How to set a forward declaration with generic types under Delphi 2010?

Question

I run into what seems to be a very classical problem: An item and a collection class, both referencing each other, that require a forward declaration. I\'m using Delphi 2010

Answer 1

My Collection's example (based on generics)

type
  TMICustomItem = class(TPersistent)
  private
    FID: Variant;
    FCollection: TList;
    function GetContained: Boolean;
  protected
    procedure SetID(Value: Integer);
  public
    constructor Create(ACollection: TList); overload;
    constructor Create(ACollection: TList; ID: Integer); overload;
    procedure Add; //Adding myself to parent collection
    procedure Remove; //Removing myself from parent collection        
    property Contained: Boolean read GetContained; //Check contains myself in parent collection
    property ID: Variant read FID;
  end;

  TMICustomCollection = class(TList)
  private
    function GetItemByID(ID: Integer): ItemClass;
  public
    property ItemID[ID: Integer]: ItemClass read GetItemByID; //find and return Item in self by ID
  end;

...

{ TMICustomItem }

constructor TMICustomItem.Create(ACollection: TList);
begin
  FCollection := ACollection;
end;

constructor TMICustomItem.Create(ACollection: TList;
  ID: Integer);
begin
  Create(ACollection);
  FID := ID;
end;

procedure TMICustomItem.Add;
begin
  if not FCollection.Contains(Self) then
    FCollection.Add(Self)
  else
    raise EListError.CreateRes(@SGenericDuplicateItem);
end;

procedure TMICustomItem.Remove;
begin
  if FCollection.Contains(Self) then
    FCollection.Remove(Self)
  else
    raise EListError.CreateRes(@SGenericItemNotFound);
end;

function TMICustomItem.GetContained: Boolean;
begin
  Result := FCollection.Contains(Self);
end;

procedure TMICustomItem.SetID(Value: Integer);
begin
  FID := Value;
end;

{ TMICustomCollection }

function TMICustomCollection.GetItemByID(ID: Integer): ItemClass;
var
  I: Integer;
begin
  for I := 0 to Count - 1 do
    if Items[I].ID = ID then
      Exit(Items[I]);

  raise EListError.CreateRes(@SGenericItemNotFound);
end;