Moved the ColumnRulerRenderer from the AddIn to the Library and changed the line pointed out by Daniel

ICSharpCode.AvalonEdit/ICSharpCode.AvalonEdit.csproj

@@ -233,6 +233,7 @@
     <Compile Include="Rendering\ColorizingTransformer.cs">
       <DependentUpon>IVisualLineTransformer.cs</DependentUpon>
     </Compile>
+    <Compile Include="Rendering\ColumnRulerRenderer.cs" />
     <Compile Include="Rendering\DocumentColorizingTransformer.cs">
       <DependentUpon>IVisualLineTransformer.cs</DependentUpon>
     </Compile>

ICSharpCode.AvalonEdit/Rendering/ColumnRulerRenderer.cs

@@ -0,0 +1,66 @@
+// Copyright (c) AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under the GNU LGPL (for details please see \doc\license.txt)
+
+using System;
+using System.Windows;
+using System.Windows.Media;
+using ICSharpCode.AvalonEdit.Rendering;
+using ICSharpCode.AvalonEdit.Utils;
+
+namespace ICSharpCode.AvalonEdit
+{
+	/// <summary>
+	/// Redners a ruler at a certain colum
+	/// </summary>
+	public class ColumnRulerRenderer : IBackgroundRenderer
+	{
+		
+		Pen pen;
+		int column;
+		TextView textView;
+		
+		public ColumnRulerRenderer(TextView textView)
+		{
+			if (textView == null)
+				throw new ArgumentNullException("textView");
+			
+			this.pen = new Pen(Brushes.LightGray, 1);
+			this.pen.Freeze();
+			this.textView = textView;
+			this.textView.BackgroundRenderers.Add(this);
+		}
+		
+		public KnownLayer Layer {
+			get {
+				return KnownLayer.Background;
+			}
+		}
+		
+		public void SetRuler(int column, Brush brush)
+		{
+			if (this.column != column) {
+				this.column = column;
+				textView.InvalidateLayer(this.Layer);
+			}
+			if (pen.Brush != brush) {
+				this.pen = new Pen(brush, 1);
+				this.pen.Freeze();
+				textView.InvalidateLayer(this.Layer);
+			}
+		}
+		
+		public void Draw(TextView textView, System.Windows.Media.DrawingContext drawingContext)
+		{
+			if(column < 1)
+				return;
+			
+			double offset = textView.WideSpaceWidth * column;
+			System.Windows.Size pixelSize = PixelSnapHelpers.GetPixelSize(textView);
+			double markerXPos = PixelSnapHelpers.PixelAlign(offset, pixelSize.Width);
+			Point start = new Point(markerXPos, 0);
+			Point end = new Point(markerXPos, Math.Max(textView.DocumentHeight, textView.ActualHeight));
+			
+			drawingContext.DrawLine(pen, start, end);
+		}
+	}
+}

ICSharpCode.AvalonEdit/Rendering/TextView.cs

@@ -40,6 +40,8 @@ namespace ICSharpCode.AvalonEdit.Rendering
 			FocusableProperty.OverrideMetadata(typeof(TextView), new FrameworkPropertyMetadata(Boxes.False));
 		}
 		
+		ColumnRulerRenderer columnRulerRenderer;
+		
 		/// <summary>
 		/// Creates a new TextView instance.
 		/// </summary>
@@ -59,6 +61,8 @@ namespace ICSharpCode.AvalonEdit.Rendering
 			this.hoverLogic = new MouseHoverLogic(this);
 			this.hoverLogic.MouseHover += (sender, e) => RaiseHoverEventPair(e, PreviewMouseHoverEvent, MouseHoverEvent);
 			this.hoverLogic.MouseHoverStopped += (sender, e) => RaiseHoverEventPair(e, PreviewMouseHoverStoppedEvent, MouseHoverStoppedEvent);
+			this.columnRulerRenderer = new ColumnRulerRenderer(this);
+			this.columnRulerRenderer.SetRuler(Options.ColumnRulerPosition, ColumnRulerBrush);
 		}
 
 		#endregion
@@ -198,6 +202,14 @@ namespace ICSharpCode.AvalonEdit.Rendering
 			if (OptionChanged != null) {
 				OptionChanged(this, e);
 			}
+			
+			if (e.PropertyName == "ColumRulerPosition" || e.PropertyName == "ShowColumnRuler") {
+				if (Options.ShowColumnRuler)
+					columnRulerRenderer.SetRuler(Options.ColumnRulerPosition, ColumnRulerBrush);
+				else
+					columnRulerRenderer.SetRuler(-1, ColumnRulerBrush);
+			}
+			
 			UpdateBuiltinElementGeneratorsFromOptions();
 			Redraw();
 		}
@@ -1928,5 +1940,23 @@ namespace ICSharpCode.AvalonEdit.Rendering
 				Redraw();
 			}
 		}
+		
+		public static readonly DependencyProperty ColumnRulerBrushProperty =
+			DependencyProperty.Register("ColumnRulerBrush", typeof(Brush), typeof(TextView),
+			                            new FrameworkPropertyMetadata(Brushes.LightGray, OnUpdateBrushes));
+		
+		public Brush ColumnRulerBrush {
+			get { return (Brush)GetValue(ColumnRulerBrushProperty); }
+			set { SetValue(ColumnRulerBrushProperty, value); }
+		}
+		
+		public static void OnUpdateBrushes(DependencyObject d, DependencyPropertyChangedEventArgs e)
+		{
+			TextView view = d as TextView;
+			if (view == null) return;
+			if (e.Property == ColumnRulerBrushProperty)
+				view.columnRulerRenderer.SetRuler(view.Options.ColumnRulerPosition, (Brush)e.NewValue);
+		}
+		
 	}
 }

ICSharpCode.AvalonEdit/TextEditorOptions.cs

@@ -398,5 +398,37 @@ namespace ICSharpCode.AvalonEdit
 				}
 			}
 		}
+		
+		bool showColumnRuler = false;
+		
+		/// <summary>
+		/// Gets/Sets whether the column ruler should be shown.
+		/// </summary>
+		[DefaultValue(false)]
+		public virtual bool ShowColumnRuler {
+			get { return showColumnRuler; }
+			set {
+				if (showColumnRuler != value) {
+					showColumnRuler = value;
+					OnPropertyChanged("ShowColumnRuler");
+				}
+			}
+		}
+		
+		int columnRulerPosition = 80;
+		
+		/// <summary>
+		/// Gets/Sets where the column ruler should be shown.
+		/// </summary>
+		[DefaultValue(80)]
+		public virtual int ColumnRulerPosition {
+			get { return columnRulerPosition; }
+			set {
+				if (columnRulerPosition != value) {
+					columnRulerPosition = value;
+					OnPropertyChanged("ColumnRulerPosition");
+				}
+			}
+		}
 	}
 }