：为什么我不能设置Xamarin.Forms.ListView的SelectedItem属性？

Question

object lastItem = null;
foreach (object item in listView.ItemsSource)
    lastItem = item;
if (lastItem != null)
    listView.SelectedItem = lastItem;

但我得到的只是一个＆＃39;序列中没有匹配元素＆＃39;将SelectedItem设置为所需的lastItem时出现异常。这怎么可能，因为我从ItemsSource？

抓取了lastItem

编辑：已添加上下文... ListView XAML和一个模型，表示值为IsoStore中的不同文本文件（是的，我知道，设置将是更好的选择）

XAML：

    <ListView Grid.Row="6" Grid.Column="0" Grid.ColumnSpan="3" x:Name="listView" IsVisible="true">
        <ListView.ItemTemplate>
            <DataTemplate>
                <ViewCell>
                    <ViewCell.View>
                        <StackLayout Orientation="Horizontal" HorizontalOptions="StartAndExpand" Padding="5">
                            <Label Text="{Binding Datum, StringFormat='{0:d}'}" />
                            <StackLayout Orientation="Vertical" Spacing="0" VerticalOptions="StartAndExpand">
                                <StackLayout Orientation="Horizontal" HorizontalOptions="StartAndExpand">
                                    <Label Text="{Binding KmBeginn}" />
                                    <Label Text="-" />
                                    <Label Text="{Binding KmEnde}" />
                                    <Label Text=" (" />
                                    <Label Text="{Binding Entfernung}" />
                                    <Label Text=" )" />
                                    <Label Text="{Binding Kategorie}" />
                                </StackLayout>
                                <Label Text="{Binding Reisezweck}" VerticalOptions="Fill" />
                            </StackLayout>
                        </StackLayout>
                    </ViewCell.View>
                </ViewCell>
            </DataTemplate>
        </ListView.ItemTemplate>
    </ListView>

C＃：

using System;
using System.Collections;
using System.Collections.Generic;
using System.ComponentModel;
using System.Text;
using System.IO;
using System.IO.IsolatedStorage;
using System.Runtime.CompilerServices;
using System.Diagnostics;

namespace EasyRideTouch.Models
{
public class Eintrag : INotifyPropertyChanged
{
    public string FilePrefix { get; set; }

    public Eintrag(string prefix = "")
    {
        FilePrefix = prefix;
    }

    public DateTime Datum
    {
        get { 
            string datestring = ReadStringFromIsoStore();
            if (datestring == "")
                return DateTime.Today;
            else
                return new DateTime(Int32.Parse(datestring.Substring(0, 4)), Int32.Parse(datestring.Substring(4, 2)), Int32.Parse(datestring.Substring(6, 2)));
        }
        set {
            string datestring = String.Format("{0:yyyyMMdd}", value);
            if (datestring != ReadStringFromIsoStore()) 
            {
                WriteStringToIsoStore(datestring); 
                OnPropertyChanged(); 
            } 
        }
    }

     public string Kategorie
    {
        get { return ReadStringFromIsoStore(); }
        set { if (value != ReadStringFromIsoStore()) { WriteStringToIsoStore(value); OnPropertyChanged(); } }
    }

    public string KmBeginn
    {
        get { return ReadStringFromIsoStore(); }
        set { if (value != ReadStringFromIsoStore()) { WriteStringToIsoStore(value); OnPropertyChanged(); OnPropertyChanged("Entfernung"); } }
    }

    public string KmEnde
    {
        get { return ReadStringFromIsoStore(); }
        set { if (value != ReadStringFromIsoStore()) { WriteStringToIsoStore(value); OnPropertyChanged(); OnPropertyChanged("Entfernung"); } }
    }

    public string Entfernung
    {
        get
        {
            if (KmEnde == "" || KmBeginn == "")
                return "";
            else
            {
                int ende;
                int beginn;
                try
                {
                    ende = Int32.Parse(KmEnde);
                    beginn = Int32.Parse(KmBeginn);
                }
                catch 
                {
                    return "";
                }
                int entfernung = ende - beginn;
                if (entfernung < 0)
                    return "";
                else
                    return entfernung.ToString() + "km";
            }
        }
    }

    public string Reisezweck
    {
        get { return ReadStringFromIsoStore(); }
        set { if (value != ReadStringFromIsoStore()) { WriteStringToIsoStore(value); OnPropertyChanged(); } }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        var handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    private string ReadStringFromIsoStore()
    {
        IsolatedStorageFile isoStore = IsolatedStorageFile.GetUserStoreForApplication();
        if (isoStore.FileExists(GetPropertyName()))
        {
            using (IsolatedStorageFileStream isoStream = new IsolatedStorageFileStream(GetPropertyName(), FileMode.Open, isoStore))
            {
                using (StreamReader reader = new StreamReader(isoStream))
                {
                    return reader.ReadToEnd(); 
                }
            }
        }
        else
            return "";
    }

    private void WriteStringToIsoStore(string value)
    {
        IsolatedStorageFile isoStore = IsolatedStorageFile.GetUserStoreForApplication();
        using (IsolatedStorageFileStream isoStream = new IsolatedStorageFileStream(GetPropertyName(), FileMode.OpenOrCreate|(isoStore.FileExists(GetPropertyName()) ? FileMode.Truncate : 0), isoStore))
        {
            using (StreamWriter writer = new StreamWriter(isoStream))
            {
                writer.WriteLine(value);
            }
        }
    }

    string GetPropertyName()
    {
        StackTrace callStackTrace = new StackTrace();
        StackFrame propertyFrame = callStackTrace.GetFrame(2); // 2: below GetPropertyName frame
        string properyAccessorName = propertyFrame.GetMethod().Name;

        return FilePrefix + (properyAccessorName.Replace("get_","").Replace("set_",""));
    }
}

}