重新加载列表视图项时“指定的演员表无效” - Xamarin Forms

如何解决重新加载列表视图项时“指定的演员表无效” - Xamarin Forms

我正在尝试制作一个将主题保存到 sqlite 数据库中的应用程序，但我遇到了一个异常：“指定的强制转换无效”。当我重新加载列表视图项目时会发生这种情况。 (Subjectviewmodel.UpdateSubjects())

我通过异步连接从 SubjectServices 静态类接收数据。

我将 contentpage 绑定上下文设置为此视图模型，然后将列表视图项源的绑定设置为 Subjectviewmodel 的 SubjectList。

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:resx="clr-namespace:MyApp.Resources"
             x:Class="MyApp.View.ShellPages.SubjectsPage">
    <ContentPage.toolbaritems>
        <ToolbarItem Text="{x:Static resx:AppResources.NewSubject}"
                     Command="{Binding SubjectNewSCommand}"/>
    </ContentPage.toolbaritems>
    <ContentPage.Content>
        <ListView x:Name="subjectListView" ItemsSource="{Binding SubjectList}"
                  IsRefreshing="True" RefreshCommand="{Binding SubjectRefreshCommand}">
            <ListView.ItemTemplate>
                <DataTemplate>
                    <TextCell Text="{Binding Name}" Detail="{Binding AddedTime,StringFormat='{0:d}'}"/>
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
    </ContentPage.Content>
</ContentPage>

主题视图模型：

using MyApp.Model;
using MyApp.View.ShellPages;
using MyApp.viewmodel.Commands;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Text;

namespace MyApp.viewmodel
{
    public class Subjectviewmodel
    {
        public ObservableCollection<Subject> SubjectList { get; set; }
        public SubjectPageRefreshCommand SubjectRefreshCommand { get; set; }
        public SubjectNewSubjectCommand SubjectNewSCommand { get; set; }
        public Subjectviewmodel()
        {
            SubjectRefreshCommand = new SubjectPageRefreshCommand(this);
            SubjectNewSCommand = new SubjectNewSubjectCommand(this);
            SubjectList = new ObservableCollection<Subject>();
        }

        public async void UpdateSubjects()
        {
            var subjectList = await SubjectServices.GetSubjects();
            SubjectList.Clear();
            foreach (var subject in subjectList)
                SubjectList.Add(subject);
        }

        public async void Navigate()
        {
            await App.Current.MainPage.Navigation.PushAsync(new NewSubjectPage());
        }
    }
}

学科类别：

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Text;
using System.Threading.Tasks;
using sqlite;

namespace MyApp.Model
{
    public class Subject
    {
        [PrimaryKey,AutoIncrement]
        public int ID { get; set; }

        [MaxLength(50)]
        public string Name { get; set; }

        public DateTime AddedTime { get; set; }
    }
}

可能是什么问题？我试图找到任何解决方案，但没有成功。我感谢任何形式的帮助。