private int? _employeeId;
private int? EmployeeId
{
    get
    {
        if (_employeeId == null)
        {
            if (ViewState["_employeeId"] != null)
            {
                _employeeId = (int)ViewState["_employeeId"];
            }
            else
            {
                _employeeId = null;
            }
        }
        return _employeeId;
    }
    set
    {
        _employeeId = value;
        if (value == null)
        {
            ViewState.Remove("_employeeId");
        }
        else
        {
            ViewState["_employeeId"] = value;
        }
    }
}