public class EmployeeViewModel : IDataErrorInfo
{
    private IDataContext Context;
    private IWindowService Service;

    public EmployeeViewModel()
    {
        Context = new DataContext(); // todo IoC resolve
        Service = new WindowService(); // todo IoC resolve
    }

    public int EmployeeId { get; set; }

    public string FirstName { get; set; }

    public string LastName { get; set; }

    public DateTime? Birthday { get; set; }

    public bool Fired { get; set; }

    public EmployeeStage Stage { get; set; }

    public ObservableCollection<EmployeeStage> Stages
    {
        get
        {
            return new ObservableCollection<EmployeeStage>(Context.GetEmployeeStages());
        }
    }

    private ICommand _saveCommand;
    public ICommand SaveCommand
    {
        get
        {
            if (_saveCommand == null)
            {
                _saveCommand = new RelayCommand(action =>
                {
                    if (Service.IsValid)
                    {
                        var vm = action as EmployeeViewModel;
                        Context.UpdateEmployee(vm.EmployeeId, vm.FirstName, vm.LastName, vm.Birthday, vm.Fired, vm.Stage);
                        Service.CloseActiveWindow(true);
                    }
                });
            }

            return _saveCommand;
        }
    }
        
    public string Error
    {
        get { return null; }
    }

    public string this[string columnName]
    {
        get
        {
            if (columnName == "FirstName")
            {
                if (string.IsNullOrEmpty(this.FirstName))
                {
                    return "Значение не должно быть пустым";
                }
            }

            if (columnName == "LastName")
            {
                if (string.IsNullOrEmpty(this.LastName))
                {
                    return "Значение не должно быть пустым";
                }
            }

            if (columnName == "Stage")
            {
                if (this.Stage == null || !new[] { "10", "20" }.Contains(this.Stage.Value))
                {
                    return "Значение должно быть 10 или 20";
                }
            }

            if (columnName == "Birthday")
            {
                if (this.Birthday == null || this.Birthday > DateTime.Today)
                {
                    return "Значение должно быть меньше сегодняшней даты";
                }
            }
                

            return null;
        }
    }
}