Открытия файла

char c;
ifstream f("C:\\WinApi.txt");
while (!f.eof())
{
  f >> c;
}

wm_paint

#include <windows.h>
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
 
int WINAPI WinMain(HINSTANCE, HINSTANCE, LPSTR, int)
{
    char szClassName[] = "mywindowclass";
    WNDCLASSEX wc = {};
    wc.cbSize = sizeof(WNDCLASSEX);
    wc.hbrBackground = (HBRUSH) (COLOR_BTNFACE + 1);
    wc.hCursor =  LoadCursor(NULL, IDC_ARROW);
    wc.lpszClassName = szClassName;
    wc.lpfnWndProc = WndProc;
    wc.style = CS_HREDRAW|CS_VREDRAW;
    wc.hIcon = LoadIcon(NULL, IDI_QUESTION);
 
    RegisterClassEx(&wc);
    
    HWND hMainWnd = CreateWindow(szClassName, "File open", WS_OVERLAPPEDWINDOW | WM_HSCROLL | WM_VSCROLL, CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, NULL, NULL);
 
    ShowWindow(hMainWnd, SW_SHOW);
    // открытия файла
    CreateFile("C:\\1.txt", GENERIC_READ, NULL, NULL,NULL, NULL, NULL);
    
    MSG msg;
    while (GetMessage(&msg, NULL, 0, 0))
        DispatchMessage(&msg);
    return 0;
}
 
 
 
LRESULT CALLBACK WndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    switch (uMsg)
    {
    case WM_PAINT:
        PAINTSTRUCT ps;
        BeginPaint(hWnd, &ps);
        //DrawText(ps.hdc, "Hello Word", -1, &ps.rcPaint, DT_SINGLELINE | DT_CENTER | DT_VCENTER);
        EndPaint(hWnd, &ps);
        break;
    case WM_CLOSE:
        PostQuitMessage(0);
        break;
    }
    return DefWindowProc(hWnd, uMsg, wParam, lParam);
}