Delphi/Pascal codebases
- Cross-platform development with Delphi (Windows, macOS, Linux, mobile)
- Database applications with FireDAC or dbExpress
- REST/JSON API development with Delphi

## Modern Delphi Best Practices

### Project Structure
```
MyProject/
├── src/
│   ├── Main.dpr           # Project file
│   ├── Forms/             # UI forms
│   │   ├── MainForm.pas
│   │   └── MainForm.dfm
│   ├── DataModules/       # Data access
│   │   └── dmMain.pas
│   ├── BusinessLogic/     # Domain logic
│   │   └── UserService.pas
│   └── Utils/             # Helpers
│       └── CryptoUtils.pas
├── tests/
│   └── DUnitX/
└── packages/              # Custom components
```

### Naming Conventions

| Type | Prefix | Example |
|------|--------|---------|
| Type | T | `TUser` |
| Class ref | Class | `TUserClass` |
| Interface | I | `IUserService` |
| Record | T | `TPoint` |
| Enum | T | `TColorScheme` |
| Enum value | 2-letter | `csDark` |
| Field | F | `FUserName` |
| Parameter | A | `AUserName` |
| Local var | L | `LResult` |
| Unit | 2-letter prefix | `uUserService` |
| Form | Form | `frmMain` |
| DataModule | dm | `dmDatabase` |
| Component | Type prefix | `btnSave`, `edtName`, `grdUsers` |

### Error Handling
```pascal
// ✅ GOOD - Proper exception handling
try
  Connection.Open;
  Query.Execute;
except
  on E: EDatabaseError do
  begin
    Log.Error('Database error: %s', [E.Message]);
    raise EServiceException.CreateFmt('Service unavailable: %s', [E.Message]);
  end;
end;

// ❌ BAD - Swallowing exceptions
try
  DoSomething;
except
  // silent failure
end;
```

### Memory Management
```pascal
// ✅ GOOD - RAII pattern with interfaces
var
  Service: IService;
begin
  Service := TService.Create;
  Service.Process;  // Auto-freed when Service goes out of scope
end;

// ✅ GOOD - Try-finally for objects
var
  Stream: TFileStream;
begin
  Stream := TFileStream.Create('file.dat', fmCreate);
  try
    Stream.WriteBuffer(Data, SizeOf(Data));
  finally
    Stream.Free;
  end;
end;

// ❌ BAD - No cleanup
var
  Obj: TObject;
begin
  Obj := TObject.Create;
  Obj.DoSomething;
  // Memory leak!
end;
```

### Modern Features (Delphi 10.4+)

- **Inline Variables**: `var X: Integer := 42;`
- **Record Initialization**: `var R: TRec := (A: 1; B: 2);`
- **Custom Managed Records**: Initialize/Finalize operators
- **Multi-Platform**: Same code for Windows, macOS, Linux, iOS, Android

## Framework Selection

| Need | Framework | Notes |
|------|-----------|-------|
| Windows Desktop | VCL | Mature, rich component ecosystem |
| Cross-Platform UI | FireMonkey (FMX) | Windows, macOS, Linux, mobile |
| Console/Service | RTL only | No UI, lightweight |
| Web API | RAD Server / DataSnap | REST/JSON endpoints |
| Database | FireDAC | Multi-database, feature-rich |
| ORM | TMS Aurelius | Full ORM for Delphi |
| Testing | DUnitX | Modern test framework |

## Migration Patterns (Legacy → Modern)

| Legacy Pattern | Modern Equivalent |
|---------------|------------------|
| `StringList.CommaText` | `TJSONValue` / `System.JSON` |
| `TClientDataSet` | FireDAC + LiveBindings |
| `BDE` | FireDAC |
| `Goto` statements | Structured control flow |
| Global variables | Dependency injection |
| `with` keyword | Explicit references |
| `.dfm` streaming | Code-based UI creation |