Why do interface implementations based on TComponent leak memory?

Question

This Delphi code will show a memory leak for an instance of TMyImplementation:

program LeakTest;

uses
  Classes;

type
  MyInterface = interface
  end;

  TMyImplementation = class(TComponent, MyInterface)
  end;

  TMyContainer = class(TObject)
  private
    FInt: MyInterface;
  public
    property Impl: MyInterface read FInt write FInt;
  end;

var
  C: TMyContainer;
begin
  ReportMemoryLeaksOnShutdown := True;

  C := TMyContainer.Create;
  C.Impl := TMyImplementation.Create(nil);
  C.Free;
end.

If TComponent is replaced by TInterfacedObject and the constructor changed to Create(), the leak disappears. What is different with TComponent here?

Many thanks for the answers. To sum up: it is easy, but wrong, to say "If you are using interfaces, they are reference counted and hence they are freed for you." - Actually any class which implements an interface can break this rule. (And there will be no compiler hint or warning be shown.)

Answer 1

Differences in implementation

• TComponent._Release does not free your instance.
• TInterfacedObject._Release does free your instance.

Perhaps someone can chime in but my take on this is that TComponent is not meant to be used as a reference counted object the way we normally use interfaces.

Implementation of TComponent._Release

function TComponent._Release: Integer;
begin
  if FVCLComObject = nil then
    Result := -1   // -1 indicates no reference counting is taking place
  else
    Result := IVCLComObject(FVCLComObject)._Release;
end;