HelloWin32.c

// clang-format off

// Compile the program linking user32.lib
#pragma comment(lib, "user32.lib")
#include <Windows.h>

// Event handler.
LRESULT CALLBACK WndProc(
    HWND   hWnd,    // Window the event was assigned to.
    UINT   Msg,     // Message type.
    WPARAM wParam,  // 1st parameter to the message passed by Windows.
    LPARAM lParam   // 2nd parameter to the message passed by Windows. 
)
{
    switch (Msg) {
    case WM_KEYDOWN: // On key down.
        if (wParam == VK_ESCAPE) {
            PostQuitMessage(0);
        }
        return 0;
    case WM_COMMAND: // When we interact with some sort of control, like a button.
        UINT Command = HIWORD(wParam);
        if (Command == BN_CLICKED) {
            MessageBox(hWnd, "Ow.", "Button", MB_ICONWARNING);
        }
        return 0;
    case WM_DESTROY: // On window destroy.
        PostQuitMessage(0);
        return 0;
    }

    // Let Windows handle the rest.
    return DefWindowProcA(hWnd, Msg, wParam, lParam);
}


// Entry point.
int APIENTRY WinMain(
    HINSTANCE hInstance,        // Current .exe
    HINSTANCE hPrevInstance,    // Parent .exe (explorer, cmd, &c.)
    LPSTR     lpCmdLine,        // Command line parmeters all as a single string.
    int       nCmdShow          // Whether to show/hide windows we create.
)
{
    // Window classes are to identify types of windows.
    WNDCLASSA WindowClass = { 0 };
    WindowClass.lpszClassName = "YourWindowClassName";
    WindowClass.lpfnWndProc = WndProc;

    if (!RegisterClassA(&WindowClass)) {
        MessageBoxA(NULL, "Couldn't register window class", "Windows app", MB_ICONERROR);
    }

    // Create the window.
    HWND Window = CreateWindowA(
        WindowClass.lpszClassName,    
        "Your title",                 
        WS_OVERLAPPEDWINDOW,          // How window appears. This value will show default window appearance.
        CW_USEDEFAULT, CW_USEDEFAULT, // X and Y, CW_USEDEFAULT will put the window wherever the system chooses.
        800, 600,                     // Window size.
        NULL,                         // Parent window, we have none.
        NULL,                         // Menu, we have none (this is also a bit outdated nowadays).
        hInstance,                    // Current .exe instance.
        NULL                          // Your custom parameter to WndProc upon creation.
    );

    if (Window == NULL) {
        MessageBoxA(NULL, "Couldn't create a window.", "Windows app", MB_ICONERROR);
    }

    ShowWindow(Window, nCmdShow);

    // Create a button (Yes, those are also Windows =D).
    UINT ButtonStyle = WS_VISIBLE | WS_CHILD | BS_DEFPUSHBUTTON;
    HWND Button = CreateWindowA(
        "BUTTON",    
        "Dont click me!!",                 
        ButtonStyle,        
        10, 10,                       // X and Y.
        100, 20,                      // Size.
        Window,                       // Parent window.
        NULL,                         // Not used here.
        hInstance,                    // Current .exe instance.
        NULL                          // Your custom parameter to WndProc upon creation.
    );

    if (Button == NULL) {
        MessageBoxA(NULL, "Couldn't create a button.", "Windows app", MB_ICONERROR);
    }

    // Poll for events.
    // This will block the thread until a message is received, such as moving the mouse.
    MSG Msg = { 0 };
    while (GetMessageA(&Msg, NULL, 0, 0)) {
        TranslateMessage(&Msg); // Translates key messages to character output. 
        DispatchMessage(&Msg);  // Runs our WndProc function!
    }

    return 0;
}