нарисовать линию

#include <Windows.h>
#include <cmath>
 
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
 
BOOL DrawLine(HDC, int, int, int, int);
 
HDC hdc;
PAINTSTRUCT ps;
 
static int x, y;
static int a, b;
 
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPreInstance, PSTR szCmdLine, int iCmdShow)
{
    static char szAppName[] = "laba2";
    HWND hwnd;
    MSG msg;
    WNDCLASSEX wndclass;
 
    wndclass.cbSize = sizeof(wndclass);
    wndclass.style = CS_HREDRAW | CS_VREDRAW;
    wndclass.lpfnWndProc = WndProc;
    wndclass.cbClsExtra = 0; 
    wndclass.cbWndExtra = 0; 
    wndclass.hInstance = hInstance; 
    wndclass.hIcon = LoadIcon(NULL, IDI_APPLICATION); 
    wndclass.hCursor = LoadCursor(NULL, IDC_ARROW); 
    wndclass.hbrBackground =(HBRUSH) GetStockObject(WHITE_BRUSH); 
    wndclass.lpszMenuName = NULL; 
    wndclass.lpszClassName = szAppName; 
    wndclass.hIconSm = LoadIcon(NULL, IDI_APPLICATION); 
 
    RegisterClassEx(&wndclass);
 
    hwnd = CreateWindow( 
        szAppName, // window class name 
        "laba2", // window caption 
        WS_OVERLAPPEDWINDOW | WS_VISIBLE, // window style 
        CW_USEDEFAULT, // initial x position 
        CW_USEDEFAULT, // initial y position 
        CW_USEDEFAULT, // initial x size 
        CW_USEDEFAULT, // initial y size 
        NULL, // parent window handle 
        NULL, // window menu handle 
        hInstance, // program instance handle 
        NULL 
        );
    while(GetMessage(&msg, NULL, 0, 0)) 
    { 
        TranslateMessage(&msg); 
        DispatchMessage(&msg); 
    } 
    return msg.wParam; 
}
 
LRESULT CALLBACK WndProc(HWND hwnd, UINT iMsg, WPARAM wParam, LPARAM lParam) 
{ 
    switch(iMsg) {
    case WM_PAINT:
        hdc = BeginPaint(hwnd, &ps); 
        GetUpdateRect(hwnd, NULL, TRUE);
        EndPaint(hwnd, &ps);
        return 0;
    case WM_LBUTTONDOWN:
        x = LOWORD(lParam);
        y = HIWORD(lParam);
        break;
    /*case WM_MOUSEMOVE:
        if (MK_LBUTTON == wParam)
        {
            a = LOWORD(lParam);
            b = HIWORD(lParam);
            hdc = GetDC(hwnd);
            MoveToEx(hdc, x, y, NULL);
            LineTo(hdc, a, b);
            ReleaseDC(hwnd, hdc);
        }
        return 0;*/
    case WM_LBUTTONUP:
        {
            //if (MK_LBUTTON == wParam)
            //{
        a = LOWORD(lParam);
        b = HIWORD(lParam);
        hdc = GetDC(hwnd);
        //MoveToEx(hdc, x, y, NULL);
        //LineTo(hdc, a, b);
        DrawLine(hdc, x, y, a, b);
        ReleaseDC(hwnd, hdc);
        //}
            //}
        break;
        }
    case WM_DESTROY:
        PostQuitMessage(0);
        return 0;
    }
    return DefWindowProc(hwnd, iMsg, wParam, lParam);
}
BOOL DrawLine(HDC hdc, int x0, int y0, int x, int y) 
{
    POINT pt;
    MoveToEx(hdc, x0, y0, &pt);
    return LineTo(hdc, x, y);
}