unit Unit1;

interface

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

type
   // late binding, that is, the dynamic call of external functions mainly uses the following three commands:
  // LoadLibrary: Get DLL
  // GetProcAddress: Get function
  // FreeLibrary: release

  // Define a procedure type, the parameters must be consistent with the required function
  TMB = function(hWnd: HWND; lpText, lpCaption: PChar; uType: UINT): Integer; stdcall;

  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
  private 
    MB: TMB;   {declaration function MB}
    inst: LongWord;   {declare a variable to record the DLL handle to use}
  public
    { Public declarations }
  end;

where
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
begin
  inst := LoadLibrary('user32.dll');
  if inst <> 0 then
    MB := GetProcAddress(inst, 'MessageBoxW');
 // MB: = GetProcAddress (inst, 'MessageBoxA'); {Delphi 2009 version used this sentence}
end;

// Call test:
procedure TForm1.Button1Click(Sender: TObject);
where
  t,b: PChar;
begin
  t := 'title';
  b := 'content';
  MB(0, b, t, 0);
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin 
  FreeLibrary (inst);   {Remember to release}
end;

end.