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How to send a windows message with SendMessage

Title: How to send a windows message with SendMessage const MY_MESSAGE = WM_USER + 4242; type TForm1 = class(TForm) Button1: TButton; procedure Button1Click(Sender: TObject); // Handler that receive the Message procedure MessageReceiver(var msg: TMessage); message MY_MESSAGE; end; var Form1: TForm1; implementation {$R *.DFM} procedure TForm1.Button1Click(Sender: TObject); var txt: string; begin txt := 'Hello World'; SendMessage(Form1.Handle, MY_MESSAGE, 0, DWORD(PChar(txt))); end; // To receive this custom Message that is addressed to form1.handle // you need a message handler. procedure TForm1.MessageReceiver(var msg: TMessage); var txt: PChar; begin txt := PChar(msg.lParam); msg.Result := 1; ShowMessage(txt); end; end.