Why VLC make the event of WinForm not work?

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C++/CLI

C#

I designd a WinForm, using C#. I want play a media, so I used VLC. I invoked it by P/invoke.
At first, all are right. But after I play a movie for a while. All event didn't work, include the butten for closing window.
Here is the code

#include "stdafx.h"
#include "MagicPlayer.h"
#include <vlc\vlc.h>

libvlc_instance_t *inst = 0;
libvlc_media_t *m = 0;
libvlc_media_player_t *mp = 0;
libvlc_time_t movie_time = 0;

int VLC_load(void *handle)
{
	inst = libvlc_new(0, NULL);        /* Load the VLC engine */
	if (inst == NULL)
		return 1;
	mp = libvlc_media_player_new(inst);
	libvlc_media_player_set_hwnd(mp, handle); /* Make the video played in the window */
	return 0;
}

int VLC_pause()
{
	if (inst == NULL)
		return 1;
	if (mp == 0)
		return 1;
	if (m == 0)
		return 1;
	if (libvlc_media_player_get_state(mp) == libvlc_Playing)
	{
		movie_time = libvlc_media_player_get_time(mp);
		libvlc_media_player_pause(mp);
		return 0;
	}
	else if (libvlc_media_player_get_state(mp) == libvlc_Paused)
	{
		libvlc_media_player_pause(mp);
		return 0;
	}
	
	return 0;
}

int VLC_play(char *file_name)
{
	if (inst == NULL)
		return 1;
	if (mp == NULL)
		return 1;
	m = libvlc_media_new_path(inst, file_name);
	libvlc_media_player_set_media(mp, m);
	if (m == NULL)
		return 2;
	return libvlc_media_player_play(mp);
}

int VLC_go_back_to_last_stop()
{
	if (mp == NULL)
		return 1;
	if (libvlc_media_player_get_state(mp) == libvlc_Playing){
		libvlc_media_player_pause(mp);
		libvlc_media_player_set_time(mp, movie_time);
		return 0;
	}
	
	return 0;
}

int VLC_release()
{
	if (mp != 0)
		libvlc_media_player_release(mp);
	if (m != 0)
		libvlc_media_release(m);
	if (inst != 0)
		libvlc_release(inst);
	return 0;
}