System.ComponentModel.DataAnnotations

 namespace System.ComponentModel.DataAnnotations {
+    public class AssociatedMetadataTypeTypeDescriptionProvider : TypeDescriptionProvider {
+        public AssociatedMetadataTypeTypeDescriptionProvider(Type type);
+        public AssociatedMetadataTypeTypeDescriptionProvider(Type type, Type associatedMetadataType);
+        public override ICustomTypeDescriptor GetTypeDescriptor(Type objectType, object instance);
+    }
+    public sealed class MetadataTypeAttribute : Attribute {
+        public MetadataTypeAttribute(Type metadataClassType);
+        public Type MetadataClassType { get; }
+    }
 }