FilteredTextBox

gistfile1.txt

using Avalonia;
using Avalonia.Controls;
using Avalonia.Input;
using System.Reflection;

namespace CosmoPlanner.Infrastructure.Controls
{
    public class FilteredTextBox : TextBox
    {
        public static readonly AvaloniaProperty<string> MaskPatternProperty =
            AvaloniaProperty.Register<FilteredTextBox, string>("MaskPattern", string.Empty);

        /// <summary>
        /// Indicate the type of the filter to apply.
        /// </summary>
        public string MaskPattern
        {
            get { return GetValue(MaskPatternProperty); }
            set { SetValue(MaskPatternProperty, value); }
        }

        protected override void OnTextInput(TextInputEventArgs e)
        {
            if (!e.Handled)
            {
                base.OnTextInput(e);
                e.Handled = true;

                if (CaretIndex > MaskPattern.Length)
                {
                    SetText(Text.Remove(Text.Length - e.Text.Length, e.Text.Length));
                    return;
                }

                var pattern = MaskPattern[CaretIndex - 1];
                if (pattern == '0' && e.Text.Length == 1 && char.IsDigit(e.Text[0]))
                {
                    return;
                }
                if (e.Text.Length == 1 && e.Text[0] == pattern)
                {
                    return;
                }

                SetText(Text.Remove(Text.Length - e.Text.Length, e.Text.Length));
                return;
            }
        }

        private void SetText(string text)
        {
            var methodInfo = typeof(TextBox).GetMethod("SetTextInternal",
                BindingFlags.NonPublic | BindingFlags.Instance);
            methodInfo.Invoke(this, new object[] { text });
        }
    }
}