Fix exception when rotating skewed item.

WpfDesign.Designer/Tests/Designer/TransformTests.cs

@@ -0,0 +1,28 @@
+using System.Windows;
+using System.Windows.Media;
+using ICSharpCode.WpfDesign.Designer;
+using NUnit.Framework;
+
+namespace ICSharpCode.WpfDesign.Tests.Designer
+{
+	public class TransformTests : ModelTestHelper
+	{
+		internal static void Rotate(double angle, DesignItem item)
+		{
+			ModelTools.ApplyTransform(item, new RotateTransform(angle, 0.5, 0.5));
+		}
+
+		[Test]
+		public void RotateSkewedButton()
+		{
+			DesignItem button = CreateCanvasContext(
+				"<Button Content='Button' Width='75' Height='23' RenderTransformOrigin='0.5,0.5'>"
+				+ "<Button.RenderTransform>"
+				+ "<SkewTransform AngleY='-45' />"
+				+ "</Button.RenderTransform>"
+				+ "</Button>");
+			Rotate(30, button);
+			//AssertCanvasDesignerOutput();
+		}
+	}
+}

WpfDesign.XamlDom/Project/XamlProperty.cs

@@ -505,12 +505,10 @@ internal void InsertNodeInCollection(XmlNode newChildNode, int index)
 					collection = parentObject.XmlElement;
 				}
 			}
-			if (collectionElements.Count == 0) {
-				// collection is empty -> we may insert anywhere
+			if (collectionElements.Count == 0 || index == collectionElements.Count) {
+				// collection is empty -> we may insert anywhere, thus we choose the end
+				// or the index is the end of the list:
 				collection.AppendChild(newChildNode);
-			} else if (index == collectionElements.Count) {
-				// insert after last element in collection
-				collection.InsertAfter(newChildNode, collectionElements[collectionElements.Count - 1].GetNodeForCollection());
 			} else {
 				// insert before specified index
 				collection.InsertBefore(newChildNode, collectionElements[index].GetNodeForCollection());