Creating ComboBox in Win32 not working properly - winapi

Question

I am creating a Win32 ComboBox for the first time. And I have a problem here.
When calling CreateWindow for the ComboBox, it calls the WndProc callback function again with the WM_CREATE message, so what happens is the ComboBox makes a child ComboBox, again and again like recursion.
Here is the code:
#include <stdio.h>
#include <conio.h>
#include <Windows.h>
#include <random>
#include <time.h>
#include <string>
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
HINSTANCE g_hInst;
LPCTSTR lpszClass = L"ComboBox";
const WCHAR *items[] = { L"Apple", L"Orange", L"Melon", L"Grape", L"Strawberry" };
HWND hwnd;
enum COMMAND_ID {
COMMAND_ID_CONTROL_COMBO_0
};
int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpszCmdParam, int nCmdShow)
{
srand(time(NULL));
g_hInst = hInstance;
WNDCLASS wndClass;
wndClass.cbClsExtra = 0;
wndClass.cbWndExtra = 0;
wndClass.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);
wndClass.hCursor = LoadCursor(NULL, IDC_ARROW);
wndClass.hIcon = LoadIcon(NULL, IDI_APPLICATION);
wndClass.hInstance = hInstance;
wndClass.lpfnWndProc = WndProc;
wndClass.lpszClassName = lpszClass;
wndClass.lpszMenuName = NULL;
wndClass.style = CS_HREDRAW | CS_VREDRAW;
RegisterClass(&wndClass);
hwnd = CreateWindow(
lpszClass,
lpszClass,
WS_CAPTION | WS_SYSMENU | WS_THICKFRAME,
CW_USEDEFAULT, CW_USEDEFAULT,
CW_USEDEFAULT, CW_USEDEFAULT,
NULL,
(HMENU)NULL,
hInstance,
NULL);
ShowWindow(hwnd, nCmdShow);
MSG msg;
while (true)
{
GetMessage(&msg, NULL, 0, 0);
if (msg.message == WM_QUIT)
break;
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return (int)msg.wParam;
}
LRESULT CALLBACK WndProc(HWND hWnd, UINT msg, WPARAM wparam, LPARAM lparam)
{
static HWND hCombo;
static WCHAR str[128];
switch (msg)
{
case WM_CREATE:
{
hCombo = CreateWindow(
L"combobox",
NULL,
WS_CHILD | WS_VISIBLE | WS_BORDER | CBS_DROPDOWN,
10, 10, 200, 200,
hWnd,
(HMENU)COMMAND_ID_CONTROL_COMBO_0,
g_hInst,
NULL);
for (int i = 0; i < 5; ++i)
{
SendMessage(hCombo, CB_ADDSTRING, 0, (LPARAM)items[i]);
}
SendMessage(hCombo, CB_SETCURSEL, 0, NULL);
}
break;
case WM_COMMAND:
{
switch (LOWORD(wparam))
{
case COMMAND_ID_CONTROL_COMBO_0:
switch (HIWORD(wparam))
{
case CBN_SELCHANGE:
{
int iCurSel = SendMessage(hCombo, CB_GETCURSEL, NULL, NULL);
SendMessage(hCombo, CB_GETLBTEXT, iCurSel, (LPARAM)str);
SetWindowText(hWnd, str);
}
break;
case CBN_EDITCHANGE:
GetWindowText(hCombo, str, 128);
SetWindowText(hWnd, str);
break;
}
break;
default:
break;
}
}
return 0;
}
return DefWindowProc(hWnd, msg, wparam, lparam);
}
And here is the result:
I tried to put some boolean flag to execute WM_CREATE only once, and it works, I mean only creating one ComboBox without any child in it.
But, it just looked like only a white window with a border mark, there's no arrow button or anything to dropdown page that the ComboBox is supposed to have.
This recursive case never happened when I was creating different controls like Buttons, CheckBoxes, ListBoxes, etc.
And the ComboBox created doesn't look like it has the proper shape, too.
Hope I am just missing something simple.

Answer 1

When calling CreateWindow for the ComboBox, it calls the WndProc
callback function again with the WM_CREATE message, so what happens
is the ComboBox makes a child ComboBox, again and again like
recursion.
WM_CREATE message is sent to the window procedure of the new window after the window is created. Your first WM_CREATE message is generated by this line hwnd = CreateWindow(). Then you create another window in first WM_CREATE message, so it will generate second WM_CREATE message. Because you use the same registered class ("ComboBox" / "combobox", it is not case sensitive) to create all these windows, all of them use the same one window procedure. So you receive WM_CREATE message again and again until CreateWindow fail to create a window and return NULL.
But, it just looked like only a white window with a border mark,
there's no arrow button or anything to dropdown page that the ComboBox
is supposed to have.
The root cause is you register a class with the same name as the existing system class: "ComboBox" / "combobox". This new registered class override the existing one. It is just a common window instead of a predefined Combobox control as #RemyLebeau pointed out.
An application can register an application local class having the same
name as a system class. This replaces the system class in the context
of the application but does not prevent other applications from using
the system class.
Refer to "How the System Locates a Window Class".
To make the Combobox display in expected shape, what you need to do is changing the lpszClass to a non-predefined one, for example, like "SimpleComboBoxExample".
It is suggested to use predefined macro of Combobox Class Name: WC_COMBOBOX instead of L"combobox".
More reference: "How to Create a Simple Combo Box".

Answer 2

You are not actually creating a Win32 ComboBox at all. You are registering your own class named "ComboBox", and then creating a window of that class, which creates a window of that class, which creates a window of that class, and so on recursively.
You need to change this line:
LPCTSTR lpszClass = L"ComboBox";
To a different unique name, such as "MyWindowClass".
On a side note, your message loop is structured wrong. It should look like this instead:
MSG msg;
while (GetMessage(&msg, NULL, 0, 0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
And in your WndProc(), the return 0; statement underneath the WM_COMMAND handler is in the wrong place. It needs to be moved inside of the WM_COMMAND handler instead:
case WM_COMMAND:
{
switch (...)
{
...
}
return 0; // <-- moved here
}
//return 0; // <-- from here