Set the background color of a MDI Form

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Set the background color of a MDI Form
* Setting the Background color of a MDI Form without changing the color
* of overlayed objects like TGroupBox or TPageControl.

unit Unit1;

interface

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

type
  TForm1 = class(TForm)
  private
    FClientInstance: TFarProc;
    FPrevClientProc: TFarProc;
    BkBrush: HBRUSH;
    procedure ClientWndProc(var Message: TMessage);
  public
    constructor Create(AOwner: TComponent); override;
    destructor Destroy; override;
  end;

implementation

{$R *.dfm}

constructor TForm1.Create(AOwner: TComponent);
begin
  inherited;
  BkBrush := CreateSolidBrush(clGray);
  FClientInstance := Classes.MakeObjectInstance(ClientWndProc);
  FPrevClientProc := Pointer(GetWindowLong(ClientHandle, GWL_WNDPROC));
  SetWindowLong(ClientHandle, GWL_WNDPROC, Longint(FClientInstance));
end;

destructor TForm1.Destroy;
begin
  DeleteObject(BkBrush);
  inherited;
end;

procedure TForm1.ClientWndProc(var Message: TMessage);
var
  DC: HDC;
  BrushOld: HBRUSH;
begin
  with Message do
  begin
    case Msg of
      WM_ERASEBKGND:
        begin
          DC := TWMEraseBkGnd(Message).DC;
          BrushOld := SelectObject(DC, BkBrush);
          FillRect(DC, ClientRect, BkBrush);
          SelectObject(DC, BrushOld);
          Result := 1;
        end;
      else
        Result := CallWindowProc(FPrevClientProc, ClientHandle, Msg, wParam, lParam);
    end;
  end;
end;

end.