Added 2 more benefits of sealed classes with performance stats.

rules/seal-your-classes-by-default/rule.md

@@ -26,3 +26,22 @@ Watch this video by Nick Chapsas, to see the performance benefits of sealing you
 
 `youtube: https://www.youtube.com/embed/d76WWAD99Yo`
 **Video: Why all your classes should be sealed by default in C# by Nick Chapsas (11 min)**
+
+### Avoid unnecessary covariance checks in Array
+
+Arrays in .NET are covariant. This means arrays enable implicit conversion of an array of a more derived type to an array of a less derived type. 
+This operation is not type safe. To make sure it’s type safe JIT checks the type of the object before an item is assigned which is a performance cost. 
+When the array is an array of sealed types, JIT knows there won’t be any covariance involved when sealed types are used, so it skips covariance checks. 
+This improves performance with arrays. 
+
+![Figure: Array covariance - sealed Vs non-sealed. See the next figure for performance results.](ArrayCovariance_code.png)
+
+![Figure: Performance results show arrays with sealed types show improved performance compared to arrays with non-sealed types.](ArrayCovariance_PerfResults.png)
+
+### Skip hierarchy checks of inheritance in runtime for is/as cast operations
+
+During a cast operation, JIT needs to know the type of the object at runtime. When casting to a non-sealed type, the runtime must check all the types in the inheritance hierarchy, which can be time consuming. When casting to a sealed type, the runtime only checks the type of the object. Hence the performance gain.
+
+![Figure: Casting sealed Vs non-sealed. See the next figure for performance results.](Casting_code.png)
+
+![Figure: Performance results show casting of sealed has improved performance compared to non-sealed.](Casting_PerfResult.png)