From 0663b300ceb8670c78e05d2f1122512c1c987b35 Mon Sep 17 00:00:00 2001
From: Steve Molloy <smolloy@microsoft.com>
Date: Wed, 14 Jul 2021 23:59:35 -0700
Subject: [PATCH] Avoid nullref while trying to throw a more helpful exception.
 (#55316)

---
 .../src/Resources/Strings.resx                              | 3 +++
 .../Runtime/Serialization/XmlObjectSerializerReadContext.cs | 6 +++---
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/src/libraries/System.Private.DataContractSerialization/src/Resources/Strings.resx b/src/libraries/System.Private.DataContractSerialization/src/Resources/Strings.resx
index 2af9be2de6213..b1fd067338933 100644
--- a/src/libraries/System.Private.DataContractSerialization/src/Resources/Strings.resx
+++ b/src/libraries/System.Private.DataContractSerialization/src/Resources/Strings.resx
@@ -1167,4 +1167,7 @@
   <data name="ReadOnlyCollectionDeserialization" xml:space="preserve">
     <value>Collection type '{0}' cannot be deserialized since it</value>
   </data>
+  <data name="UnknownNullType" xml:space="preserve">
+    <value>Unknown Type for null value</value>
+  </data>
 </root>
\ No newline at end of file
diff --git a/src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/XmlObjectSerializerReadContext.cs b/src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/XmlObjectSerializerReadContext.cs
index 4ba8da9b6cc30..0fa085b0f755c 100644
--- a/src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/XmlObjectSerializerReadContext.cs
+++ b/src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/XmlObjectSerializerReadContext.cs
@@ -361,9 +361,9 @@ public void ReplaceDeserializedObject(string id, object? oldObj, object? newObj)
                 // throw in such cases to allow us add fix-up support in the future if we need to.
                 if (DeserializedObjects.IsObjectReferenced(id))
                 {
-                    // https://github.com/dotnet/runtime/issues/41465 - oldObj or newObj may be null below - suppress compiler error by asserting non-null
-                    Debug.Assert(oldObj != null && newObj != null);
-                    throw DiagnosticUtility.ExceptionUtility.ThrowHelperError(XmlObjectSerializer.CreateSerializationException(SR.Format(SR.FactoryObjectContainsSelfReference, DataContract.GetClrTypeFullName(oldObj.GetType()), DataContract.GetClrTypeFullName(newObj.GetType()), id)));
+                    string oldType = (oldObj != null) ? DataContract.GetClrTypeFullName(oldObj.GetType()) : SR.UnknownNullType;
+                    string newType = (newObj != null) ? DataContract.GetClrTypeFullName(newObj.GetType()) : SR.UnknownNullType;
+                    throw DiagnosticUtility.ExceptionUtility.ThrowHelperError(XmlObjectSerializer.CreateSerializationException(SR.Format(SR.FactoryObjectContainsSelfReference, oldType, newType, id)));
                 }
                 DeserializedObjects.Remove(id);
                 DeserializedObjects.Add(id, newObj);