A Resource Dictionary can not only contain Objects wich a Key or Styles.

For Example a DataTemplate may be in the ResourceDictionary and has no Key set and is no Style.

src/AddIns/DisplayBindings/WpfDesign/WpfDesign.XamlDom/Project/CollectionSupport.cs

@@ -68,10 +68,14 @@ namespace ICSharpCode.WpfDesign.XamlDom
 			} else if (collectionInstance is ResourceDictionary) {
 				object val = newElement.GetValueFor(null);
 				object key = newElement is XamlObject ? ((XamlObject)newElement).GetXamlAttribute("Key") : null;
-				if (key == null) {
+				if (key == null || key == "") {
 					if (val is Style)
 						key = ((Style)val).TargetType;
+					else if (val is DataTemplate)
+						key = ((DataTemplate)val).DataType;					
 				}
+				if (key == null || key == "")
+					key = val;
 				((ResourceDictionary)collectionInstance).Add(key, val);
 			} else {
 				collectionType.InvokeMember(