Interfaces, Anonymous Methods and Memory Leaks

Question

this is a constructed example. I don't want to post the original code here. I tried to extract the relevant parts though.

I have an interface that manages a list of listeners.

TListenerProc = reference to procedure (SomeInt : ISomeInterface);

ISomeInterface = interface
   procedure AddListener (Proc : TListenerProc);   
end;

Now I register a listener:

SomeObj.AddListener (MyListener);

procedure MyListener (SomeInt : ISomeInterface);
begin
  ExecuteSynchronized (procedure
                       begin
                       DoSomething (SomeInt);
                       end);
end;

I do get memory leaks. Both the anonymous method and the interfaces are never freed. I suspect that this is due to some kind of circular reference here. The anonymous method keeps the interface alife and the interface keeps the anonymous method alife.

Two questions:

1. Do you support that explanation? Or am I missing something else here?
2. Is there anything I can do about it?

Thanks in advance!

---

EDIT: It's not so easy to reproduce this in an application small enough to post it here. The best I can do by now is the following. The anonymous method does not get released here:

program TestMemLeak;

{$APPTYPE CONSOLE}

uses
  Generics.Collections, SysUtils;

type
  ISomeInterface = interface;
  TListenerProc  = reference to procedure (SomeInt : ISomeInterface);

  ISomeInterface = interface
  ['{DB5A336B-3F79-4059-8933-27699203D1B6}']
    procedure AddListener (Proc : TListenerProc);
    procedure NotifyListeners;
    procedure Test;
  end;

  TSomeInterface = class (TInterfacedObject, ISomeInterface)
  strict private
    FListeners          : TList <TListenerProc>;
  protected
    procedure AddListener (Proc : TListenerProc);
    procedure NotifyListeners;
    procedure Test;
  public
    constructor Create;
    destructor  Destroy; override;
  end;


procedure TSomeInterface.AddListener(Proc: TListenerProc);
begin
FListeners.Add (Proc);
end;

constructor TSomeInterface.Create;
begin
FListeners := TList <TListenerProc>.Create;
end;

destructor TSomeInterface.Destroy;
begin
FreeAndNil (FListeners);
  inherited;
end;

procedure TSomeInterface.NotifyListeners;

var
  Listener : TListenerProc;

begin
for Listener in FListeners do
  Listener (Self);
end;

procedure TSomeInterface.Test;
begin
// do nothing
end;

procedure Execute (Proc : TProc);

begin
Proc;
end;

procedure MyListener (SomeInt : ISomeInterface);
begin
Execute (procedure
         begin
         SomeInt.Test;
         end);
end;

var
  Obj     : ISomeInterface;

begin
  try
    ReportMemoryLeaksOnShutdown := True;
    Obj := TSomeInterface.Create;
    Obj.AddListener (MyListener);
    Obj.NotifyListeners;
    Obj := nil;
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.

Answer 1

Your code is far from minimal. The following:

program AnonymousMemLeak;

{$APPTYPE CONSOLE}

uses
  SysUtils;

type
  TListenerProc  = reference to procedure (SomeInt : IInterface);

procedure MyListener (SomeInt : IInterface);
begin
end;

var
  Listener: TListenerProc;

begin
  try
    ReportMemoryLeaksOnShutdown := True;

    Listener := MyListener;
    Listener := nil;
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.

has the very same problem (Delphi 2009 here). This can't be worked or designed around. Looks to me like a bug in the compiler.

Edit:

Or maybe this is a problem of the memory leak detection. It has nothing to do with the parameter being an interface, a parameterless procedure leads to the same "leak". Very strange.

Answer 2

Looks to me like a definite circular reference issue. Anonymous methods are managed through hidden interfaces, and if the TList<TListenerProc> is owned by the object that ISomeInterface is implemented on, then you've got a circular reference issue.

One possible solution would be to put a ClearListeners method on ISomeInterface which calls .Clear on the TList<TListenerProc>. As long as nothing else is holding a reference to the anonymous methods, that would make them all vanish and drop their references to the ISomeInterface.

I've done a few articles about the structure and implementation of anonymous methods that might help you understand what you're really working with and how they operate a little bit better. You can find them at http://tech.turbu-rpg.com/category/delphi/anonymous-methods.