Editor.cs

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

namespace WpfRichTextBox
{

    public class Editor : RichTextBox
    {
        public int _fontsize = 16;

        public List<TextLine> TextLines { get; private set; } = new List<TextLine>();

        public Editor()
        {
            FontManager.SetProgrammingFont(this, _fontsize);
            TextChanged += Editor_TextChanged;
            DataObject.AddPastingHandler(this, OnPasteHandler);
        }

        private void OnPasteHandler(object sender, DataObjectPastingEventArgs e)
        {
            SyncTextLines();
            InvalidateVisual();
        }

        private void Editor_TextChanged(object sender, TextChangedEventArgs e)
        {
            SyncTextLines();
            InvalidateVisual();
        }

        private void SyncTextLines()
        {
            TextLines.Clear();
            var text = new TextRange(Document.ContentStart, Document.ContentEnd).Text;
            var lines = text.Split('\n');
            for (int lineIndex = 0; lineIndex < lines.Length; lineIndex++)
            {
                var line = lines[lineIndex];
                var textLine = new TextLine();
                for (int i = 0; i < line.Length; i++)
                {
                    textLine.RichChars.Add(new RichChar
                    {
                        Character = line[i],
                        Position = new Point(i * _fontsize, lineIndex * _fontsize),
                        Color = Colors.Black
                    });
                }
                TextLines.Add(textLine);
            }
        }

        protected override void OnRender(DrawingContext drawingContext)
        {
            base.OnRender(drawingContext);
            foreach (var textLine in TextLines)
            {
                foreach (var richChar in textLine.RichChars)
                {
                    if (richChar.Position.X >= 0 && richChar.Position.X + _fontsize <= ActualWidth && richChar.Position.Y >= 0 && richChar.Position.Y + _fontsize <= ActualHeight)
                    {
                        var formattedText = new FormattedText(
                            richChar.Character.ToString(),
                            System.Globalization.CultureInfo.CurrentCulture,
                            FlowDirection.LeftToRight,
                            new Typeface(this.FontFamily, this.FontStyle, this.FontWeight, this.FontStretch),
                            _fontsize,
                            new SolidColorBrush(richChar.Color),
                            VisualTreeHelper.GetDpi(this).PixelsPerDip);

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

}