Delphi 3,4,5 TCollection Performance Issues and Solutions

Problem/Question/Abstract:

Delphi 3,4,5 TCollection Performance Issues and Solutions

Answer:

If you are using TCollection classes in your Delphi 3,4 or 5 applications than you will find this article quite interesting.

Firstly let us try fairly simple code:

procedure TForm1.Button1Click(Sender: TObject);
var
  old: TCollection;
  i: integer;
begin
  old := TCollection.Create(TCollectionItem);
  for i := 0 to 100000 do
  begin
    old.Add;
  end;
  Windows.beep(900, 1000); // hi-freq beep after we done with adding empty items
  old.Free;
  Windows.beep(100, 1000); // low-freq beep after we done with destroying empty items
end;

You might think that low-freq beep will follow right after hi-freq beep (well what can be faster that just simply destroy all collection items) - but IT IS not!

In fact it takes 10-20 seconds to destroy collection which hold few dozen thousands items - and worse of all your CPU will be 100% busy. We bumped into this problem when our clients complains that application is "freeze PC for a few minutes".

To understand why it is happening you need take closer look at TCollectionItem.Destroy, TCollectionItem.SetCollection and TCollection.RemoveItem functions which located at classes.pas - last one is the key to understanding this problem.

You also might want to compare your TCollection.RemoveItem version to Delphi 6 TCollection.RemoveItem code:

{ classes.pas from Delphi 6 }

procedure TCollection.RemoveItem(Item: TCollectionItem);
begin
  Notify(Item, cnExtracting);
  if Item = FItems.Last then
    FItems.Delete(FItems.Count - 1) // that will fix original problem
  else
    FItems.Remove(Item);
  Item.FCollection := nil;
  NotifyDesigner(Self, Item, opRemove);
  Changed;
end;

Now you probably will want to fix it. But seems it is not so easy because TCollection.RemoveItem is not a virtual or dynamic function.

Here is two solutions:

You will need to alter classes.pas - put that TCollection.RemoveItem code from Delphi 6 into your version of classes.pas.

Copy new (fixed) classes.pas into your project directory and put it at the first position in your .dpr uses section like this:

program Project1;

uses
  classes in 'classes.pas' // new classes.pas with fixed TCollection.RemoveItem
  Forms,
  Unit1 in 'Unit1.pas' {Form1};

{$R *.res}

begin
  Application.Initialize;
  Application.CreateForm(TForm1, Form1);
  Application.Run;
end.

Now your project will be compiled with new version of TCollection.

In some situations it is not so convenient or even not possible to use altered classes.pas and in this case we have other trick for you.

type
  { TFixCollection - fix TCollection.RemoveItem issue in Delphi 3,4,5 }
  TFixCollection = class(TCollection)
  public
    { Unfortunately Clear is not a virtual or dynamic procedure so we will
                have to reintroduce it }
    procedure Clear;
    destructor Destroy; override;
  end;

procedure TFixCollection.Clear;
var
  i: integer;
  AList, OrgList: TList;
begin
  AList := TList.Create;
  try
    OrgList := TList(PDWORD(DWORD(Self) + $4 + SizeOf(TPersistent))^);

    { Save original pointers to collection items }
    for i := 0 to OrgList.Count - 1 do
      AList.Add(OrgList[i]);

    OrgList.Clear;

    { Destroy collection items }
    for i := 0 to AList.Count - 1 do
      TCollectionItem(AList[i]).Free;
  finally
    AList.Free;
  end;
  inherited;
end;

destructor TFixCollection.Destroy;
begin
  Clear;
  inherited;
end;

{ Let's try again ! }

procedure TForm1.Button2Click(Sender: TObject);
var
  old: TFixCollection;
  i: integer;
begin
  old := TFixCollection.Create(TCollectionItem);
  for i := 0 to 100000 do
    old.Add;
  Windows.beep(900, 1000); // hi-freq beep after we done with adding empty items
  old.Free;
  Windows.beep(100, 1000); // low-freq beep after we done with destroying empty items
end;

As you can see now it works just fine. We used one trick which gives us access to protected section of TCollection.

You can use both techniques in your applications written on Delphi versions 3,4,5.

For your convenience you can download Delphi 3,4,5 TCollection performance issue demo sources here CollectionPerformanceDemo.zip.