c# - 运行MVVM时，EventHandler为Null

Question

因此，我试图在WPF应用程序中使用共享变量。为此，我使用ViewModelController实现了eventhandler类来存储影响应用程序其余部分的变量。

我的ViewModelController类是这样的:

    public class ViewModelController
    {

        private DataRowView _SelectedUniverse;

        public MenuViewModel MenuViewModel { get { return new MenuViewModel(this); } }

        public GamePageViewModel GamePageViewModel { get { return new GamePageViewModel(this); } }

        public PlayersViewModel PlayersViewModel { get { return new PlayersViewModel(this); } }

        public event EventHandler UniverseChanged;

        public DataRowView SelectedUniverse
        {
            get { return _SelectedUniverse; }
            set
            {
                _SelectedUniverse = value;
                UniverseChanged.Invoke(this, EventArgs.Empty);
                Debug.Print("Universe Changed in ViewModelController");
            }
        }

    }

我正在菜单ViewModel上对其进行测试，以查看是否传递给另一个类:
MenuViewModel

    public class MenuViewModel : ObservableObject
    {

        ViewModelController _ViewModelController;
        private DataView _Universes;
        private DataRowView _SelectedUniverse;

        public MenuViewModel(ViewModelController controller)
        {
            _ViewModelController = controller;
            _Universes = Controller.UniverseTableAdapter.GetData().DefaultView;
        }

        public DataView Universes
        {
            get { return _Universes; }
            set
            {
                _Universes = value;
                RaisePropertyChangedEvent("Universes");
            }
        }

        public DataRowView SelectedUniverse
        {
            get { return _SelectedUniverse; }
            set
            {
                _SelectedUniverse = value;
                Debug.Print("Universe Changed from Menu");
                _ViewModelController.SelectedUniverse = _SelectedUniverse;
                RaisePropertyChangedEvent("SelectedUniverse");
            }
        }
    }

PlayerViewModel类别

public class PlayersViewModel : ObservableObject
{

    private ViewModelController _ViewModelController;

    #region Variables
    private ObservableCollection<Player> _Players = Controller.GetPlayers(0);
    private Player _SelectedPlayer;
    private DataView _Colleges = Controller.CollegeTableAdapter.GetData().DefaultView;
    private DataView _Universes = Controller.UniverseTableAdapter.GetData().DefaultView;

    private long _PlayerID;
    private string _PlayerFirstName;
    private string _PlayerLastName;
    private string _PlayerFullName;
    private DataRowView _SelectedCollege;
    private DataRowView _SelectedUniverse;
    private long _PlayerCollegeID;
    private long _UniverseID;
    private int _PlayerEnteredYear;

    private DataView _RegularSeasonStats = Controller.SeasonView.GetData().DefaultView;
    private DataRowView _SelectedSeason;
    private DataView _PlayoffStats = Controller.PlayoffsView.GetData().DefaultView;
    private DataView _PlayerSchedule = Controller.PlayerScheduleView.GetDataByPersonAndYear(0, 0).DefaultView;
    #endregion 

    public PlayersViewModel(ViewModelController controller)
    {
        _ViewModelController = controller;
        _ViewModelController.UniverseChanged += OnUniverseChanged;
        Debug.Print("PlayerViewModel Constructed");
    }

    public DataRowView SelectedUniverse
    {
        get { return _SelectedUniverse; }
        set
        {
            _SelectedUniverse = value;
            if(_SelectedUniverse != null)
            {
                _UniverseID = _SelectedUniverse.Row.Field<long>("UniverseID");
                RaisePropertyChangedEvent("SelectedUniverse");
            }
        }
    }

    private void OnUniverseChanged(object sender, EventArgs e)
    {
        SelectedUniverse = _ViewModelController.SelectedUniverse;
        Debug.Print("Universe was changed");
    }
}

更改 SelectedUniverse时，它将变量发送到 ViewModelController，后者应调用 UniverseChanged事件。但是，我得到了 Object Reference Not Set to an Instance of an Object。

我知道我应该实例化 EventHandler，但是我没有找到一种无需构造函数即可实例化它的方法。我能解决所有这些错误吗？

Answer 1

这里的问题是您要强制调用UniverseChanged事件，但是没有监听器来处理它，因此该事件实际上可以为null。

调用之前，应检查是否有监听器。像这样:

public DataRowView SelectedUniverse
{
    get { return _SelectedUniverse; }
    set
    {
        _SelectedUniverse = value;
        UniverseChanged?.Invoke(this, EventArgs.Empty);
        // Or if you're using a C# version< 6:
        // var evtHandlers = UniverseChanged;
        // if(evtHandlers != null) evtHandlers.Invoke(this, EventArgs.Empty);
        Debug.Print("Universe Changed in ViewModelController");
    }
}

希望对您有所帮助。