UWP ICommand参数始终为null

Question

当尝试执行ICommand时，由于参数为null，它总是抛出异常。

当我手动打印使用Debug.WriteLine传递给ICommand的变量时，它可以正常工作。

这是命令本身

    private CatalogBase<T> _catalogBase;

    public DeleteCommand()
    {
        _catalogBase = new CatalogBase<T>();
    }
    public bool CanExecute(object parameter)
    {
        return true;
    }

    public void Execute(object parameter)
    {
        _catalogBase.Delete((int)parameter);
    }

    public event EventHandler CanExecuteChanged;

带参数和命令绑定的按钮：

<AppBarButton Icon="Delete" Label="Delete" CommandParameter="{Binding CustomerId}" Command="{Binding DeleteCommand}"/>

这是上面引用的 CustomerId ：

    public int CustomerId
    {
        get { return _customer.CustomerId; }
    }

同样，手动打印 _customer.CustomerId 会返回正确的，而不是null。

任何人都知道是什么原因引起的？提前谢谢。

下面的完整xaml和viewmodel：

<Page
x:Class="Hovmand.View.Domain.CustomersPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:page="using:Hovmand.ViewModel.Page"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">

<Page.DataContext>
    <page:CustomerViewModel/>
</Page.DataContext>

<Page.BottomAppBar>
    <CommandBar>
        <AppBarButton Icon="Add" Label="Create" />
        <AppBarButton Icon="Edit" Label="Edit" />
        <AppBarButton Icon="Delete" Label="Delete" DataContext="" CommandParameter="{Binding CustomerDomainObject.CustomerId}" Command="{Binding DeleteCommand}"/>
    </CommandBar>
</Page.BottomAppBar>

<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
    <ListView ItemsSource="{Binding Customers}" SelectedItem="{Binding CustomerDomainObject, Mode=TwoWay}">
        <ListView.HeaderTemplate>
            <DataTemplate>
                <Grid Padding="0,12,0,12" >
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="1*"/>
                        <ColumnDefinition Width="1*"/>
                        <ColumnDefinition Width="1*"/>
                        <ColumnDefinition Width="1*"/>
                        <ColumnDefinition Width="1*"/>
                    </Grid.ColumnDefinitions>
                    <TextBlock Grid.Column="0" Padding="20,0,0,0" Text="Name"  Style="{ThemeResource CaptionTextBlockStyle}"/>
                    <TextBlock Grid.Column="1" Text="Phone" Style="{ThemeResource CaptionTextBlockStyle}"/>
                    <TextBlock Grid.Column="2" Text="Mail" Style="{ThemeResource CaptionTextBlockStyle}"/>
                    <TextBlock Grid.Column="3" Text="Company" Style="{ThemeResource CaptionTextBlockStyle}"/>
                    <TextBlock Grid.Column="4" Text="Adress" Style="{ThemeResource CaptionTextBlockStyle}"/>
                </Grid>
            </DataTemplate>
        </ListView.HeaderTemplate>
        <ListView.ItemTemplate>
            <DataTemplate>
                <Grid Height="48">
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="1*"/>
                        <ColumnDefinition Width="1*"/>
                        <ColumnDefinition Width="1*"/>
                        <ColumnDefinition Width="1*"/>
                        <ColumnDefinition Width="1*"/>
                    </Grid.ColumnDefinitions>
                    <TextBlock Grid.Column="0" VerticalAlignment="Center" Text="1" />
                    <TextBlock Grid.Column="1" VerticalAlignment="Center" Text="2"/>
                    <TextBlock Grid.Column="2" VerticalAlignment="Center" Text="3"/>
                    <TextBlock Grid.Column="3" VerticalAlignment="Center" Text="4"/>
                    <TextBlock Grid.Column="4" VerticalAlignment="Center" Text="5"/>
                </Grid>

            </DataTemplate>
        </ListView.ItemTemplate>

    </ListView>

</Grid>

视图模型

using System.Collections.Generic;
using System.ComponentModel;
using System.Diagnostics;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Windows.Input;
using Hovmand.Annotations;
using Hovmand.Model.Database;
using Hovmand.ViewModel.Base;
using Hovmand.ViewModel.Commands;

namespace Hovmand.ViewModel.Page
{
    public class CustomerViewModel
    {
        public event PropertyChangedEventHandler PropertyChanged;
        private ICommand _createCommand = new CreateCommand<Customer>();
        private ICommand _deleteCommand = new DeleteCommand<Customer>();
        private ICommand _updateCommand = new UpdateCommand<Customer>();
        private HovmanddbContext dbContext = HovmanddbContext.Instance;
        private Customer _customer = new Customer(); 

        public CustomerViewModel()
        {
        }

        public Customer CustomerDomainObject
        {
            get { return _customer; }
            set
            {
                _customer = value;
                OnPropertyChanged();
                Debug.WriteLine(_customer.CustomerId);
            }
        }

        public int CustomerId
        {
            get { return _customer.CustomerId; }
        }

        public int Cvr
        {
            get { return CustomerDomainObject.Cvr; }
            set
            {
                CustomerDomainObject.Cvr = value;
                OnPropertyChanged();
            }
        }

        public string CompanyName
        {
            get { return CustomerDomainObject.CompanyName; }
            set
            {
                CustomerDomainObject.CompanyName = value; 
                OnPropertyChanged();
            }
        }

        public string Address
        {
            get { return CustomerDomainObject.Address; }
            set
            {
                CustomerDomainObject.Address = value;
                OnPropertyChanged();
            }
        }

        public int Phone
        {
            get { return CustomerDomainObject.Phone; }
            set
            {
                CustomerDomainObject.Phone = value;
                OnPropertyChanged();
            }
        }

        public string Mail
        {
            get { return CustomerDomainObject.Mail; }
            set
            {
                CustomerDomainObject.Mail = value; 
                OnPropertyChanged();
            }
        }

        public int FkContactId
        {
            get { return CustomerDomainObject.FkContactId; }
            set
            {
                CustomerDomainObject.FkContactId = value;
                OnPropertyChanged();
            }
        }

        public int FkLocationId
        {
            get { return CustomerDomainObject.FkLocationId; }
            set
            {
                CustomerDomainObject.FkLocationId = value;
                OnPropertyChanged();
            }
        }

        public List<Customer> Customers
        {
            get { return dbContext.Customers.ToList(); }
        }

        public ICommand CreateCommand
        {
            get { return _createCommand; }
        }

        public ICommand DeleteCommand
        {
            get { return _deleteCommand; }
        }

        public ICommand UpdateCommand
        {
            get { return _updateCommand; }
        }

        [NotifyPropertyChangedInvocator]
        private void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}