I make dialogbox-base program

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WinAPI

(First, I'm worry about my suck English. If you can understand, could you help me?)

#include <Windows.h>
#include "resource.h"

HINSTANCE hInst;
HWND hDigMain;

BOOL CALLBACK MainDigProc(HWND, UINT, WPARAM, LPARAM);
BOOL CALLBACK testProc(HWND, UINT, WPARAM, LPARAM);

int APIENTRY wWinMain(_In_ HINSTANCE hInstance,
	_In_opt_ HINSTANCE hPrevInstance,
	_In_ LPWSTR    lpCmdLine,
	_In_ int       nCmdShow)
{
	hInst = hInstance;
	DialogBox(hInst, MAKEINTRESOURCE(IDD_DIALOG1), HWND_DESKTOP, MainDigProc);
	return 0;
}

BOOL CALLBACK MainDigProc(HWND hWnd, UINT iMessage, WPARAM wParam, LPARAM lParam)
{
	//Search Files ..
	switch (iMessage) {
	case WM_INITDIALOG:
		return TRUE;
	case WM_COMMAND:
		switch (LOWORD(wParam)) {
		case IDSTART:
			if (!IsWindow(hDigMain)) {
				hDigMain = CreateDialog(hInst, MAKEINTRESOURCE(IDD_DIALOG2), HWND_DESKTOP, testProc);
				ShowWindow(hDigMain, SW_SHOW);
			}
			//EndDialog(hWnd, IDSTART);
			return TRUE;
		case IDLOAD:
			EndDialog(hWnd, IDLOAD);
			return TRUE;
		case ID_OPTION:
			EndDialog(hWnd, ID_OPTION);
			return TRUE;
		case IDCANCEL:
			EndDialog(hWnd, IDLOAD);
			return TRUE;
		}
		break;
	}
	return FALSE;
}

BOOL CALLBACK testProc(HWND hWnd, UINT iMessage, WPARAM wParam, LPARAM lParam)
{
	switch (iMessage) {
	case WM_INITDIALOG:
		return TRUE;
	case WM_COMMAND:
		switch (LOWORD(wParam)) {
		case IDOK:
		case IDCANCEL:
			EndDialog(hWnd, IDLOAD);
			return TRUE;
		}
		break;
	}
	return FALSE;
}

I tried popup new window(IDD_DIALOG2) when clicked button ':IDSTART', and close (automatic) old window(IDD_DIALOG1).

What I have tried:

In MainDigProc - WM_COMMAND, EndDialog make close total program. (now I think 'IsWindow' is unnecessary, isn't it?) Did I tried close Parent window, so it make to shut-down program? I'm not sure. Opinion please.