Add setter to TextDocument.UndoStack.

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Document/TextDocument.cs

@@ -722,13 +722,23 @@ namespace ICSharpCode.AvalonEdit.Document
 			}
 		}
 		
-		readonly UndoStack undoStack;
+		UndoStack undoStack;
 		
 		/// <summary>
 		/// Gets the <see cref="UndoStack"/> of the document.
 		/// </summary>
+		/// <remarks>This property can also be used to set the undo stack, e.g. for sharing a common undo stack between multiple documents.</remarks>
 		public UndoStack UndoStack {
 			get { return undoStack; }
+			set {
+				if (value == null)
+					throw new ArgumentNullException();
+				if (value != undoStack) {
+					undoStack.ClearAll(); // first clear old undo stack, so that it can't be used to perform unexpected changes on this document
+					// ClearAll() will also throw an exception when it's not safe to replace the undo stack (e.g. update is currently in progress)
+					undoStack = value;
+				}
+			}
 		}
 		
 		/// <summary>

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Utils/Deque.cs

@@ -12,7 +12,7 @@ namespace ICSharpCode.AvalonEdit.Utils
 	/// </summary>
 	[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Naming", "CA1710:IdentifiersShouldHaveCorrectSuffix")]
 	[Serializable]
-	public class Deque<T> : ICollection<T>
+	public sealed class Deque<T> : ICollection<T>
 	{
 		T[] arr = Empty<T>.Array;
 		int size, head, tail;