How do I capture desktop screenshot behind full screen form?

Question

I have a full screen form to block all inputs. How to capture screenshot of all desktop behind this form? In other words, how to take a printscreen without appearing this form that is in front of the screen?

I'm writing a remote access software. I need show info of technical support in screen and block all inputs during access for analysts work quietly - VNC for example have option to turn off the monitor, Dameware have option to block input.

There is another way to work remotely behind this lock screen?

Answer 1

While I am fully in line with the commenters, that it is not a good idea to block user input with a full screen form, here is some code to get you started. The code assumes a screen size of 1920 x 1200.

unit Unit152;

interface

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

type
  TForm152 = class( TForm )
    Timer1: TTimer;
    procedure Timer1Timer( Sender: TObject );
    procedure FormCreate( Sender: TObject );
  private
    { Private declarations }
    DesktopBMP: TBitmap;
    procedure WMEraseBkgnd( var Message: TWMEraseBkgnd ); message WM_ERASEBKGND;
  protected
    procedure Paint; override;
  public
    { Public declarations }

  end;

var
  Form152: TForm152;

implementation

{$R *.dfm}
{ TForm152 }

procedure TForm152.FormCreate( Sender: TObject );
begin
  DesktopBMP := TBitmap.Create;
  DesktopBMP.SetSize( 1920, 1200 );
end;

procedure TForm152.Paint;
begin
  inherited;
  Canvas.Draw( 0, 0, DesktopBMP );
end;

procedure TForm152.Timer1Timer( Sender: TObject );
begin
  // alternatively a simple Invalidate would do here, but then
  // all other windows would not redraw
  Width := 0;
  Height := 0;
  Width := 1920;
  Height := 1200;
end;

procedure TForm152.WMEraseBkgnd( var Message: TWMEraseBkgnd );
var
  DesktopDC: HDC;
  DesktopHwnd: Hwnd;
  DesktopCanvas: TCanvas;
begin
  DesktopHwnd := GetDesktopWindow;
  DesktopDC := GetDC( DesktopHwnd );
  try
    DesktopCanvas := TCanvas.Create;
    DesktopCanvas.Handle := DesktopDC;
    DesktopBMP.Canvas.CopyRect( Rect( 0, 0, 1919, 1199 ), DesktopCanvas, Rect( 0, 0, 1919, 1199 ) );
  finally
    DesktopCanvas.Free;
    ReleaseDc( DesktopHwnd, DesktopDC );
  end;
  Message.Result := 1;
  inherited;
end;

end.

The DFM looks like this:

object Form152: TForm152
  Left = 2296
  Top = 103
  BorderStyle = bsNone
  Caption = 'Form152'
  ClientHeight = 699
  ClientWidth = 1289
  Color = clBtnFace
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'Tahoma'
  Font.Style = []
  OldCreateOrder = False
  WindowState = wsMaximized
  OnCreate = FormCreate
  PixelsPerInch = 96
  TextHeight = 13
  object Timer1: TTimer
    Interval = 10
    OnTimer = Timer1Timer
    Left = 640
    Top = 360
  end
end