Delphi: How to send command to other application?

Question

How to send & receive commands from other Delphi created applications? I want to send command to another application that I've written.

Answer 1

Sender:

unit Unit1;

interface

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

const
  WM_MY_MESSAGE = WM_USER + 1;

type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
var
  h: HWND;
begin
  h := FindWindow(nil, 'My Second Window');
  if IsWindow(h) then
    SendMessage(h, WM_MY_MESSAGE, 123, 520);
end;

end.

Receiver:

unit Unit1;

interface

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

const
  WM_MY_MESSAGE = WM_USER + 1;

type
  TForm1 = class(TForm)
  private
    { Private declarations }
  protected
    procedure WndProc(var Message: TMessage); override;    
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.WndProc(var Message: TMessage);
begin
  inherited;
  case Message.Msg of
    WM_MY_MESSAGE:
      ShowMessageFmt('The other application sent the data %d and %d.', [Message.WParam, Message.LParam]);
  end;
end;

end.

Make sure that the caption of the receiving form is 'My Second Window'.