winmain 无法重载

初学win32编程，抄的书上随机生成线段的代码，但是报错
winmain 无法重载
全代码如下，请朋友们救我

#define _CRT_SECURE_NO_WARNINGS 1
#define WIN32_LEAN_AND_MEAN
#include<Windows.h>
#include<windowsx.h>
#include<time.h>
#include<stdlib.h>
#define MAX_STRING 124

//全局变量
HINSTANCE hInst;
TCHAR szTitle[MAX_STRING]; //标题栏文字
TCHAR szWindowClass[MAX_STRING];
HWND hWnd; //主窗口句柄

//函数声明
ATOM RegisterWindowClass(HINSTANCE hInstance);
BOOL InitInstance(HINSTANCE, int);
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
void OnPaint(HDC pDC);

//主函数
int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPTSTR lpCmdLine, int nCmdShow)
{
MSG msg;
wcscpy_s(szTitle, L"GDIDemo");
wcscpy_s(szWindowClass, L"Win32");
RegisterWindowClass(hInstance);
hInst = hInstance;

if (!InitInstance(hInstance, nCmdShow))
{
    return FALSE;
}

while (GetMessage(&msg, NULL, 0, 0))
{
    TranslateMessage(&msg);
    DispatchMessage(&msg);
}
return msg.wParam;

}

//注册窗口类

ATOM RegisterWindowClass(HINSTANCE hInstance)
{
WNDCLASSEXW wcex;

wcex.cbSize = sizeof(WNDCLASSEX);
wcex.style = CS_HREDRAW | CS_VREDRAW;
wcex.lpfnWndProc =(WNDPROC)WndProc;
wcex.cbClsExtra = 0;
wcex.cbWndExtra = 0;
wcex.hInstance = hInstance;
wcex.hIcon = LoadIcon(NULL,IDI_APPLICATION);
wcex.hCursor = LoadCursor(NULL,IDC_ARROW);
wcex.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH); ;
wcex.lpszMenuName = NULL;
wcex.lpszClassName = szWindowClass;
wcex.hIconSm = LoadIcon(NULL,IDI_APPLICATION);

return RegisterClassEx(&wcex);

}

//创建主窗口
BOOL InitInstance(HINSTANCE hInstance, int nCmdShow)
{
hWnd = CreateWindowEx(NULL,szWindowClass, szTitle,
WS_OVERLAPPEDWINDOW,
CW_USEDEFAULT, 0,
CW_USEDEFAULT, 0,NULL,
NULL, hInstance, NULL);

if (!hWnd)
{
    return FALSE;
}

ShowWindow(hWnd, nCmdShow);
UpdateWindow(hWnd);

return TRUE;

}

//消息处理
LRESULT CALLBACK WndProc(HWND hWnd, UINT message,
WPARAM wParam, LPARAM lParam)
{
PAINTSTRUCT ps;
HDC hdc;

switch (message)
{
case WM_PAINT:
    hdc = BeginPaint(hWnd, &ps);
    //绘图
    OnPaint(hdc);
    EndPaint(hWnd, &ps);
    break;
case WM_DESTROY:
    PostQuitMessage(0);
    break;
case WM_LBUTTONDOWN:
    MessageBox(hWnd, L"Mouse left button down", L"win32sample", MB_OK);
default:
    return DefWindowProc(hWnd, message, wParam, lParam);
}
return 0;

}
void OnPaint(HDC hdc)
{
srand((unsigned)time(NULL));
RECT clientRect;
GetClientRect(hWnd, &clientRect);
int maxX = clientRect.right;
int maxY = clientRect.bottom;

for (int i = 0; i < 20; i++)
{
    int r = rand() % 256;
    int g = rand() % 256;
    int b = rand() % 256;

    HPEN newPen = CreatePen(PS_SOLID, 1, RGB(r, g, b));
    HPEN oldPen = (HPEN)SelectObject(hdc,newPen);
    int x0 = rand() % maxX;
    int y0 = rand() % maxY;
    MoveToEx(hdc, x0, y0, NULL);
    int x1 = rand() % maxX;
    int y1 = rand() % maxY;
    LineTo(hdc, x1, y1);
    DeleteObject(newPen);
    SelectObject(hdc, oldPen);
}
HBRUSH newbrush = CreateSolidBrush(RGB(255, 0, 0));
HBRUSH oldbrush = (HBRUSH)SelectObject(hdc,newbrush);

Ellipse(hdc, 20, 20, 40, 40);
Ellipse(hdc, 100, 100, 200, 140);

SelectObject(hdc, oldbrush);
DeleteObject(newbrush);

}