块愁死我了,大家帮帮我！

我想当鼠标按下时随便画一条直线，可是就这么简单的一个功能居然实现不了，大家帮我看看我的代码是哪里的问题，快愁死我了！谢谢
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
HDC hdc;
int wmId, wmEvent;
PAINTSTRUCT ps;
TCHAR szHello[MAX_LOADSTRING];

HPEN hPen;
int nDraw = 0;

switch (message)
{
case WM_COMMAND:
wmId    = LOWORD(wParam);
wmEvent = HIWORD(wParam);
// Parse the menu selections:
switch (wmId)
{
case IDM_HELP_ABOUT:
DialogBox(g_hInst, (LPCTSTR)IDD_ABOUTBOX, hWnd, (DLGPROC)About);
    break;
case IDOK:
SendMessage(hWnd, WM_ACTIVATE, MAKEWPARAM(WA_INACTIVE, 0), (LPARAM)hWnd);
SendMessage (hWnd, WM_CLOSE, 0, 0);
break;
default:
   return DefWindowProc(hWnd, message, wParam, lParam);
}
break;
case WM_CREATE:
g_hwndCB = CreateRpCommandBar(hWnd);
            // Initialize the shell activate info structure
            memset (&s_sai, 0, sizeof (s_sai));
            s_sai.cbSize = sizeof (s_sai);

hPen = (HPEN)CreatePen(PS_SOLID, 1, RGB(255, 0, 0));
break;
case WM_PAINT:
RECT rt;
hdc = BeginPaint(hWnd, &ps);
//GetClientRect(hWnd, &rt);
//LoadString(g_hInst, IDS_HELLO, szHello, MAX_LOADSTRING);
//DrawText(hdc, szHello, _tcslen(szHello), &rt,
//DT_SINGLELINE | DT_VCENTER | DT_CENTER);
EndPaint(hWnd, &ps);
break;
case WM_LBUTTONDOWN :
hdc = BeginPaint(hWnd, &ps);
g_pt[0].x = 10;
g_pt[0].y = 10;
g_pt[1].x = 100;
g_pt[1].y = 100;
SelectObject(hdc, hPen);
Polyline(hdc, g_pt, 2);
EndPaint(hWnd, &ps);
break;
case WM_DESTROY:
CommandBar_Destroy(g_hwndCB);
PostQuitMessage(0);
break;
case WM_ACTIVATE:
            // Notify shell of our activate message
SHHandleWMActivate(hWnd, wParam, lParam, &s_sai, FALSE);
      break;
case WM_SETTINGCHANGE:
SHHandleWMSettingChange(hWnd, wParam, lParam, &s_sai);
      break;
default:
return DefWindowProc(hWnd, message, wParam, lParam);
   }
   return 0;
}

看看把作图的部分
hdc = BeginPaint(hWnd, &ps);
g_pt[0].x = 10;
g_pt[0].y = 10;
g_pt[1].x = 100;
g_pt[1].y = 100;
SelectObject(hdc, hPen);
Polyline(hdc, g_pt, 2);
EndPaint(hWnd, &ps);
移到WM_PAINT这个消息处理中，WM_LBUTTONDOWN 中向主窗口发送一个WM_PAINT消息.

还有注意端点不要超过显示区，线的颜色不要和底色一样了。

谢谢楼上的，以后还要向你多多请教！

Windows 编程上有个例子你抄一下。