# System.Runtime.CompilerServices

``` diff
 namespace System.Runtime.CompilerServices {
+    public sealed class PreserveBaseOverridesAttribute : Attribute {
+        public PreserveBaseOverridesAttribute();
+    }
     public static class RuntimeFeature {
+        public const string CovariantReturnsOfClasses = "CovariantReturnsOfClasses";
     }
 }
```

