delphi-memory-and-exceptions

🧠 Memory and Exceptions Management in Delphi

Contexto

Delphi has manual memory management for class instances (not derived from interfaces) and uses ARC (Automatic Reference Counting) only for interfaces (IInterface), Strings, Dynamic Arrays and anonymous types. Poor exception handling and forgetting to release memory result in chronic Memory Leaks, catastrophic production failures and systemic instability.

Like AI, you must proactively ensure that every object you create is freed regardless of error streams.

Objectives of this Skill

• Teach how to generate safe blocks of try..finally.
• Prevent Memory Leaks by advising on Free and FreeAndNil.
• Promote the use of Interfaces for memory automation.
• Institute defensive and typed handling of Exceptions (try..except).
• Introduce custom Domain Exceptions.

🛑 Memory Management: Critical Rules

1. The Gold Standard: try..finally

Whenever an object instance is created and does not have an Owner who manages it, instantiate it in a try..finally block. try must occur IMMEDIATELY on the line following creation.

var
  LList: TStringList;
begin
  LList := TStringList.Create;
  try
    LList.Add('Item 1');
    // ...
  finally
    LList.Free;
  end;
end;

Anti-Pattern (DO NOT USE): Code between Create and try may generate an exception, leaking the newly created object.

  //WRONG - potential leak!
  LList := TStringList.Create;
  LList.Add('Item 1');
  try
    // ...

2. Multiple Objects in the Same Block

When allocating multiple temporary resources in the same method, do not nest dozens of try..finally if it is not strictly necessary. But be careful to initialize them all with nil beforehand if there is a chance of leakage, or nest them prudently. The ideal pattern is guaranteed sequential release, but strict nesting is safest for chained allocations:

var
  LStream: TMemoryStream;
  LReader: TStreamReader;
begin
  LStream := TMemoryStream.Create;
  try
    LReader := TStreamReader.Create(LStream);
    try
      //logics with both
    finally
      LReader.Free;
    end;
  finally
    LStream.Free;
  end;
end;

3. Avoid Creating Objects for Single Passage

If an API takes a class parameter, declare an interface or instantiate it before the method with try..finally. Never pass an inline .Create to a parameter in a method if you do not have an absolute guarantee that the consuming function will free the memory.

4. Garbage Collection via Intefaces

For Dependency Injection, Repository/Service Patterns or Temporary Functional Classes, use inheritance from TInterfacedObject linked to a Interface. Delphi will kill the instance when the reference counter reaches zero.

var
  LService: ICustomerService;
begin
  //No try..finally and no .Free calls.
  //Memory is scanned when leaving the scope of this procedure.
  LService := TCustomerService.Create; 
  LService.ProcessDailyBatch;
end;

🚨 Exception Handling: The Transparent Standard

1. Specific, Not Generic Catches

Use try..except primarily to trap recoverable errors, log failures without breaking loops, or transform infrastructure exceptions into more semantic domain exceptions. Never "Swallow" an exception without logical justification.

try
  PerformDatabaseCommit;
except
  //SPECIFIC database capture
  on E: EFDDBEngineException do
  begin
    Logger.Error('Falha no banco de dados [Cód: %d]: %s', [E.ErrorCode, E.Message]);
    raise EDatabaseConnectionException.Create('Serviço temporariamente indisponível.');
  end;
  //Validation SPECIFIC Capture
  on E: EValidationException do
  begin
    ShowWarning(E.Message);
  end;
end;

Anti-Pattern (DO NOT USE): This blinds the application trace (hides AccessViolations and Out of Memory).

try
  ProcessData;
except
  //Wrong! Hides any developer errors during debugging!
end;

2. Creating Exceptions Based on Business Logic (DDD)

Do not use raise Exception.Create(str). Declare cohesive exceptions to enable elegant interception by upper layers (REST Controllers, UI Interface).

type
  //Domain/Essence of the Rules
  EBusinessRuleException = class(Exception);
  ECustomerLimitReachedException = class(EBusinessRuleException);
  
  //Infrastructure
  EInfrastructureException = class(Exception);
  EDatabaseConnectionException = class(EInfrastructureException);

3. Encapsulating Errors and Raise without Modifying Context

If you only need to perform a one-off log but want the exception to flow naturally to the global UI, just use pure raise;.

try
  SomeDangerousCall;
except
  on E: Exception do
  begin
    Logger.LogError('Critical failure', E);
    raise; //REPROJECT the original exception with the same stack-trace
  end;
end;

💡 AI Action Flow

When asked to write/refactor code:

1. Review whether each TObject.Create will result in a deallocation (.Free or Third-Party Ownership).
2. Inject try..finally if you notice legacy code without it.
3. If you generate Services, recommend Dependency Injection via Interfaces (IService) to simplify garbage scanning (GC).
4. In logic that may fail, create Typological Exceptions to validate flow and prevent error code spaghetti conditionals (if return = -1 then).