Merge pull request #101 from dotnet-campus/t/lindexi/replaceIsAssignable

Replace IsAssignableFrom to is pattern

src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/Context/ObjectWriterContext.cs

@@ -391,7 +391,7 @@ internal XamlType GetDestinationType()
                                     // One last thing to check:  If the object we are inside is a ME
                                     // then we are inside a call to ProvideValue and we don't want to
                                     // return a reference to ourselves to ourselves.
-                                    if (!typeof(MarkupExtension).IsAssignableFrom(lowerFrame.Instance.GetType()))
+                                    if (!(lowerFrame.Instance is MarkupExtension))
                                     {
                                         returnAmbientValue = true;
                                         value = lowerFrame.Instance;