[angvelem]

Вариация, использующая WM_COPYDATA:

Вызывающее приложение

Код:

program Test;

uses
  Windows, Messages;

type
  TSetMessage	= procedure; stdcall;

const
  szAppName     = 'MainForm';
  szCaptionName = 'test';

var
  Window        : HWND;
  Msg           : TMsg;
  WndClass      : TWndClassEX;
  SizeX, SizeY  : Integer;
  hDll		: HMODULE;
  hBtn		: HWND;

  SetMessage	: TSetMessage;

//---------------------------------------------------------

procedure InitApp(Wnd : HWND);
begin
  hDLL := LoadLibrary(PChar('mydll.dll'));
  if hDLL = 0 then
    Exit;

  @SetMessage := GetProcAddress(hDLL, 'SetMessage');
  if Addr(SetMessage) = NIL then
    Exit;

  hBtn := CreateWindow('BUTTON', 'Жми', WS_CHILD or WS_VISIBLE, 280, 400, 80, 25, Wnd, 100, hInstance, NIL);
end;

//---------------------------------------------------------

procedure DeInitApp(Wnd : HWND);
begin
  DestroyWindow(hBtn);

  if hDll <> 0 then
    FreeLibrary(hDLL);
end;

//---------------------------------------------------------

function MainProc(Wnd : HWND; Msg : Integer;  wParam, lParam : Longint) : Integer; stdcall;
begin
  Result := 0;

  case Msg of
    WM_CREATE : InitApp(Wnd);

    WM_COMMAND :
    case LOWORD(wParam) of
      100 :
      if hDll <> 0 then
        SetMessage;
    end;
    
    WM_CLOSE :  DestroyWindow(Wnd);

    WM_DESTROY :
    begin
      DeInitApp(Wnd);
      PostQuitMessage(0);
      Exit;
    end;
  end;
  Result := DefWindowProc(Wnd, Msg, wParam, lParam);
end;

//---------------------------------------------------------

begin
  SizeX := 640;
  SizeY := 480;

  FillChar(WndClass, SizeOf(TWndClassEx), 0);
  WndClass.cbSize        := SizeOf(TWndClassEx);
  WndClass.style         := CS_HREDRAW or CS_VREDRAW;
  WndClass.lpfnWndProc   := @MainProc;
  WndClass.cbClsExtra    := 0;
  WndClass.cbWndExtra    := 0;
  WndClass.hInstance     := hInstance;
  WndClass.hCursor       := LoadCursor(0, IDC_ARROW);
  WndClass.hbrBackGround := GetSysColorBrush(COLOR_BTNFACE);
  WndClass.lpszClassName := szAppName;

  if RegisterClassEx(WndClass) = 0 then
    Halt(255);

  Window := CreateWindowEx(0, szAppName, szCaptionName,
                           WS_DLGFRAME or WS_SYSMENU or WS_MINIMIZEBOX,
                           0, 0, SizeX, SizeY, 0, 0, hInstance, NIL);

  ShowWindow(Window, SW_SHOW);

  while(GetMessage(Msg, 0, 0, 0)) do
  begin
    TranslateMessage(Msg);
    DispatchMessage(Msg);
  end;
  Halt(Msg.wParam);
end.

Получающее приложение

Код:

program Main;

uses
  Windows, Messages;

const
  szAppName     = 'SuperMainForm';
  szCaptionName = 'test';

var
  Window        : HWND;
  Msg           : TMsg;
  WndClass      : TWndClassEX;
  SizeX, SizeY  : Integer;

//---------------------------------------------------------

function MainProc(Wnd : HWND; Msg : Integer;  wParam, lParam : Longint) : Integer; stdcall;
var
  cds : PCOPYDATASTRUCT;
  St  : AnsiString;
  Buf : array[Word] of AnsiChar;
begin
  Result := 0;

  case Msg of
    WM_COPYDATA :
    begin
      cds := PCopyDataStruct(lParam);
      if (cds^.dwData = DWORD($12345678)) then
      begin
        lstrcpynA(@Buf[0], cds^.lpData, cds^.cbData);
        SetString(St, Buf, cds^.cbData);
        if St <> '' then
          MessageBox(Wnd, PChar(St), '', MB_OK);
      end;
    end;

    WM_CLOSE : DestroyWindow(Wnd);

    WM_DESTROY :
    begin
      PostQuitMessage(0);
      Exit;
    end;
  end;
  Result := DefWindowProc(Wnd, Msg, wParam, lParam);
end;

//---------------------------------------------------------

begin
  SizeX := 320;
  SizeY := 240;

  FillChar(WndClass, SizeOf(TWndClassEx), 0);
  WndClass.cbSize        := SizeOf(TWndClassEx);
  WndClass.style         := CS_HREDRAW or CS_VREDRAW;
  WndClass.lpfnWndProc   := @MainProc;
  WndClass.cbClsExtra    := 0;
  WndClass.cbWndExtra    := 0;
  WndClass.hInstance     := hInstance;
  WndClass.hCursor       := LoadCursor(0, IDC_ARROW);
  WndClass.hbrBackGround := GetSysColorBrush(COLOR_BTNFACE);
  WndClass.lpszClassName := szAppName;

  if RegisterClassEx(WndClass) = 0 then
    Halt(255);

  Window := CreateWindowEx(0, szAppName, szCaptionName,
                           WS_DLGFRAME or WS_SYSMENU or WS_MINIMIZEBOX,
                           800, 0, SizeX, SizeY, 0, 0, hInstance, NIL);

  ShowWindow(Window, SW_SHOW);

  while(GetMessage(Msg, 0, 0, 0)) do
  begin
    TranslateMessage(Msg);
    DispatchMessage(Msg);
  end;
  Halt(Msg.wParam);
end.

dll

Код:

library mydll;

uses
  Windows, Messages;

const
  St = 'Hello World';

procedure SetMessage; stdcall; export;
var
  Wnd : HWND;
  cds : COPYDATASTRUCT;
begin
  Wnd := FindWindow('SuperMainForm', NIL);
  if Wnd <> 0 then
  begin
    cds.dwData := $12345678;
    cds.lpData := @St[1];
    cds.cbData := Length(St) + 1;
    SendMessage(Wnd, WM_COPYDATA, 0, DWORD(@cds));
  end
  else
    MessageBox(0, 'fail', 'Error', MB_OK);
end;

exports
  SetMessage;

begin
end.

Кроме передачи через WM_COPYDATA, можно также использовать:
1. Обмен через файлы
2. Каналы передачи данных Pipe
3. Каналы передачи данных Mailslot

Добавлено:
Статью с примерами, можно прочитать здесь.