Csharp .NET MAUI 项目始终为空

我正在使用毛伊岛开发一款应用程序.我有四页几乎一模一样的书.在第四个示例中，对于从视图传递的模型，我总是得到NULL.我try 了各种方法，从创建另一个页面到复制，再到将工作页面粘贴到XAML和C #中.我知道这可能是一件简单的事情，但它让我难以理解.

XAML

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:models="clr-namespace:WhatGoesIn.型号s"
             x:Class="WhatGoesIn.Views.ExportPage"
             xmlns:ios="clr-namespace:Microsoft.Maui.Controls.PlatformConfiguration.iOSSpecific;assembly=Microsoft.Maui.Controls"
             ios:Page.UseSafeArea="True"
             Title="Export">
    <ContentPage.BindingContext>
        <models:ExportItem />
    </ContentPage.BindingContext>
    <ScrollView>
        <VerticalStackLayout
                             Padding="20"
                             Spacing="10"
                             VerticalOptions="StartAndExpand">
            <Label Text="Date" FontAttributes="Bold"/>
            <DatePicker Date="{Binding StartDate}" MinimumDate="01/01/2023"/>
            <Label Text="Time" FontAttributes="Bold"/>
            <DatePicker Date="{Binding ToDate}"/>
            <Label Text="Recipient" FontAttributes="Bold" FontSize="16"/>
            <Entry Text="{Binding Recipient}" />
            <Button Text="Export" Clicked="OnExportClicked" />
            <Button Text="Cancel"
                    Clicked="OnCancelClicked" />
        </VerticalStackLayout>
    </ScrollView>
</ContentPage>

C #

using WhatGoesIn.型号s;

namespace WhatGoesIn.Views;

[QueryProperty("Item", "Item")]
public partial class ExportPage : ContentPage
{
    WhatGoesInDatabase database;

    public ExportItem Item
    {
        get => BindingContext as ExportItem;
        set => BindingContext = value;
    }

    public ExportPage(WhatGoesInDatabase whatGoesInDatabase)
    {
        InitializeComponent();
        BindingContext = this;
        database = whatGoesInDatabase;
    }

    private async void OnExportClicked(object sender, EventArgs e)
    {
        ExportItem item = Item as ExportItem; // <= always NULL!

        string body = await database.ExportItemAsync(item);

        if (string.IsNullOrEmpty(Item.Recipient))
        {
            await DisplayAlert("No Recipient", "Please choose the recipient of the report.", "OK");
            return;
        }

        if (Email.Default.IsComposeSupported)
        {

            string subject = "What Goes In Food Log!";
            string[] recipients = new[] { Item.Recipient };

            var message = new EmailMessage
            {
                Subject = subject,
                Body = body,
                BodyFormat = EmailBodyFormat.PlainText,
                To = new List<string>(recipients)
            };

            await Email.Default.ComposeAsync(message);
        }

        await Shell.Current.GoToAsync("..");
    }

    private async void OnCancelClicked(object sender, EventArgs e)
    {
        await Shell.Current.GoToAsync("..");
    }
}

型号

using SQLite;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace WhatGoesIn.型号s
{
    public class ExportItem
    {
        [PrimaryKey, AutoIncrement]
        public long ID { get; set; }
        public DateTime StartDate { get; set; }
        public DateTime ToDate { get; set; }
        public string Recipient { get; set; }
    }
}