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

Befunge/Editor/CharStyles/BefungeStyler.cs

@@ -0,0 +1,38 @@
+using System.Collections.Generic;
+using System.Linq;
+using System.Windows;
+using System.Windows.Documents;
+using System.Windows.Media;
+
+namespace Befunge.Editor.CharStyles
+{
+    public class BefungeStyler : ITextStyler
+    {
+        private string _directionals = "<>^v?r";
+
+        public IEnumerable<Run> StyledString(string input)
+        {
+            var runs = new List<Run>();
+
+            foreach (var c in input)
+            {
+                if (_directionals.Contains(c))
+                    runs.Add(new Run(c.ToString())
+                    {
+                        Foreground = new SolidColorBrush(Colors.CornflowerBlue),
+                        FontWeight = FontWeights.Bold
+                    });
+                else
+                {
+                    runs.Add(new Run(c.ToString())
+                    {
+                        Foreground = new SolidColorBrush(Colors.Black),
+                        FontWeight = FontWeights.Normal
+                    });
+                }
+            }
+
+            return runs;
+        }
+    }
+}

Befunge/Editor/CharStyles/ITextStyler.cs

@@ -1,14 +1,13 @@
 using System;
 using System.Collections.Generic;
-using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
 using System.Windows.Documents;
 
 namespace Befunge.Editor.CharStyles
 {
-    public interface ICharStyler
+    public interface ITextStyler
     {
-        Run StyledString(string input);
+        IEnumerable<Run> StyledString(string input);
     }
-}
+}

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));
+            nameof(Text), typeof(string), typeof(CodeTextBox),
-
+            new PropertyMetadata(default(string), OnTextPropertyChanged));
-        private static void OnTextPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
-        {
-            throw new System.NotImplementedException();
-        }
 
         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;
+        }
     }
 }

Befunge/Editor/Editor.csproj

@@ -63,8 +63,10 @@
       <DependentUpon>App.xaml</DependentUpon>
       <SubType>Code</SubType>
     </Compile>
-    <Compile Include="CharStyles\ICharStyler.cs" />
+    <Compile Include="CharStyles\BefungeStyler.cs" />
+    <Compile Include="CharStyles\ITextStyler.cs" />
     <Compile Include="Controls\CodeTextBox.cs" />
+    <Compile Include="Utils\Extensions.cs" />
     <Compile Include="MainWindow.xaml.cs">
       <DependentUpon>MainWindow.xaml</DependentUpon>
       <SubType>Code</SubType>

Befunge/Editor/MainWindow.xaml

@@ -7,6 +7,6 @@
         mc:Ignorable="d"
         Title="Befunge Editor" Height="350" Width="525">
     <Grid>
-        
+        <loc:CodeTextBox x:Name="EditRegion" Margin="10,10,10,10.5" Text="Hello, world!"/>
     </Grid>
 </Window>

Befunge/Editor/MainWindow.xaml.cs

@@ -1,12 +1,16 @@
 using System.Windows;
+using Befunge.Editor.CharStyles;
 
 namespace Befunge.Editor
 {
     public partial class MainWindow
     {
-        public MainWindow ()
+        public MainWindow()
         {
-            InitializeComponent ();
+            InitializeComponent();
+
+            EditRegion.TextStyler = new BefungeStyler();
+            EditRegion.UpdateColors();
         }
     }
-}
+}

Befunge/Editor/Utils/Extensions.cs

@@ -0,0 +1,14 @@
+using System.Windows.Documents;
+using Befunge.Editor.CharStyles;
+
+namespace Befunge.Editor.Utils
+{
+    internal static class Extensions
+    {
+        public static void AppendStylizedText(this Paragraph p, ITextStyler styler, char text)
+            => AppendStylizedText(p, styler, text.ToString());
+
+        public static void AppendStylizedText(this Paragraph p, ITextStyler styler, string text)
+            => p.Inlines.AddRange(styler?.StyledString(text) ?? new[] {new Run(text)});
+    }
+}