Added some proposals.

ICSharpCode.Editor/IDocumentLine.cs

@@ -15,6 +15,11 @@ namespace ICSharpCode.Editor
 		/// </summary>
 		int TotalLength { get; }
 		
+		/// <summary>
+		/// Gets the length of this line, excluding the line delimiter.
+		/// </summary>
+		int EditableLength { get; }
+		
 		/// <summary>
 		/// Gets the length of the line terminator.
 		/// Returns 1 or 2; or 0 at the end of the document.

ICSharpCode.Editor/ISegment.cs

@@ -27,4 +27,29 @@ namespace ICSharpCode.Editor
 		/// <remarks>EndOffset = Offset + Length;</remarks>
 		int EndOffset { get; }
 	}
+	
+	public static class ISegmentExtensions
+	{
+		/// <summary>
+		/// True, if the segment contains the specified offset, false otherwise.
+		/// </summary>
+		/// <param name='offset'>
+		/// The offset.
+		/// </param>
+		public static bool Contains (this ISegment segment, int offset)
+		{
+			return segment.Offset <= offset && offset < segment.EndOffset;
+		}
+		
+		/// <summary>
+		/// True, if the segment contains the specified segment, false otherwise.
+		/// </summary>
+		/// <param name='segment'>
+		/// The segment.
+		/// </param>
+		public static bool Contains (this ISegment thisSegment, ISegment segment)
+		{
+			return  segment != null && thisSegment.Offset <= segment.Offset && segment.EndOffset <= thisSegment.EndOffset;
+		}
+	}
 }

ICSharpCode.Editor/ITextBuffer.cs

@@ -91,6 +91,22 @@ namespace ICSharpCode.Editor
 		/// <param name="count">Length of the area to search.</param>
 		/// <returns>The first index where any character was found; or -1 if no occurrence was found.</returns>
 		int IndexOfAny(char[] anyOf, int startIndex, int count);
+		
+		/* What about:
+		void Insert (int offset, string value);
+		void Remove (int offset, int count);
+		void Remove (ISegment segment);
+		
+		void Replace (int offset, int count, string value);
+		
+		Or more search operations:
+		
+		IEnumerable<int> SearchForward (string pattern, int startIndex);
+		IEnumerable<int> SearchForwardIgnoreCase (string pattern, int startIndex);
+		
+		IEnumerable<int> SearchBackward (string pattern, int startIndex);
+		IEnumerable<int> SearchBackwardIgnoreCase (string pattern, int startIndex);
+		*/
 	}
 	
 	/// <summary>

ICSharpCode.Editor/LinkedElement.cs

@@ -5,59 +5,64 @@ using System;
 
 namespace ICSharpCode.Editor
 {
-	/// <summary>
-	/// Represents an element in the text editor that is either editable, or bound to another editable element.
-	/// Used with <see cref="ITextEditor.ShowLinkedElements"/>
-	/// </summary>
-	public class LinkedElement
-	{
-		LinkedElement boundTo;
-		
-		/// <summary>
-		/// Gets/Sets the start offset of this linked element.
-		/// </summary>
-		public int StartOffset { get; set; }
-		
-		/// <summary>
-		/// Gets/Sets the end offset of this linked element.
-		/// </summary>
-		public int EndOffset { get; set; }
-		
-		/// <summary>
-		/// Gets the linked element to which this element is bound.
-		/// </summary>
-		public LinkedElement BoundTo {
-			get { return boundTo; }
-		}
-		
-		/// <summary>
-		/// Gets whether this element is editable. Returns true if this element is not bound.
-		/// </summary>
-		public bool IsEditable {
-			get { return boundTo == null; }
-		}
-		
-		/// <summary>
-		/// Creates a new editable element.
-		/// </summary>
-		public LinkedElement(int startOffset, int endOffset)
-		{
-			this.StartOffset = startOffset;
-			this.EndOffset = endOffset;
-		}
-		
-		/// <summary>
-		/// Creates a new element that is bound to <paramref name="boundTo"/>.
-		/// </summary>
-		public LinkedElement(int startOffset, int endOffset, LinkedElement boundTo)
-		{
-			if (boundTo == null)
-				throw new ArgumentNullException("boundTo");
-			this.StartOffset = startOffset;
-			this.EndOffset = endOffset;
-			while (boundTo.boundTo != null)
-				boundTo = boundTo.boundTo;
-			this.boundTo = boundTo;
-		}
-	}
+// I'm not sure if we need this.
+// How about a method in the context - this method could wrap the internal representation.
+// public void StartTextLinkMode (int linkLength, IEnumerable<int> offsets)
+// and maybe then variations taking more than one link element ?
+	
+//	/// <summary>
+//	/// Represents an element in the text editor that is either editable, or bound to another editable element.
+//	/// Used with <see cref="ITextEditor.ShowLinkedElements"/>
+//	/// </summary>
+//	public class LinkedElement
+//	{
+//		LinkedElement boundTo;
+//		
+//		/// <summary>
+//		/// Gets/Sets the start offset of this linked element.
+//		/// </summary>
+//		public int StartOffset { get; set; }
+//		
+//		/// <summary>
+//		/// Gets/Sets the end offset of this linked element.
+//		/// </summary>
+//		public int EndOffset { get; set; }
+//		
+//		/// <summary>
+//		/// Gets the linked element to which this element is bound.
+//		/// </summary>
+//		public LinkedElement BoundTo {
+//			get { return boundTo; }
+//		}
+//		
+//		/// <summary>
+//		/// Gets whether this element is editable. Returns true if this element is not bound.
+//		/// </summary>
+//		public bool IsEditable {
+//			get { return boundTo == null; }
+//		}
+//		
+//		/// <summary>
+//		/// Creates a new editable element.
+//		/// </summary>
+//		public LinkedElement(int startOffset, int endOffset)
+//		{
+//			this.StartOffset = startOffset;
+//			this.EndOffset = endOffset;
+//		}
+//		
+//		/// <summary>
+//		/// Creates a new element that is bound to <paramref name="boundTo"/>.
+//		/// </summary>
+//		public LinkedElement(int startOffset, int endOffset, LinkedElement boundTo)
+//		{
+//			if (boundTo == null)
+//				throw new ArgumentNullException("boundTo");
+//			this.StartOffset = startOffset;
+//			this.EndOffset = endOffset;
+//			while (boundTo.boundTo != null)
+//				boundTo = boundTo.boundTo;
+//			this.boundTo = boundTo;
+//		}
+//	}
 }

ICSharpCode.Editor/TextLocation.cs

@@ -22,6 +22,16 @@ namespace ICSharpCode.Editor
 		/// </summary>
 		public static readonly TextLocation Empty = new TextLocation(0, 0);
 		
+		/// <summary>
+		/// Constant of the minimum line.
+		/// </summary>
+		public const int MinLine   = 1;
+		
+		/// <summary>
+		/// Constant of the minimum column.
+		/// </summary>
+		public const int MinColumn = 1;
+		
 		/// <summary>
 		/// Creates a TextLocation instance.
 		/// </summary>
@@ -52,7 +62,7 @@ namespace ICSharpCode.Editor
 		/// </summary>
 		public bool IsEmpty {
 			get {
-				return column <= 0 && line <= 0;
+				return column < MinLine && line < MinColumn;
 			}
 		}