奇怪，将ComboBox放入StringGrid中，ComboBox的下拉列表不能展开！

Question

代码如下：
//自定义表格
unit Unit2;interfaceuses classes, grids, StdCtrls;  TCustomGrid = class(TStringGrid)
  private
    FComboBox : TComboBox;
    FPickList : TStrings;
  protected
    procedure DrawCell(ACol, ARow: Longint; ARect: TRect;
      AState: TGridDrawState); override;
    procedure InitialComboBox(ARect: TRect);
    procedure OnCustomSelectCell(Sender: TObject; ACol, ARow: Longint; var
        CanSelect: Boolean); virtual;
  public
    constructor Create(AOwner: TComponent); override;
    destructor Destroy; override;    property PickList : TStrings read FPickList write FPickList;
  end;
{ TCustomGrid }constructor TCustomGrid.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  DefaultColWidth := 64;
  DefaultRowHeight := 18;
  Options := Options + [goEditing];  FComboBox := TComboBox.Create(self);  FPickList := TStringList.Create;  OnSelectCell := OnCustomSelectCell;
end;destructor TCustomGrid.Destroy;
begin
  FPickList.Free;  FComboBox.Free;
  inherited;
end;procedure TCustomGrid.DrawCell(ACol, ARow: Integer; ARect: TRect;
  AState: TGridDrawState);
begin
  inherited;
end;procedure TCustomGrid.InitialComboBox(ARect: TRect);
begin
  with ARect do
    FComboBox.SetBounds(Left, Top, (Right - Left), (Bottom - Top));
  FComboBox.Parent := self;
  FComboBox.Items.Assign(FPickList);
  FComboBox.Visible := True;
  FComboBox.SetFocus;
end;procedure TCustomGrid.OnCustomSelectCell(Sender: TObject; ACol, ARow: Longint;
    var CanSelect: Boolean);
var
  R : TRect;
begin
  R := CellRect(ACol, ARow);
  InitialComboBox(R);
end;
end.//应用自定义表格
var
  aGrid : TCustomGrid;
begin
  aGrid := TCustomGrid.Create(self);
  aGrid.SetBounds(20,20,450,300);
  aGrid.Parent := self;  aGrid.PickList.Add('aa');
  aGrid.PickList.Add('bb');
end;

Answer 1

如果再TCustomGrid.Create中对ComboBox的Items进行操作，会产生一个错误"control '' not has no a parent window",不知道为什么？

Answer 2

unit Unit2;interfaceuses classes, grids, StdCtrls, windows, Controls;type
  TCustomGrid = class(TStringGrid)
  private
    FComboBox : TComboBox;
    FPickList : TStrings;
  protected
    procedure DrawCell(ACol, ARow: Longint; ARect: TRect;
      AState: TGridDrawState); override;
    procedure InitialComboBox(ARect: TRect);
    procedure OnCustomSelectCell(Sender: TObject; ACol, ARow: Longint; var
        CanSelect: Boolean); virtual;
  public
    constructor Create(AOwner: TComponent); override;
    destructor Destroy; override;
    property PickList : TStrings read FPickList write FPickList;
  end;implementationconstructor TCustomGrid.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  DefaultColWidth := 64;
  DefaultRowHeight := 18;
  Options := Options + [goEditing];  FComboBox := TComboBox.Create(nil);
  FComboBox.Parent:=TWinControl(AOwner);
  FComboBox.Visible:=false;
  FComboBox.Items.Add('a');  FPickList := TStringList.Create;  OnSelectCell := OnCustomSelectCell;
end;destructor TCustomGrid.Destroy;
begin
  FPickList.Free;  //FComboBox.Free;
  inherited;
end;procedure TCustomGrid.DrawCell(ACol, ARow: Integer; ARect: TRect;
  AState: TGridDrawState);
begin
  inherited;
end;procedure TCustomGrid.InitialComboBox(ARect: TRect);
begin
  with ARect do
    FComboBox.SetBounds(Left+Self.Left, Top+Self.Top, (Right - Left), (Bottom - Top));
  //FComboBox.Parent := self;
  FComboBox.BringToFront;
  FComboBox.Items.Assign(FPickList);
  FComboBox.Visible := True;
  FComboBox.SetFocus;
end;procedure TCustomGrid.OnCustomSelectCell(Sender: TObject; ACol, ARow: Longint;
    var CanSelect: Boolean);
var
  R : TRect;
begin
  R := CellRect(ACol, ARow);
  InitialComboBox(R);
end;end.