Change window's background color with button using winapi

Question

I'm trying to make button change background color of window when clicked. I know that I need to handle this event in WM_COMMAND, where I also check ID of this button, but nothing happens. I tried to debug and my program recognizes ID correctly. The piece of code used for changing color works well when in main loop but it doesn't do anything when in WM_COMMAND. How do I solve this problem? Whole code:

#include <Windows.h>
#define BUTTON_ID 100

struct status_info {
    const char* waiting = "Waiting for connection...";
    const char* connected = "Connected.\nWaiting for frajer to copy number.";
    const char* changed = "Number changed.";

}status_info;

LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam);

int WINAPI wWinMain(HINSTANCE hInstance, HINSTANCE, PWSTR pCmdLine, int nCmdShow) {

const wchar_t CLASS_NAME[] = L"Name";

WNDCLASS wc = {};
wc.lpfnWndProc = WindowProc;
wc.hInstance = hInstance;
wc.lpszClassName = (LPCSTR)CLASS_NAME;
wc.hbrBackground = (HBRUSH)GetStockObject(GRAY_BRUSH);

RegisterClass(&wc);
//main window
HWND hwnd = CreateWindowEx(0, (LPCSTR)CLASS_NAME, (LPCSTR)"Hacker", WS_OVERLAPPEDWINDOW | WS_VISIBLE, CW_USEDEFAULT, CW_USEDEFAULT, 500, 300, NULL, NULL, hInstance, NULL);
//number window
HWND number = CreateWindowEx(WS_EX_WINDOWEDGE, TEXT("Static"), TEXT("Account number:\n00 1234 1234 1234 1234 1234 1234"), WS_CHILD | WS_VISIBLE, 5, 5, 240, 40, hwnd, NULL, NULL, NULL);
//status window
const char* status_message = status_info.waiting;
HWND status = CreateWindowEx(WS_EX_WINDOWEDGE, TEXT("Static"), TEXT(status_message), WS_CHILD | WS_VISIBLE, 5, 55, 240, 40, hwnd, NULL, NULL, NULL);
//button
HWND button = CreateWindowEx(0, "BUTTON", "Nightmode", WS_CHILD | WS_VISIBLE, 100, 100, 150, 30, hwnd, (HMENU)BUTTON_ID, hInstance, NULL);
MSG msg;
WNDCLASS okno;

while (GetMessage(&msg, (HWND)NULL, 0, 0)) {
    TranslateMessage(&msg);
    DispatchMessage(&msg);

    SetWindowText(status, status_message);
}
return msg.wParam;

return 0;
}

LRESULT CALLBACK WindowProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
switch (message)
{
case WM_CREATE:
    //MessageBox(hwnd, L"1", L"1", 0);
    return (0);

case WM_DESTROY:
    //MessageBox(hwnd, L"2", L"2", 0);
    PostQuitMessage(0);
    return (0);

case WM_COMMAND: {
    if (LOWORD(wParam) == BUTTON_ID) {
        PAINTSTRUCT ps;
        RECT rc;
        HDC hdc = BeginPaint(hwnd, &ps);
        GetClientRect(hwnd, &rc);
        SetBkColor(hdc, BLACK_BRUSH);
        ExtTextOut(hdc, 0, 0, ETO_OPAQUE, &rc, 0, 0, 0);
        EndPaint(hwnd, &ps);
    }
    break;
}

default:
    return DefWindowProc(hwnd, message, wParam, lParam);
}
}

Answer 1

BeginPaint/EndPaint should be used in response to WM_PAINT only.

You can use GetDC(hwnd)/ReleaseDC(hwnd, hdc) to obtain hdc for painting on device context outside of WM_PAINT, but this will be temporary. The next refresh message causes the window to be erased and repainted according to what's in WM_PAINT

SetDCBrushColor can be used if the goal is to avoid creating brush handle.

static COLORREF bkcolor = RGB(255,255,255);
switch(message)
{
case WM_COMMAND:
    if(LOWORD(wparam) == BUTTON_ID)
    {
        bkcolor = RGB(255, 0, 0);
        InvalidateRect(hwnd, NULL, TRUE);
    }
    break;

case WM_PAINT:
{
    PAINTSTRUCT ps;
    RECT rc;
    HDC hdc = BeginPaint(hwnd, &ps);
    GetClientRect(hwnd, &rc);
    SetDCBrushColor(hdc, bkcolor);
    FillRect(hdc, &rc, (HBRUSH)GetStockObject(DC_BRUSH));

    //or use ps.rcPaint to repaint only the section which requires update
    //FillRect(hdc, &ps.rcPaint, (HBRUSH)GetStockObject(DC_BRUSH));

    EndPaint(hwnd, &ps);
    return 0;
}

case WM_ERASEBKGND:
    //return 0 means WM_PAINT handles the background
    return 0;

Alternatively, use SetClassLongPtr to replace the background brush:

static HBRUSH bkbrush = NULL;
switch(message)
{
case WM_COMMAND:
    if(LOWORD(wparam) == BUTTON_ID)
    {
        COLORREF bkcolor = RGB(rand() % 256, rand() % 256, rand() % 256);
        if(bkbrush)
            DeleteObject(bkbrush);
        bkbrush = CreateSolidBrush(bkcolor);
        SetClassLongPtr(hwnd, GCL_HBRBACKGROUND, (LONG)bkbrush);
        InvalidateRect(hwnd, NULL, TRUE);
    }
    break;