WinUI自定义仅支持Nullable Long的文本框首次加载后端数据时UI不显示值的问题

我最近在WinUI里做了个自定义文本框，专门用来输入可空长整型（nullable long）的值，结果碰到个奇怪的问题：第一次从后端加载数据的时候，明明绑定的属性已经拿到值了，UI上却完全不显示；不过之后在UI上手动修改值的时候，一切又都正常工作。下面是我的实现代码：

自定义控件代码（Code-behind） #

using Microsoft.UI.Xaml.Controls;
using Microsoft.UI.Xaml;

namespace Conversion.Common
{
    public class NullableLongNumberBox : TextBox
    {
        public static readonly DependencyProperty ValueProperty =
            DependencyProperty.Register(
                nameof(Value),
                typeof(long?),
                typeof(NullableLongNumberBox),
                new PropertyMetadata(null, OnValuePropertyChanged));

        public NullableLongNumberBox()
        {
            BeforeTextChanging += NullableLongNumberBox_BeforeTextChanging;
            TextChanging += NullableLongNumberBox_TextChanging;
        }

        public long? Value
        {
            get => (long?)GetValue(ValueProperty);
            set => SetValue(ValueProperty, value);
        }

        private int NextCaretPosition { get; set; }

        private static void OnValuePropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if (d is NullableLongNumberBox nullableLongNumberBox)
            {
                nullableLongNumberBox.Text = e.NewValue?.ToString() ?? string.Empty;
            }
        }

        private void NullableLongNumberBox_TextChanging(TextBox sender, TextBoxTextChangingEventArgs args)
        {
            sender.SelectionStart = NextCaretPosition;
        }

        private void NullableLongNumberBox_BeforeTextChanging(TextBox sender, TextBoxBeforeTextChangingEventArgs args)
        {
            if (string.IsNullOrEmpty(args.NewText))
            {
                Value = null;
                return;
            }

            if (!long.TryParse(args.NewText, out long value))
            {
                args.Cancel = true;
                return;
            }

            NextCaretPosition = sender.SelectionStart;
            Value = value;
        }
    }
}

XAML 使用代码 #

<common:NullableLongNumberBox
    x:Name="longPengeoUpdate"
    Grid.Column="5"
    Width="150"
    Style="{StaticResource NumericTextBoxRightAlignStyle}"
    Value="{Binding PengeoUpdate, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}"
/>

备注：内容来源于stack exchange，提问作者SANJEEV PRATAP Singh