If I assign wndproc function to wndclass then error occur - winapi

Question

I coded simple windowing app with rust and winapi. When I assigned DefWindowProcW function to lpfnWndProc field of WndClass structure, code worked fine. However, when I assign the WndProc function I defined, the window does not appear and even message box for exception handling also does not appear. I want to know why it doesn't work and how fix it.
This is the WndClass structure I defined.
let WndClass = WNDCLASSW {
style: CS_HREDRAW | CS_VREDRAW,
lpfnWndProc: Some( DefWindowProcW ),
cbClsExtra: 0,
cbWndExtra: 0,
hInstance,
hIcon: LoadIconW( null_mut(), IDC_ARROW ),
hCursor: LoadCursorW( null_mut() , IDI_APPLICATION ),
hbrBackground: GetStockObject(WHITE_BRUSH as i32) as HBRUSH,
lpszMenuName: null_mut(),
lpszClassName:lpszClass.as_ptr(),
};
And this is whole code.
extern crate winapi;
use self::winapi::{
shared::{
windef::{HWND, HBRUSH},
minwindef::{LPARAM, WPARAM, UINT, LRESULT},
},
um::{
wingdi::{GetStockObject, WHITE_BRUSH},
libloaderapi::GetModuleHandleW,
winuser::{
RegisterClassW,
CreateWindowExW,
GetMessageW,
TranslateMessage,
DispatchMessageW,
PostQuitMessage,
DefWindowProcW,
LoadIconW,
LoadCursorW,
MessageBoxW,
ShowWindow,
MSG,
WS_OVERLAPPEDWINDOW,
WS_VISIBLE,
CW_USEDEFAULT,
WNDCLASSW,
SW_SHOWDEFAULT,
CS_HREDRAW,
CS_VREDRAW,
MB_OK,
IDC_ARROW,
IDI_APPLICATION
},
}
};
use std::ptr::{ null, null_mut };
use std::ffi::OsStr;
use std::os::windows::ffi::OsStrExt;
use std::iter::once;
#[cfg(windows)]
fn win32_string( value : &str ) -> Vec<u16> {
OsStr::new( value ).encode_wide().chain( once( 0 ) ).collect()
}
unsafe fn handle_message( handle: HWND, message: &mut MSG ) -> bool {
if GetMessageW(message, handle, 0, 0) > 0 {
TranslateMessage(message);
DispatchMessageW(message);
true
} else {
false
}
}
fn main() {
unsafe{
let mut hWnd: HWND;
let mut Message: MSG = std::mem::zeroed();
let mut hInstance = GetModuleHandleW( std::ptr::null_mut() );
let lpszClass = win32_string("First Window");
let mut g_hInst = hInstance;
let WndClass = WNDCLASSW {
style: CS_HREDRAW | CS_VREDRAW,
lpfnWndProc: Some( DefWindowProcW ),
cbClsExtra: 0,
cbWndExtra: 0,
hInstance,
hIcon: LoadIconW( null_mut(), IDC_ARROW ),
hCursor: LoadCursorW( null_mut() , IDI_APPLICATION ),
hbrBackground: GetStockObject(WHITE_BRUSH as i32) as HBRUSH,
lpszMenuName: null_mut(),
lpszClassName:lpszClass.as_ptr(),
};
RegisterClassW( &WndClass );
hWnd = CreateWindowExW(
0,
lpszClass.as_ptr(),
lpszClass.as_ptr(),
WS_OVERLAPPEDWINDOW,
CW_USEDEFAULT,
CW_USEDEFAULT,
CW_USEDEFAULT,
CW_USEDEFAULT,
null_mut(),
null_mut(),
hInstance,
null_mut()
);
if hWnd.is_null() {
let title = win32_string("Error!");
let text = win32_string("Creating window is faield");
let ret = MessageBoxW(0 as HWND, text.as_ptr(), title.as_ptr(), MB_OK);
} else {
loop {
if !handle_message(0 as HWND, &mut Message) {
break;
}
}
}
}
}
#[cfg(windows)]
unsafe extern "system" fn WndProc(hWnd: HWND, iMessage: UINT, wParam: WPARAM, lParam: LPARAM) -> LRESULT {
match iMessage {
WM_DESTROY => {
PostQuitMessage(0);
0
},
_ => DefWindowProcW(hWnd, iMessage, wParam, lParam)
}
}

Answer 1

You did not quote this win32 constant: WM_DESTROY, if you use the if condition like:
if iMessage == WM_DESTROY {
PostQuitMessage(0);
}
Then you will get the compile error:
error[E0425]: cannot find value `WM_DESTROY` in this scope
Just add WM_DESTROY:
use self::winapi::{
shared::{
windef::{HWND, HBRUSH},
minwindef::{LPARAM, WPARAM, UINT, LRESULT,ATOM},
},
um::{
errhandlingapi::GetLastError,
wingdi::{GetStockObject, WHITE_BRUSH},
libloaderapi::GetModuleHandleW,
winuser::{
RegisterClassW,
...,
IDI_APPLICATION,
WM_DESTROY
},
}
};
In addition, you need to show the window with ShowWindow after create it:
...
lpfnWndProc: Some( WndProc ),
...
if hWnd.is_null() {
let title = win32_string("Error!");
let text = win32_string("Creating window is faield");
let ret = MessageBoxW(0 as HWND, text.as_ptr(), title.as_ptr(), MB_OK);
} else {
println!("successful");
ShowWindow(hWnd, SW_SHOWDEFAULT);
loop {
if !handle_message(0 as HWND, &mut Message) {
break;
}
}
}
Then it works for me.