Basic Syntax highlighting, added CodeTextBox to MainWindow to test it.

Befunge/Editor/Controls/CodeTextBox.cs

@@ -1,26 +1,89 @@
 using System.ComponentModel;
+using System.Data.SqlTypes;
 using System.Windows;
 using System.Windows.Controls;
+using System.Windows.Documents;
+using System.Windows.Media;
 using Befunge.Editor.CharStyles;
+using Befunge.Editor.Utils;
 
 namespace Befunge.Editor.Controls
 {
     public class CodeTextBox : RichTextBox
     {
         public static readonly DependencyProperty TextProperty = DependencyProperty.Register(
-            nameof(Text), typeof(string), typeof(CodeTextBox), new PropertyMetadata(default(string), OnTextPropertyChanged));
-
-        private static void OnTextPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
-        {
-            throw new System.NotImplementedException();
-        }
+            nameof(Text), typeof(string), typeof(CodeTextBox),
+            new PropertyMetadata(default(string), OnTextPropertyChanged));
 
         public string Text
         {
             get { return (string) GetValue(TextProperty); }
-            set { SetValue(TextProperty, value); }
+            set
+            {
+                SetValue(TextProperty, value);
+                UpdateColors();
+            }
         }
 
-        public ICharStyler TextStyler { get; set; }
+        private static void OnTextPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
+        {
+            var ctb = d as CodeTextBox;
+            var val = e.NewValue as string;
+
+            if (ctb == null) return;
+            var start = ctb.Document.ContentStart;
+            var end = ctb.Document.ContentEnd;
+            var range = new TextRange(start, end);
+            range.Text = val + "\r\n";
+        }
+
+        public ITextStyler TextStyler { get; set; }
+        private bool _changingColors;
+
+        public CodeTextBox()
+        {
+            var s = new Style {TargetType = typeof(Paragraph)};
+            s.Setters.Add(new Setter(Block.MarginProperty, new Thickness(0)));
+            Resources.Add(typeof(Paragraph), s);
+
+            FontFamily = new FontFamily("Consolas");
+        }
+
+        protected override void OnTextChanged(TextChangedEventArgs e)
+        {
+            UpdateColors();
+        }
+
+        public void UpdateColors()
+        {
+            if (_changingColors) return;
+            _changingColors = true;
+
+            var start = Document.ContentStart;
+            var caret = CaretPosition;
+            var end = Document.ContentEnd;
+
+            var before = new TextRange(start, caret);
+            var after = new TextRange(caret, end);
+
+            var beforeText = before.Text.Replace("\r\n", "\n");
+            var afterText = after.Text.Replace("\r\n", "\n");
+            if (afterText.Length > 0)
+                afterText = afterText.Substring(0, afterText.Length - 1);
+            var allText = beforeText + afterText;
+
+            Text = allText;
+
+            Document.Blocks.Clear();
+
+            var p = new Paragraph();
+            Document.Blocks.Add(p);
+
+            p.AppendStylizedText(TextStyler, beforeText);
+            CaretPosition = p.ContentEnd;
+            p.AppendStylizedText(TextStyler, afterText);
+
+            _changingColors = false;
+        }
     }
 }