Adding test to check if OnDeserialized doesn't throw if serialization… (#21357)

* Adding test to check if OnDeserialized doesn't throw if serialization fields aren't initialized yet. Relates to #21322

* Skipping for netfx as this only tests core impl

* Update ObservableCollection_Serialization.cs

Author: ViktorHofer

src/System.ObjectModel/tests/ObservableCollection/ObservableCollection_Serialization.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Collections.Generic;
+using System.Reflection;
 using System.Runtime.Serialization.Formatters.Tests;
 using Xunit;
 
@@ -26,5 +27,17 @@ public void SerializeDeserialize_Roundtrips(ObservableCollection<int> c)
             Assert.NotSame(c, clone);
             Assert.Equal(c, clone);
         }
+
+        [Fact]
+        [SkipOnTargetFramework(TargetFrameworkMonikers.NetFramework)]
+        public void OnDeserialized_MonitorNotInitialized_ExpectSuccess()
+        {
+            var observableCollection = new ObservableCollection<int>();
+            MethodInfo onDeserializedMethodInfo = observableCollection.GetType().GetMethod("OnDeserialized",
+                BindingFlags.Instance | Reflection.BindingFlags.NonPublic);
+
+            Assert.NotNull(onDeserializedMethodInfo);
+            onDeserializedMethodInfo.Invoke(observableCollection, new object[] { null });
+        }
     }
 }