How to process use defined message in forms(overriding WndProc method)

In TCustomForm class, there exists "WndProc" method which processes window messages.

  procedure TCustomForm.WndProc(var Message: TMessage);

You can override this method to process user defined message.

[Delphi]

unit Unit1;

interface

uses

  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,

  Dialogs, StdCtrls;

type

  TForm1 = class(TForm)

    Button1: TButton;

    procedure Button1Click(Sender: TObject);

  private

    { Private declarations }

  public

    { Public declarations }

    procedure WndProc(var Message: TMessage); override;

  end;

var

  Form1: TForm1;

implementation

{$R *.dfm}

const

  WM_MY_MESSAGE = WM_USER + 10;

procedure TForm1.WndProc(var Message: TMessage);

begin

  case Message.Msg of

    WM_MY_MESSAGE:

      ShowMessage('WM_MY_MESSAGE');

    else

      inherited WndProc(Message);

  end;

end;

procedure TForm1.Button1Click(Sender: TObject);

begin

  SendMessage(Handle, WM_MY_MESSAGE, 0, 0);

end;

end.