[WinAPI] undefined reference to `WndProc(HWND__*, unsigned int, unsigned int, long)@16'

#include <windows.h>

HWND Ramka, Pole_tekstowe, Pole_opcji_1, Pole_opcji_2, Lista_rozwijana,
Pole_wyboru_1, Pole_wyboru_2, Przycisk;

LPSTR NazwaKlasy = "KlasaOkna";
MSG Komunikat;

LRESULT CALLBACK WndProc( HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam );

int WINAPI WinMain( HINSTANCE hInstance, HINSTANCE hPrevInstance,
LPSTR lpCmdLine, int nCmdShow )
{
   
    WNDCLASSEX ko;
    ko.cbSize = sizeof( WNDCLASSEX );
    ko.lpfnWndProc = WndProc;
    ko.hInstance = hInstance;
    ko.hCursor = LoadCursor( NULL, IDC_ARROW );
    ko.hbrBackground =( HBRUSH )( COLOR_WINDOW + 1 );
    ko.lpszClassName = "WindowClass";
    ko.hIcon = LoadIcon( NULL, IDI_APPLICATION );
    ko.hIconSm = LoadIcon( NULL, IDI_APPLICATION );
   
    if( !RegisterClassEx( & ko ) )
         return 0;
   
    HWND hwnd;
    hwnd = CreateWindowEx( WS_EX_CLIENTEDGE, "Nazwa Klasy", "Tytuł okna",
    WS_VISIBLE | WS_OVERLAPPEDWINDOW,
    CW_USEDEFAULT,
    CW_USEDEFAULT,
    375,
    330,
    NULL, NULL, hInstance, NULL );
   
    Ramka = CreateWindowEx( 0, "BUTTON", "Ramka",
    WS_CHILD | WS_VISIBLE | BS_GROUPBOX, 10, 10, 335, 270,
    hwnd, NULL, hInstance, NULL );
   
    Pole_tekstowe = CreateWindowEx( WS_EX_CLIENTEDGE, "COMBOBOX", NULL,
    WS_CHILD | WS_VISIBLE | WS_BORDER, 20, 30, 150, 30,
    hwnd, NULL, hInstance, NULL );
    Lista_rozwijana = CreateWindowEx( WS_EX_CLIENTEDGE, "COMBOBOX", NULL,
    WS_CHILD | WS_VISIBLE | WS_BORDER | CBS_DROPDOWN, 20, 80, 150, 200,
    hwnd, NULL, hInstance, NULL );
   
    SendMessage( Lista_rozwijana, CB_ADDSTRING, 0,( LPARAM ) "Opcja1" );
    SendMessage( Lista_rozwijana, CB_ADDSTRING, 0,( LPARAM ) "Opcja2" );
    SendMessage( Lista_rozwijana, CB_ADDSTRING, 0,( LPARAM ) "Opcja3" );
   
    Pole_wyboru_1 = CreateWindowEx( 0, "BUTTON", "Pole wyboru 1",
    WS_CHILD | WS_VISIBLE | BS_CHECKBOX,
    210, 30, 125, 30, hwnd, NULL, hInstance, NULL );
   
    Pole_wyboru_2 = CreateWindowEx( 0, "BUTTON", "Pole wyboru 2",
    WS_CHILD | WS_VISIBLE | BS_CHECKBOX,
    210, 80, 125, 30, hwnd, NULL, hInstance, NULL );
   
    Pole_opcji_1 = CreateWindowEx( 0, "BUTTON", "Pole opcji 1",
    WS_CHILD | WS_VISIBLE | BS_RADIOBUTTON,
    210, 130, 125, 30, hwnd, NULL, hInstance, NULL );
   
    Pole_opcji_2 = CreateWindowEx( 0, "BUTTON", "Pole opcji 2",
    WS_CHILD | WS_VISIBLE | BS_RADIOBUTTON,
    210, 180, 125, 30, hwnd, NULL, hInstance, NULL );
   
    Przycisk = CreateWindowEx( 0, "BUTTON", "Przycisk",
    WS_CHILD | WS_VISIBLE, 127, 230, 70, 30,
    hwnd, NULL, hInstance, NULL );
   
    ShowWindow( hwnd, nCmdShow );
    UpdateWindow( hwnd );
   
    while( GetMessage( & Komunikat, NULL, 0, 0 ) )
    {
       
        TranslateMessage( & Komunikat );
        DispatchMessage( & Komunikat );
       
    }
    return Komunikat.wParam;
   
}

LRESULT CALLBACK WinProc( HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam )

{
   
    switch( msg )
    {
       
    case WM_CLOSE:
        DestroyWindow( hwnd );
        break;
    case WM_DESTROY:
        PostQuitMessage( 0 );
        break;
        defaylt:
        return DefWindowProc( hwnd, msg, wParam, lParam );
       
    case WM_COMMAND:
        if(( HWND ) lParam == Przycisk )
        MessageBox( hwnd, "Nacisnięto pszycisk!", "Komunikat",
             MB_ICONINFORMATION );
       
        break;
       
    }
   
   
    return 0;
   
}