Go Premium for a chance to win a PS4. Enter to Win

x
?
Solved

"control '' has no parent"  error...

Posted on 2001-08-07
4
Medium Priority
?
267 Views
Last Modified: 2010-04-04
Hi,
  I'm writing a visual component based on TPaintBox.
but the create method raise the error :"control '' has no parent", I don't understant why. Can you help me ?

  I reduced the code to the max in order to keep the error :

unit PaintBox1;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  ExtCtrls;

type
  TPaintBox1 = class(TPaintBox)
  private
  protected
  public
    constructor create(AOwner:Tcomponent);Override;
  published
  end;

procedure Register;
implementation

constructor TPaintBox1.Create(AOwner:Tcomponent);
var R:TRect;
begin
  inherited;
  R.Top:=5;
  R.Left:=5;
  R.Bottom:=Height-5;
  R.Right:=width-5;
  Self.Canvas.Brush.color:=clLime;
  Self.Canvas.FillRect(R);
end;

procedure Register;
begin
  RegisterComponents('Additional', [TPaintBox1]);
end;

end.

Thanks for help.

Mike
0
Comment
Question by:gklux
  • 2
4 Comments
 
LVL 11

Expert Comment

by:robert_marquardt
ID: 6358750
Try
  inherited Create(AOwner);
0
 

Author Comment

by:gklux
ID: 6358760
Sorry, the full error message is :
"control '' has no parent window"

Mike
0
 
LVL 20

Accepted Solution

by:
Madshi earned 400 total points
ID: 6358821
constructor TPaintBox1.Create(AOwner:Tcomponent);
var R:TRect;
begin
 inherited;
 R.Top:=5;
 R.Left:=5;
 R.Bottom:=Height-5;
 R.Right:=width-5;

This is alright. But this...

 Self.Canvas.Brush.color:=clLime;
 Self.Canvas.FillRect(R);

... fails, because in the moment where you access the Canvas property, a handle to the paintbox is needed. But a handle can only be created if the parent window of the paint box is known. But the parent is known later, not in the constructor.
So I recommend to move the canvas stuff to a "later location". Maybe you can overwrite the "SetParent" method and do it there, after the parent was set to a non-nil value.

Regards, Madshi.
0
 

Author Comment

by:gklux
ID: 6359270
Very helpfull answer Madshi, has always  :-)

regards

Mike
0

Featured Post

[Webinar] Cloud and Mobile-First Strategy

Maybe you’ve fully adopted the cloud since the beginning. Or maybe you started with on-prem resources but are pursuing a “cloud and mobile first” strategy. Getting to that end state has its challenges. Discover how to build out a 100% cloud and mobile IT strategy in this webinar.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Introduction Raise your hands if you were as upset with FireMonkey as I was when I discovered that there was no TListview.  I use TListView in almost all of my applications I've written, and I was not going to compromise by resorting to TStringGrid…
In my programming career I have only very rarely run into situations where operator overloading would be of any use in my work.  Normally those situations involved math with either overly large numbers (hundreds of thousands of digits or accuracy re…
Want to learn how to record your desktop screen without having to use an outside camera. Click on this video and learn how to use the cool google extension called "Screencastify"! Step 1: Open a new google tab Step 2: Go to the left hand upper corn…
In a question here at Experts Exchange (https://www.experts-exchange.com/questions/29062564/Adobe-acrobat-reader-DC.html), a member asked how to create a signature in Adobe Acrobat Reader DC (the free Reader product, not the paid, full Acrobat produ…
Suggested Courses

824 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question