Modeless Dialog

Most sample code shows DialogBox, which blocks until the user closes the dialog. This project shows the other half of the API: CreateDialog returns immediately and hands you the dialog's HWND, so your main window stays alive and responsive while the dialog sits on top - necessary when the dialog must coexist with other windows, timers, or background work. The key piece is the message loop in WinMain: pump messages with GetMessage, route them through IsDialogMessage (which gives the dialog keyboard navigation like Tab), and only translate/dispatch what the dialog did not handle. The dialog procedure itself is the standard shape - WM_INITDIALOG to wire up controls, WM_COMMAND for buttons, EndDialog to close.

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevious, LPSTR CmdLine, int CmdShow)
{
	AppInstance = hInstance;

	HANDLE xMutex = NULL;

    if(!InitializeApp())
    {
        return 1;
    }

	if( (xMutex = OpenMutex(SYNCHRONIZE, FALSE, gsTitleCaption)) )
	{
		MessageBox (NULL, "Only one instance can run at a time!" , gsTitleCaption, MB_ICONASTERISK + MB_SYSTEMMODAL);
		ReleaseMutex(xMutex);
		CloseHandle(xMutex);
		return 0;
	}
	else xMutex = CreateMutex(0, 0, gsTitleCaption);

	HWND MyDialog_hWnd = CreateDialog(AppInstance, MAKEINTRESOURCE(IDD_MAIN), 0, MainDialog);

    BOOL bRet;
    MSG msg;

    while ( (bRet = GetMessage(&msg, MyDialog_hWnd, 0, 0)) != 0 )
    {
        if(bRet == -1)
        {
    		MessageBox(NULL, "GetMessage Error."  , gsTitleCaption, MB_SYSTEMMODAL);
            return 0;
            // handle the error and possibly exit
        }
        else if(!IsWindow(MyDialog_hWnd) || !IsDialogMessage(MyDialog_hWnd, &msg))
        {
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }
    }

    if(!UninitializeApp())
    {
        return 1;
    }

	ReleaseMutex(xMutex);
	CloseHandle(xMutex);

	return 0;
}

The dialog procedure handles the usual messages; note that unlike a modal dialog it must keep returning control to the pump, and closing it with EndDialog makes IsWindow(MyDialog_hWnd) go false so the loop exits cleanly:

BOOL CALLBACK MainDialog(HWND xHandle, UINT xMessage, WPARAM wParam, LPARAM lParam)
{
    static HWND TextEdit_hWnd   = NULL;
    static HWND TextStatic_hWnd = NULL;

    static HMENU SystemMenu     = NULL;

    if(xMessage == WM_INITDIALOG)
    {
        MainDialog_hWnd = xHandle;

        SystemMenu = GetSystemMenu(xHandle, FALSE);
        AppendMenu(SystemMenu, MF_SEPARATOR, 0, 0);
        AppendMenu(SystemMenu, MF_STRING, 1000, "About");

        SendMessage(xHandle, (UINT)WM_SETTEXT, (WPARAM)0, (LPARAM)gsTitleCaption);
        SendMessage(xHandle, WM_SETICON, TRUE, (LPARAM) LoadIcon(AppInstance, MAKEINTRESOURCE(IDI_MAIN)) );

        TextEdit_hWnd   = GetDlgItem(xHandle, IDC_TEXTBOX);
        TextStatic_hWnd = GetDlgItem(xHandle, IDC_STATICTEXT);

        Set_Text(TextEdit_hWnd, "Text here");
        Set_Text(TextStatic_hWnd, "will appear here.");

        CenterWindow(xHandle);

        return TRUE; // Return TRUE to set the keyboard focus, Otherwise return FALSE
    }

    if(xMessage == WM_COMMAND)
    {
        if(wParam == IDC_OK) //- Ok (Dialog Command)
        {
            char TempText[256];
            Get_TextEx(TextEdit_hWnd, TempText, 255);
            Set_Text(TextStatic_hWnd, TempText);
            return TRUE;
        }

        if(wParam == IDC_CANCEL) //- Cancel (Dialog Command)
        {
            EndDialog(xHandle, 0);
            return TRUE;
        }

        return FALSE;
    }

	if(xMessage == WM_PAINT)
    {
        HDC ThisHDC;
        PAINTSTRUCT ThisPS;

        ThisHDC = BeginPaint(xHandle, &ThisPS);

        // Any painting should be done here

        EndPaint(xHandle, &ThisPS);
        return TRUE;
    }

    if(xMessage == WM_CLOSE)
    {
        EndDialog(xHandle,0);
        return TRUE;
    }

    return FALSE;
}

The single-instance mutex check around the whole thing is the same pattern as the modal examples - open first, create only if nobody else holds it.