uses Dialogs; << Add dialogs to your uses list

  TWorkFlowShape = class(TPaintBox)
  public
    procedure DblClick; override; <<< Add New Method
  end;

procedure TWorkFlowShape.DblClick;
var S: String;
begin
  FIgnoreClick := True; // Ignore a mouse down click. Read below
  if InputQuery(Application.Title, 'Enter Text', S) then
  begin
    Text := S;
    Invalidate;
  end;
end;

if FIgnoreClick then
  begin
    FIgnoreClick := False;
    Exit;
  end;

 
P := ClientToScreen(Point(Width div 2, Height div 2));
    for I := 0 to Parent.ControlCount - 1 do
    try
      LShape := Parent.Controls[i] as TWorkFlowShapeGripped; //<-- error : invalid class typecast
      LGripCode := LShape.GetGripFromScreenPoint(P);
      if LGripCode > 0 then
      begin

...
TWorkFlow = class(TScrollBox)
  private
    FPaintBox: TPaintBox;
  public
    LConnector: TWorkFlowShapeConnector;
  public
    constructor Create(AOwner: TComponent); override;
    procedure Paint(ASender: TObject);
    procedure FPaintBoxDblClick(Sender: TObject);
  end;
...
 
Implementation
...
procedure TWorkFlow.FPaintBoxDblClick(Sender: TObject);
begin
  ShowMessage('Do Something'); // do something
end;
..

// main program
L := TLine.Create(LWorkFlow);
L.SetLine(250, 250, 550, 550); L.Parent := LWorkFlow;
 
// class workflow
type
TLine = class(TPaintBox)
  private
    FP1, FP2: TPoint;       
    FIsButtonDown: array[TMouseButton] of Boolean; 
    FGripCodeLine: Integer;
  public
    constructor Create(AOwner: TWorkFlow); reintroduce; virtual;
    procedure Paint; override;
    procedure DblClick; override;   
    procedure SetLine(X1, Y1, X2, Y2 : integer);
  end;
 
Implementation
constructor TLine.Create(AOwner: TWorkFlow);
begin inherited Create(AOwner);
 
end;
 
procedure TLine.SetLine(X1, Y1, X2, Y2 : integer);
begin inherited;
  FP1.x := X1; FP1.Y := Y1;
  FP2.x := X2; FP2.Y := Y2;
end;
 
procedure TLine.Paint;
var I: Integer; R: TRect;
begin inherited;
  Canvas.Pen.Color := clRed; Canvas.Pen.Width := 5;
  Canvas.MoveTo(FP1.X, FP1.Y);
  Canvas.LineTo(FP2.X, FP2.Y);
end;
 
procedure TLine.DblClick;
var S: String;
begin
  FIgnoreClick := True; // Ignore a mouse down click. Read below
  if InputQuery(Application.Title, 'Enter Text', S) then
  begin
    Text := S;
    Invalidate;
  end;
end;