CustomRichTextBox.cs

using System.Collections.Generic;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Media;
using System.Windows;

namespace WpfRichTextBox
{

    public class CustomRichTextBox : RichTextBox
    {
        public List<RichChar> RichChars { get; private set; } = new List<RichChar>();

        public CustomRichTextBox()
        {
            TextChanged += CustomRichTextBox_TextChanged;
        }

        private void CustomRichTextBox_TextChanged(object sender, TextChangedEventArgs e)
        {
            SyncRichChars();
            InvalidateVisual();
        }

        private void SyncRichChars()
        {
            RichChars.Clear();
            var text = new TextRange(Document.ContentStart, Document.ContentEnd).Text;
            for (int i = 0; i < text.Length; i++)
            {
                RichChars.Add(new RichChar
                {
                    Character = text[i],
                    Position = new Point(i * 16, 10), // 横向排列，纵坐标可调整
                    Color = Colors.Black // 默认颜色
                });
            }
        }

        protected override void OnRender(DrawingContext drawingContext)
        {
            base.OnRender(drawingContext);
            foreach (var richChar in RichChars)
            {
                var formattedText = new FormattedText(
                    richChar.Character.ToString(),
                    System.Globalization.CultureInfo.CurrentCulture,
                    FlowDirection.LeftToRight,
                    new Typeface("Segoe UI"),
                    16,
                    new SolidColorBrush(richChar.Color),
                    VisualTreeHelper.GetDpi(this).PixelsPerDip);

                drawingContext.DrawText(formattedText, richChar.Position);
            }
        }
    }

}