我需要根据条目的文本长度更改其掩码和文本。当我更改文本时，会导致无限循环。
查看我的代码：

async void DocNum_TextChanged(System.Object sender, Xamarin.Forms.TextChangedEventArgs e)
{
    string DocNumCopy = Regex.Replace(DocNum.Text,"[^0-9]","");

    if (DocNumCopy.Length > 11)
    {
        if (DocNumMask.Mask != "XX.XXX.XXX/XXXX-XX")
        {
            DocNum.TextChanged -= DocNum_TextChanged;
            DocNum.Text = DocNumCopy;
            DocNumMask.Mask = "XX.XXX.XXX/XXXX-XX";
            DocNum.TextChanged += DocNum_TextChanged;
        }
    }
    else
    {
        if (DocNumMask.Mask != "XXX.XXX.XXX-XX")
        {
            DocNum.TextChanged -= DocNum_TextChanged;
            DocNum.Text = DocNumCopy;
            DocNumMask.Mask = "XXX.XXX.XXX-XX";
            DocNum.TextChanged += DocNum_TextChanged;
        }
    }
}

我的XAML：

<Frame Padding="2" Grid.Column="0" Grid.Row="2" Grid.ColumnSpan="2" CornerRadius="5">
    <Entry x:Name="DocNum" Text="" Placeholder="CPF/CNPJ" FontSize="18" TextColor="#8C8C8C" TextChanged="DocNum_TextChanged">
        <Entry.Behaviors>
            <ContentView:MaskedBehavior x:Name="DocNumMask" Mask="XXX.XXX.XXX-XX" />
        </Entry.Behaviors>
    </Entry>
</Frame>

掩码代码：

using System.Collections.Generic;
using Xamarin.Forms;

namespace MasterDetailPageNavigation.XAML
{
    public class MaskedBehavior : Behavior<Entry>
    {
        private string _mask = "";
        public string Mask
        {
            get => _mask;
            set
            {
                _mask = value;
                SetPositions();
            }
        }

        protected override void OnAttachedTo(Entry entry)
        {
            entry.TextChanged += OnEntryTextChanged;
            base.OnAttachedTo(entry);
        }

        protected override void OnDetachingFrom(Entry entry)
        {
            entry.TextChanged -= OnEntryTextChanged;
            base.OnDetachingFrom(entry);
        }

        IDictionary<int, char> _positions;

        void SetPositions()
        {
            if (string.IsNullOrEmpty(Mask))
            {
                _positions = null;
                return;
            }

            var list = new Dictionary<int, char>();
            for (var i = 0; i < Mask.Length; i++)
                if (Mask[i] != 'X')
                    list.Add(i, Mask[i]);

            _positions = list;
        }

        private void OnEntryTextChanged(object sender, TextChangedEventArgs args)
        {
            var entry = sender as Entry;

            var text = entry.Text;

            if (string.IsNullOrWhiteSpace(text) || _positions == null)
                return;

            if (text.Length > _mask.Length)
            {
                entry.Text = text.Remove(text.Length - 1);
                return;
            }

            foreach (var position in _positions)
                if (text.Length >= position.Key + 1)
                {
                    var value = position.Value.ToString();
                    if (text.Substring(position.Key, 1) != value)
                        text = text.Insert(position.Key, value);
                }

            if (entry.Text != text)
                entry.Text = text;
        }
    }
}

编辑1：

DocNum是客户的单据编号。根据位数的不同，它应具有不同的掩码，如果编号的字符数为11个或更少，则掩码应为000.000.000-00;但是，如果编号的字符数超过11个，则掩码应为00.000.000/0000-00。
DocNumCopy只是一个没有掩码的DocNum的副本，以便于了解字符的真实的长度。