c# - ObservableCollection 对象引用

Question

我正在尝试了解在 Xamarin Forms 中的不同 View 之间传递数据的基础知识。

我有一个包含可观察集合的 session 类。在后面的代码中，我有另一个 ObservableCollection，用作 listview 的 ItemSource。后面代码中的 ObservableCollection 工作正常，但是当我尝试设置 myMeeting.MeetingInvitees = _myMeetingInvitees 时，我收到此消息:

System.NullReferenceException: Object reference not set to an instance of an object

在后面代码的构造函数中，我有:

myMeeting = this.BindingContext as Meeting;

这不会初始化 myMeeting 吗？ Topic 和 Place 属性工作正常，只是集合不行。所以当我这样做时:

myMeeting.MeetingInvitees = _myMeetingInvitees;

这不会将它们设置为相同的地址吗？

我觉得在设置 MeetingInvitees

属性的 Meeting 类中有一些我不理解的基本内容

谢谢你能给我指出正确的方向!

session .cs:

private ObservableCollection<String> meetingInvitees;
private string meetingTopic;
private string meetingPlace;

public ObservableCollection<string> MeetingInvitees
{
    get { return meetingInvitees; }
    set
    {
        meetingInvitees = value;
    }
}
public string MeetingTopic
{
    get { return meetingTopic; }
    set
    {
        meetingTopic = value;
        OnPropertyChanged("MeetingTopic");
    }
}
public string MeetingPlace
{
    get { return meetingPlace; }
    set
    {
        meetingPlace = value;
        OnPropertyChanged("MeetingPlace");
    }
}

Xamarin 代码隐藏

public Meeting myMeeting;
public ObservableCollection<string> _myMeetingInvitees;

public NewMeetingContactsView ()
{
    InitializeComponent ();
    _myMeetingInvitees = new ObservableCollection<string>();
    myMeeting = this.BindingContext as Meeting;
    listAttendees.ItemsSource = _myMeetingAttendess;
}

async void btnNext_Clicked(object sender, EventArgs e)
{
    myMeeting.MeetingInvitees = _myMeetingAttendess;
    var nextpage = new NewMeetingOverviewView();
    nextpage.BindingContext = myMeeting;
    await Navigation.PushAsync(nextpage);
}

async void btnCancel_Clicked(object sender, EventArgs e)
{
    await Navigation.PopToRootAsync();
}

void btnAdd_Clicked(object sender, EventArgs e)
{
    _myMeetingAttendess.Add(entryName.Text);
}

Answer 1

在你的构造函数中，而不是做

_myMeetingInvitees = new ObservableCollection<string>();

初始化您的 myMeeting 对象后将其更改为以下代码。

public NewMeetingContactsView ()
 {
     InitializeComponent ();
     myMeeting = new Meeting();
     myMeeting.MeetingInvites = new ObservableCollection<string>();
     this.BindingContext = myMeeting;
     listAttendees.ItemsSource = myMeeting.MeetingInvites;
 }

您似乎没有为 myMeeting 初始化您的属性 MeetingInvites，这就是引发空引用错误的原因。