diff --git a/src/runtime/Codecs/DecoderGroup.cs b/src/runtime/Codecs/DecoderGroup.cs
index cf0ee33e9..5b3d127ad 100644
--- a/src/runtime/Codecs/DecoderGroup.cs
+++ b/src/runtime/Codecs/DecoderGroup.cs
@@ -30,7 +30,7 @@ public void Add(IPyObjectDecoder item)
         public bool CanDecode(PyType objectType, Type targetType)
             => this.decoders.Any(decoder => decoder.CanDecode(objectType, targetType));
         /// <inheritdoc />
-        public bool TryDecode<T>(PyObject pyObj, out T value)
+        public bool TryDecode<T>(PyObject pyObj, out T? value)
         {
             if (pyObj is null) throw new ArgumentNullException(nameof(pyObj));
 
@@ -65,7 +65,7 @@ public static class DecoderGroupExtensions
         /// that can decode from <paramref name="objectType"/> to <paramref name="targetType"/>,
         /// or <c>null</c> if a matching decoder can not be found.
         /// </summary>
-        public static IPyObjectDecoder GetDecoder(
+        public static IPyObjectDecoder? GetDecoder(
             this IPyObjectDecoder decoder,
             PyType objectType, Type targetType)
         {
diff --git a/src/runtime/Codecs/EncoderGroup.cs b/src/runtime/Codecs/EncoderGroup.cs
index 6c40623ca..32b550eb9 100644
--- a/src/runtime/Codecs/EncoderGroup.cs
+++ b/src/runtime/Codecs/EncoderGroup.cs
@@ -28,7 +28,7 @@ public void Add(IPyObjectEncoder item)
         /// <inheritdoc />
         public bool CanEncode(Type type) => this.encoders.Any(encoder => encoder.CanEncode(type));
         /// <inheritdoc />
-        public PyObject TryEncode(object value)
+        public PyObject? TryEncode(object value)
         {
             if (value is null) throw new ArgumentNullException(nameof(value));
 
diff --git a/src/runtime/CollectionWrappers/IterableWrapper.cs b/src/runtime/CollectionWrappers/IterableWrapper.cs
index 2ddcd6190..d30e584d3 100644
--- a/src/runtime/CollectionWrappers/IterableWrapper.cs
+++ b/src/runtime/CollectionWrappers/IterableWrapper.cs
@@ -35,7 +35,7 @@ public IEnumerator<T> GetEnumerator()
                     iterObject.Dispose();
                     break;
                 }
-                yield return iterObject.Current.As<T>();
+                yield return iterObject.Current.As<T>()!;
             }
         }
     }
diff --git a/src/runtime/CollectionWrappers/ListWrapper.cs b/src/runtime/CollectionWrappers/ListWrapper.cs
index 29608bc40..41ccb8fae 100644
--- a/src/runtime/CollectionWrappers/ListWrapper.cs
+++ b/src/runtime/CollectionWrappers/ListWrapper.cs
@@ -16,7 +16,7 @@ public T this[int index]
             {
                 var item = Runtime.PyList_GetItem(pyObject, index);
                 var pyItem = new PyObject(item);
-                return pyItem.As<T>();
+                return pyItem.As<T>()!;
             }
             set
             {
diff --git a/src/runtime/CustomMarshaler.cs b/src/runtime/CustomMarshaler.cs
index 4ba603609..f544756d8 100644
--- a/src/runtime/CustomMarshaler.cs
+++ b/src/runtime/CustomMarshaler.cs
@@ -74,7 +74,7 @@ public static ICustomMarshaler GetInstance(string cookie)
             return Instance;
         }
 
-        public static string PtrToStringUni(IntPtr p)
+        public static string? PtrToStringUni(IntPtr p)
         {
             if (p == IntPtr.Zero)
             {
@@ -134,7 +134,7 @@ public static IntPtr Py3UnicodePy2StringtoPtr(string s)
         /// <returns>
         /// Managed String
         /// </returns>
-        public static string PtrToPy3UnicodePy2String(IntPtr p)
+        public static string? PtrToPy3UnicodePy2String(IntPtr p)
         {
             return PtrToStringUni(p);
         }
@@ -184,7 +184,7 @@ public override IntPtr MarshalManagedToNative(object managedObj)
             return mem;
         }
 
-        public static ICustomMarshaler GetInstance(string cookie)
+        public static ICustomMarshaler GetInstance(string? cookie)
         {
             return Instance;
         }
diff --git a/src/runtime/Python.Runtime.csproj b/src/runtime/Python.Runtime.csproj
index c90ca38e4..a90aa3aaa 100644
--- a/src/runtime/Python.Runtime.csproj
+++ b/src/runtime/Python.Runtime.csproj
@@ -60,7 +60,8 @@
   </ItemGroup>
 
   <ItemGroup>
+    <PackageReference Include="Lost.Compat.NullabilityAttributes" Version="0.0.4" PrivateAssets="All" />
+    <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.1.1" PrivateAssets="All" />
     <PackageReference Include="System.Reflection.Emit" Version="4.3.0" />
-    <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0" PrivateAssets="All" />
   </ItemGroup>
 </Project>
diff --git a/src/runtime/ReflectedClrType.cs b/src/runtime/ReflectedClrType.cs
index 3b83fb443..93c28fd87 100644
--- a/src/runtime/ReflectedClrType.cs
+++ b/src/runtime/ReflectedClrType.cs
@@ -53,7 +53,9 @@ internal void Restore(InterDomainContext context)
     {
         var cb = context.Storage.GetValue<ClassBase>("impl");
 
-        cb.Load(this, context);
+        Debug.Assert(cb is not null);
+
+        cb!.Load(this, context);
 
         Restore(cb);
     }
diff --git a/src/runtime/StateSerialization/CLRWrapperCollection.cs b/src/runtime/StateSerialization/CLRWrapperCollection.cs
index 66d5170dd..f9e2654eb 100644
--- a/src/runtime/StateSerialization/CLRWrapperCollection.cs
+++ b/src/runtime/StateSerialization/CLRWrapperCollection.cs
@@ -1,10 +1,11 @@
 using System.Collections.ObjectModel;
+using System.Diagnostics.CodeAnalysis;
 
 namespace Python.Runtime;
 
 public class CLRWrapperCollection : KeyedCollection<object, CLRMappedItem>
 {
-    public bool TryGetValue(object key, out CLRMappedItem value)
+    public bool TryGetValue(object key, [NotNullWhen(true)] out CLRMappedItem? value)
     {
         if (Dictionary == null)
         {
diff --git a/src/runtime/StateSerialization/ClassManagerState.cs b/src/runtime/StateSerialization/ClassManagerState.cs
index 70bb076cd..093e5d41c 100644
--- a/src/runtime/StateSerialization/ClassManagerState.cs
+++ b/src/runtime/StateSerialization/ClassManagerState.cs
@@ -3,6 +3,10 @@
 
 namespace Python.Runtime.StateSerialization;
 
+// Workaround for the lack of required properties: https://github.com/dotnet/csharplang/issues/3630
+// Non-nullable field must contain a non-null value when exiting constructor. Consider declaring as nullable.
+#pragma warning disable CS8618
+
 [Serializable]
 internal class ClassManagerState
 {
diff --git a/src/runtime/StateSerialization/ImportHookState.cs b/src/runtime/StateSerialization/ImportHookState.cs
index 1ade98dbf..cf87cb18b 100644
--- a/src/runtime/StateSerialization/ImportHookState.cs
+++ b/src/runtime/StateSerialization/ImportHookState.cs
@@ -3,10 +3,14 @@
 
 namespace Python.Runtime.StateSerialization;
 
+// Workaround for the lack of required properties: https://github.com/dotnet/csharplang/issues/3630
+// Non-nullable field must contain a non-null value when exiting constructor. Consider declaring as nullable.
+#pragma warning disable CS8618
+
 [Serializable]
 internal class ImportHookState
 {
-    public PyModule PyCLRModule { get; set; }
-    public PyObject Root { get; set; }
-    public Dictionary<PyString, PyObject> Modules { get; set; }
+    public PyModule PyCLRModule { get; init; }
+    public PyObject Root { get; init; }
+    public Dictionary<PyString, PyObject> Modules { get; init; }
 }
diff --git a/src/runtime/StateSerialization/MaybeMemberInfo.cs b/src/runtime/StateSerialization/MaybeMemberInfo.cs
index 0a3fbef69..aa369a5ed 100644
--- a/src/runtime/StateSerialization/MaybeMemberInfo.cs
+++ b/src/runtime/StateSerialization/MaybeMemberInfo.cs
@@ -7,8 +7,6 @@ namespace Python.Runtime
     [Serializable]
     internal struct MaybeMemberInfo<T> : ISerializable where T : MemberInfo
     {
-        public static implicit operator MaybeMemberInfo<T>(T ob) => new MaybeMemberInfo<T>(ob);
-
         // .ToString() of the serialized object
         const string SerializationDescription = "d";
         // The ReflectedType of the object
@@ -50,8 +48,8 @@ public override string ToString()
         public MaybeMemberInfo(T fi)
         {
             info = fi;
-            Description = info?.ToString();
-            if (info?.DeclaringType is not null)
+            Description = info.ToString();
+            if (info.DeclaringType is not null)
                 Description += " of " + info.DeclaringType;
             deserializationException = null;
         }
diff --git a/src/runtime/StateSerialization/MaybeMethodBase.cs b/src/runtime/StateSerialization/MaybeMethodBase.cs
index e773b8e03..a278df2cf 100644
--- a/src/runtime/StateSerialization/MaybeMethodBase.cs
+++ b/src/runtime/StateSerialization/MaybeMethodBase.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Diagnostics.CodeAnalysis;
 using System.Reflection;
 using System.Runtime.Serialization;
 using System.Linq;
@@ -21,7 +22,7 @@ internal struct MaybeMethodBase<T> : ISerializable where T: MethodBase
 
         public static implicit operator MaybeMethodBase<T> (T? ob) => new (ob);
 
-        string name;
+        string? name;
         MethodBase? info;
 
         [NonSerialized]
@@ -48,7 +49,8 @@ public T Value
         }
 
         public T UnsafeValue => (T)info!;
-        public string Name {get{return name;}}
+        public string? Name => name;
+        [MemberNotNullWhen(true, nameof(info))]
         public bool Valid => info != null;
 
         public override string ToString()
diff --git a/src/runtime/StateSerialization/MetatypeState.cs b/src/runtime/StateSerialization/MetatypeState.cs
index 3c0d55642..4e7a44d40 100644
--- a/src/runtime/StateSerialization/MetatypeState.cs
+++ b/src/runtime/StateSerialization/MetatypeState.cs
@@ -2,6 +2,10 @@
 
 namespace Python.Runtime.StateSerialization;
 
+// Workaround for the lack of required properties: https://github.com/dotnet/csharplang/issues/3630
+// Non-nullable field must contain a non-null value when exiting constructor. Consider declaring as nullable.
+#pragma warning disable CS8618
+
 [Serializable]
 internal class MetatypeState
 {
diff --git a/src/runtime/StateSerialization/PythonNetState.cs b/src/runtime/StateSerialization/PythonNetState.cs
index 66092aa42..771f92ecc 100644
--- a/src/runtime/StateSerialization/PythonNetState.cs
+++ b/src/runtime/StateSerialization/PythonNetState.cs
@@ -2,12 +2,16 @@
 
 namespace Python.Runtime.StateSerialization;
 
+// Workaround for the lack of required properties: https://github.com/dotnet/csharplang/issues/3630
+// Non-nullable field must contain a non-null value when exiting constructor. Consider declaring as nullable.
+#pragma warning disable CS8618
+
 [Serializable]
 internal class PythonNetState
 {
-    public MetatypeState Metatype { get; set; }
-    public SharedObjectsState SharedObjects { get; set; }
-    public TypeManagerState Types { get; set; }
-    public ClassManagerState Classes { get; set; }
-    public ImportHookState ImportHookState { get; set; }
+    public MetatypeState Metatype { get; init; }
+    public SharedObjectsState SharedObjects { get; init; }
+    public TypeManagerState Types { get; init; }
+    public ClassManagerState Classes { get; init; }
+    public ImportHookState ImportHookState { get; init; }
 }
diff --git a/src/runtime/StateSerialization/SharedObjectsState.cs b/src/runtime/StateSerialization/SharedObjectsState.cs
index a445c9252..0375007d6 100644
--- a/src/runtime/StateSerialization/SharedObjectsState.cs
+++ b/src/runtime/StateSerialization/SharedObjectsState.cs
@@ -3,11 +3,15 @@
 
 namespace Python.Runtime.StateSerialization;
 
+// Workaround for the lack of required properties: https://github.com/dotnet/csharplang/issues/3630
+// Non-nullable field must contain a non-null value when exiting constructor. Consider declaring as nullable.
+#pragma warning disable CS8618
+
 [Serializable]
 internal class SharedObjectsState
 {
-    public Dictionary<PyObject, CLRObject> InternalStores { get; set; }
-    public Dictionary<PyObject, ExtensionType> Extensions { get; set; }
-    public RuntimeDataStorage Wrappers { get; set; }
-    public Dictionary<PyObject, InterDomainContext> Contexts { get; set; }
+    public Dictionary<PyObject, CLRObject> InternalStores { get; init; }
+    public Dictionary<PyObject, ExtensionType> Extensions { get; init; }
+    public RuntimeDataStorage Wrappers { get; init; }
+    public Dictionary<PyObject, InterDomainContext> Contexts { get; init; }
 }
diff --git a/src/runtime/StateSerialization/TypeManagerState.cs b/src/runtime/StateSerialization/TypeManagerState.cs
index 158579549..7630990ef 100644
--- a/src/runtime/StateSerialization/TypeManagerState.cs
+++ b/src/runtime/StateSerialization/TypeManagerState.cs
@@ -3,6 +3,10 @@
 
 namespace Python.Runtime.StateSerialization;
 
+// Workaround for the lack of required properties: https://github.com/dotnet/csharplang/issues/3630
+// Non-nullable field must contain a non-null value when exiting constructor. Consider declaring as nullable.
+#pragma warning disable CS8618
+
 [Serializable]
 internal class TypeManagerState
 {
diff --git a/src/runtime/Util.cs b/src/runtime/Util.cs
index 6fd467ae7..bbed4ad0a 100644
--- a/src/runtime/Util.cs
+++ b/src/runtime/Util.cs
@@ -119,13 +119,6 @@ internal static void WriteCLong(BorrowedReference type, int offset, Int64 value)
             }
         }
 
-        /// <summary>
-        /// Null-coalesce: if <paramref name="primary"/> parameter is not
-        /// <see cref="IntPtr.Zero"/>, return it. Otherwise return <paramref name="fallback"/>.
-        /// </summary>
-        internal static IntPtr Coalesce(this IntPtr primary, IntPtr fallback)
-            => primary == IntPtr.Zero ? fallback : primary;
-
         /// <summary>
         /// Gets substring after last occurrence of <paramref name="symbol"/>
         /// </summary>
@@ -149,5 +142,14 @@ internal static string ReadStringResource(this System.Reflection.Assembly assemb
         }
 
         public static IEnumerator<T> GetEnumerator<T>(this IEnumerator<T> enumerator) => enumerator;
+
+        public static IEnumerable<T> WhereNotNull<T>(this IEnumerable<T?> source)
+            where T: class
+        {
+            foreach (var item in source)
+            {
+                if (item is not null) yield return item;
+            }
+        }
     }
 }
diff --git a/src/runtime/assemblymanager.cs b/src/runtime/assemblymanager.cs
index e39a9cd73..56c70c13a 100644
--- a/src/runtime/assemblymanager.cs
+++ b/src/runtime/assemblymanager.cs
@@ -358,7 +358,7 @@ public static List<string> GetNames(string nsname)
             //Dictionary<string, int> seen = new Dictionary<string, int>();
             var names = new List<string>(8);
 
-            List<string> g = GenericUtil.GetGenericBaseNames(nsname);
+            List<string>? g = GenericUtil.GetGenericBaseNames(nsname);
             if (g != null)
             {
                 foreach (string n in g)
diff --git a/src/runtime/classbase.cs b/src/runtime/classbase.cs
index 069757b40..9ef7c626c 100644
--- a/src/runtime/classbase.cs
+++ b/src/runtime/classbase.cs
@@ -67,7 +67,7 @@ public virtual NewReference type_subscript(BorrowedReference idx)
                 return Exceptions.RaiseTypeError(type.DeletedMessage);
             }
 
-            Type target = GenericUtil.GenericForType(type.Value, types.Length);
+            Type? target = GenericUtil.GenericForType(type.Value, types.Length);
 
             if (target != null)
             {
@@ -396,7 +396,7 @@ protected override void OnSave(BorrowedReference ob, InterDomainContext context)
             context.Storage.AddValue("impl", this);
         }
 
-        protected override void OnLoad(BorrowedReference ob, InterDomainContext context)
+        protected override void OnLoad(BorrowedReference ob, InterDomainContext? context)
         {
             base.OnLoad(ob, context);
             var gcHandle = GCHandle.Alloc(this);
diff --git a/src/runtime/classderived.cs b/src/runtime/classderived.cs
index b9b3419de..a39f23bef 100644
--- a/src/runtime/classderived.cs
+++ b/src/runtime/classderived.cs
@@ -144,7 +144,7 @@ internal static NewReference ToPython(IPythonDerivedType obj)
         internal static Type CreateDerivedType(string name,
             Type baseType,
             BorrowedReference py_dict,
-            string namespaceStr,
+            string? namespaceStr,
             string? assemblyName,
             string moduleName = "Python.Runtime.Dynamic.dll")
         {
@@ -669,7 +669,7 @@ public class PythonDerivedType
         /// method binding (i.e. it has been overridden in the derived python
         /// class) it calls it, otherwise it calls the base method.
         /// </summary>
-        public static T InvokeMethod<T>(IPythonDerivedType obj, string methodName, string origMethodName, object[] args)
+        public static T? InvokeMethod<T>(IPythonDerivedType obj, string methodName, string origMethodName, object[] args)
         {
             var self = GetPyObj(obj);
 
@@ -776,7 +776,7 @@ public static void InvokeMethodVoid(IPythonDerivedType obj, string methodName, s
                 args);
         }
 
-        public static T InvokeGetProperty<T>(IPythonDerivedType obj, string propertyName)
+        public static T? InvokeGetProperty<T>(IPythonDerivedType obj, string propertyName)
         {
             var self = GetPyObj(obj);
 
diff --git a/src/runtime/clrobject.cs b/src/runtime/clrobject.cs
index f3fed3ce2..c178ca459 100644
--- a/src/runtime/clrobject.cs
+++ b/src/runtime/clrobject.cs
@@ -59,7 +59,7 @@ internal static void Restore(object ob, BorrowedReference pyHandle, InterDomainC
             co.OnLoad(pyHandle, context);
         }
 
-        protected override void OnLoad(BorrowedReference ob, InterDomainContext context)
+        protected override void OnLoad(BorrowedReference ob, InterDomainContext? context)
         {
             base.OnLoad(ob, context);
             GCHandle gc = GCHandle.Alloc(this);
diff --git a/src/runtime/converterextensions.cs b/src/runtime/converterextensions.cs
index 7dfdebcbd..9a7ae5403 100644
--- a/src/runtime/converterextensions.cs
+++ b/src/runtime/converterextensions.cs
@@ -126,7 +126,7 @@ internal static bool TryDecode(BorrowedReference pyHandle, BorrowedReference pyT
             Debug.Assert(PyType.IsType(sourceTypeRef));
             var pyType = new PyType(sourceTypeRef, prevalidated: true);
 
-            IPyObjectDecoder decoder;
+            IPyObjectDecoder? decoder;
             lock (decoders)
             {
                 decoder = decoders.GetDecoder(pyType, targetType);
diff --git a/src/runtime/extensiontype.cs b/src/runtime/extensiontype.cs
index d583f6710..e3f049e1a 100644
--- a/src/runtime/extensiontype.cs
+++ b/src/runtime/extensiontype.cs
@@ -95,7 +95,7 @@ public static int tp_clear(BorrowedReference ob)
             return res;
         }
 
-        protected override void OnLoad(BorrowedReference ob, InterDomainContext context)
+        protected override void OnLoad(BorrowedReference ob, InterDomainContext? context)
         {
             base.OnLoad(ob, context);
             SetupGc(ob, Runtime.PyObject_TYPE(ob));
diff --git a/src/runtime/fieldobject.cs b/src/runtime/fieldobject.cs
index 0250cffc4..af772afe2 100644
--- a/src/runtime/fieldobject.cs
+++ b/src/runtime/fieldobject.cs
@@ -14,7 +14,7 @@ internal class FieldObject : ExtensionType
 
         public FieldObject(FieldInfo info)
         {
-            this.info = info;
+            this.info = new MaybeFieldInfo(info);
         }
 
         /// <summary>
diff --git a/src/runtime/genericutil.cs b/src/runtime/genericutil.cs
index 92b847e98..74db54af1 100644
--- a/src/runtime/genericutil.cs
+++ b/src/runtime/genericutil.cs
@@ -14,7 +14,7 @@ internal static class GenericUtil
         /// <summary>
         /// Maps namespace -> generic base name -> list of generic type names
         /// </summary>
-        private static Dictionary<string, Dictionary<string, List<string>>> mapping;
+        private static Dictionary<string, Dictionary<string, List<string>>> mapping = new();
 
         public static void Reset()
         {
@@ -51,7 +51,7 @@ internal static void Register(Type t)
         /// <summary>
         /// xxx
         /// </summary>
-        public static List<string> GetGenericBaseNames(string ns)
+        public static List<string>? GetGenericBaseNames(string ns)
         {
             Dictionary<string, List<string>> nsmap;
             if (!mapping.TryGetValue(ns, out nsmap))
@@ -69,7 +69,7 @@ public static List<string> GetGenericBaseNames(string ns)
         /// <summary>
         /// Finds a generic type with the given number of generic parameters and the same name and namespace as <paramref name="t"/>.
         /// </summary>
-        public static Type GenericForType(Type t, int paramCount)
+        public static Type? GenericForType(Type t, int paramCount)
         {
             return GenericByName(t.Namespace, t.Name, paramCount);
         }
@@ -77,7 +77,7 @@ public static Type GenericForType(Type t, int paramCount)
         /// <summary>
         /// Finds a generic type in the given namespace with the given name and number of generic parameters.
         /// </summary>
-        public static Type GenericByName(string ns, string basename, int paramCount)
+        public static Type? GenericByName(string ns, string basename, int paramCount)
         {
             Dictionary<string, List<string>> nsmap;
             if (!mapping.TryGetValue(ns, out nsmap))
@@ -107,7 +107,7 @@ public static Type GenericByName(string ns, string basename, int paramCount)
         /// <summary>
         /// xxx
         /// </summary>
-        public static string GenericNameForBaseName(string ns, string name)
+        public static string? GenericNameForBaseName(string ns, string name)
         {
             Dictionary<string, List<string>> nsmap;
             if (!mapping.TryGetValue(ns, out nsmap))
diff --git a/src/runtime/intern_.cs b/src/runtime/intern_.cs
index 09986593e..4884a81ad 100644
--- a/src/runtime/intern_.cs
+++ b/src/runtime/intern_.cs
@@ -3,7 +3,8 @@
 namespace Python.Runtime
 {
     static class PyIdentifier
-    {
+    {
+#pragma warning disable CS0649 // indentifier is never assigned to (assigned with reflection)
          static IntPtr f__name__;
          public static BorrowedReference __name__ => new(f__name__);
          static IntPtr f__dict__;
@@ -37,7 +38,8 @@ static class PyIdentifier
          static IntPtr f__overloads__;
          public static BorrowedReference __overloads__ => new(f__overloads__);
          static IntPtr fOverloads;
-         public static BorrowedReference Overloads => new(fOverloads);
+         public static BorrowedReference Overloads => new(fOverloads);
+#pragma warning restore CS0649        // indentifier is never assigned to (assigned with reflection)
     }
 
 
diff --git a/src/runtime/intern_.tt b/src/runtime/intern_.tt
index bb8d9f12d..03a26cb50 100644
--- a/src/runtime/intern_.tt
+++ b/src/runtime/intern_.tt
@@ -30,7 +30,8 @@ using System;
 namespace Python.Runtime
 {
     static class PyIdentifier
-    {
+    {
+#pragma warning disable CS0649 // indentifier is never assigned to (assigned with reflection)
 <#
     foreach (var name in internNames)
     {
@@ -39,7 +40,8 @@ namespace Python.Runtime
          public static BorrowedReference <#= name #> => new(f<#= name #>);
 <#
     }
-#>
+#>
+#pragma warning restore CS0649        // indentifier is never assigned to (assigned with reflection)
     }
 
 
diff --git a/src/runtime/interop.cs b/src/runtime/interop.cs
index e894fa591..adaac1b6e 100644
--- a/src/runtime/interop.cs
+++ b/src/runtime/interop.cs
@@ -22,13 +22,7 @@ public DocStringAttribute(string docStr)
             DocString = docStr;
         }
 
-        public string DocString
-        {
-            get { return docStr; }
-            set { docStr = value; }
-        }
-
-        private string docStr;
+        public string DocString { get; }
     }
 
     [Serializable]
@@ -204,15 +198,10 @@ internal class ThunkInfo
         public readonly Delegate Target;
         public readonly IntPtr Address;
 
-        public static readonly ThunkInfo Empty = new ThunkInfo(null);
-
         public ThunkInfo(Delegate target)
         {
-            if (target == null)
-            {
-                return;
-            }
-            Target = target;
+            Debug.Assert(target is not null);
+            Target = target!;
             Address = Marshal.GetFunctionPointerForDelegate(target);
         }
     }
diff --git a/src/runtime/managedtype.cs b/src/runtime/managedtype.cs
index d7472cc61..91ed43473 100644
--- a/src/runtime/managedtype.cs
+++ b/src/runtime/managedtype.cs
@@ -114,13 +114,14 @@ internal void Save(BorrowedReference ob, InterDomainContext context)
             OnSave(ob, context);
         }
 
-        internal void Load(BorrowedReference ob, InterDomainContext context)
+#warning context appears to be unused
+        internal void Load(BorrowedReference ob, InterDomainContext? context)
         {
             OnLoad(ob, context);
         }
 
         protected virtual void OnSave(BorrowedReference ob, InterDomainContext context) { }
-        protected virtual void OnLoad(BorrowedReference ob, InterDomainContext context) { }
+        protected virtual void OnLoad(BorrowedReference ob, InterDomainContext? context) { }
 
         protected static void ClearObjectDict(BorrowedReference ob)
         {
diff --git a/src/runtime/methodbinder.cs b/src/runtime/methodbinder.cs
index 34462f7c9..0569afe3e 100644
--- a/src/runtime/methodbinder.cs
+++ b/src/runtime/methodbinder.cs
@@ -3,6 +3,7 @@
 using System.Reflection;
 using System.Text;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 
 namespace Python.Runtime
@@ -23,7 +24,7 @@ internal class MethodBinder
         public List<MaybeMethodBase> list;
 
         [NonSerialized]
-        public MethodBase[] methods;
+        public MethodBase[]? methods;
 
         [NonSerialized]
         public bool init = false;
@@ -188,7 +189,7 @@ internal MethodBase[] GetMethods()
                 methods = (from method in list where method.Valid select method.Value).ToArray();
                 init = true;
             }
-            return methods;
+            return methods!;
         }
 
         /// <summary>
@@ -376,7 +377,7 @@ public MismatchedMethod(Exception exception, MethodBase mb)
                 {
                     var keyStr = Runtime.GetManagedString(Runtime.PyList_GetItem(keylist.Borrow(), i));
                     BorrowedReference value = Runtime.PyList_GetItem(valueList.Borrow(), i);
-                    kwargDict[keyStr] = new PyObject(value);
+                    kwargDict[keyStr!] = new PyObject(value);
                 }
             }
 
diff --git a/src/runtime/methodbinding.cs b/src/runtime/methodbinding.cs
index b2d23ab01..8dcd985d0 100644
--- a/src/runtime/methodbinding.cs
+++ b/src/runtime/methodbinding.cs
@@ -78,12 +78,13 @@ PyObject Signature
                     {
                         ParameterInfo[] altParamters = info.GetParameters();
                         return i < altParamters.Length ? altParamters[i] : null;
-                    }).Where(p => p != null);
+                    }).WhereNotNull();
                     using var defaultValue = alternatives
-                        .Select(alternative => alternative.DefaultValue != DBNull.Value ? alternative.DefaultValue.ToPython() : null)
-                        .FirstOrDefault(v => v != null) ?? parameterClass.GetAttr("empty");
+                        .Select(alternative => alternative!.DefaultValue != DBNull.Value ? alternative.DefaultValue.ToPython() : null)
+                        .FirstOrDefault(v => v != null) ?? parameterClass?.GetAttr("empty");
 
-                    if (alternatives.Any(alternative => alternative.Name != parameter.Name))
+                    if (alternatives.Any(alternative => alternative.Name != parameter.Name)
+                        || positionalOrKeyword is null)
                     {
                         return signatureClass.Invoke();
                     }
@@ -94,7 +95,7 @@ PyObject Signature
                     {
                         kw["default"] = defaultValue;
                     }
-                    using var parameterInfo = parameterClass.Invoke(args: args, kw: kw);
+                    using var parameterInfo = parameterClass!.Invoke(args: args, kw: kw);
                     parameters.Append(parameterInfo);
                 }
 
@@ -209,12 +210,12 @@ public static NewReference tp_call(BorrowedReference ob, BorrowedReference args,
                 // (eg if calling the base class method) then call the original base class method instead
                 // of the target method.
                 IntPtr superType = IntPtr.Zero;
-                if (target is not null && Runtime.PyObject_TYPE(target) != self.targetType)
+                if (target is not null && Runtime.PyObject_TYPE(target) != self.targetType!)
                 {
                     var inst = GetManagedObject(target) as CLRObject;
                     if (inst?.inst is IPythonDerivedType)
                     {
-                        var baseType = GetManagedObject(self.targetType) as ClassBase;
+                        var baseType = GetManagedObject(self.targetType!) as ClassBase;
                         if (baseType != null && baseType.type.Valid)
                         {
                             string baseMethodName = "_" + baseType.type.Value.Name + "__" + self.m.name;
diff --git a/src/runtime/module.cs b/src/runtime/module.cs
index 159fc6912..ada24c6cd 100644
--- a/src/runtime/module.cs
+++ b/src/runtime/module.cs
@@ -165,6 +165,9 @@ public void Import(PyObject module, string? asname = null)
             if (module is null) throw new ArgumentNullException(nameof(module));
 
             asname ??= module.GetAttr("__name__").As<string>();
+
+            if (asname is null) throw new ArgumentException("Module has no name");
+
             Set(asname, module);
         }
 
@@ -238,7 +241,7 @@ public PyObject Execute(PyObject script, PyDict? locals = null)
         /// and convert the result to a Managed Object of given type.
         /// The ast can be either an expression or stmts.
         /// </summary>
-        public T Execute<T>(PyObject script, PyDict? locals = null)
+        public T? Execute<T>(PyObject script, PyDict? locals = null)
         {
             Check();
             PyObject pyObj = Execute(script, locals);
@@ -307,7 +310,7 @@ private void Exec(string code, BorrowedReference _globals, BorrowedReference _lo
         /// Add a new variable to the variables dict if it not exist
         /// or update its value if the variable exists.
         /// </remarks>
-        public PyModule Set(string name, object value)
+        public PyModule Set(string name, object? value)
         {
             if (name is null) throw new ArgumentNullException(nameof(name));
 
@@ -425,7 +428,7 @@ public T Get<T>(string name)
 
             Check();
             PyObject pyObj = Get(name);
-            return pyObj.As<T>();
+            return pyObj.As<T>()!;
         }
 
         /// <summary>
@@ -449,13 +452,13 @@ public bool TryGet<T>(string name, out T? value)
             return true;
         }
 
-        public override bool TryGetMember(GetMemberBinder binder, out object result)
+        public override bool TryGetMember(GetMemberBinder binder, out object? result)
         {
             result = CheckNone(this.Get(binder.Name));
             return true;
         }
 
-        public override bool TrySetMember(SetMemberBinder binder, object value)
+        public override bool TrySetMember(SetMemberBinder binder, object? value)
         {
             this.Set(binder.Name, value);
             return true;
diff --git a/src/runtime/moduleobject.cs b/src/runtime/moduleobject.cs
index 21435264a..68acaf022 100644
--- a/src/runtime/moduleobject.cs
+++ b/src/runtime/moduleobject.cs
@@ -23,7 +23,7 @@ internal class ModuleObject : ExtensionType
 
         // Attributes to be set on the module according to PEP302 and 451
         // by the import machinery.
-        static readonly HashSet<string?> settableAttributes = 
+        static readonly HashSet<string?> settableAttributes =
             new () {"__spec__", "__file__", "__name__", "__path__", "__loader__", "__package__"};
 
 #pragma warning disable CS8618 // Non-nullable field must contain a non-null value when exiting constructor. Consider declaring as nullable.
@@ -142,7 +142,7 @@ public NewReference GetAttribute(string name, bool guess)
             // enough to complicate the implementation for now.
             if (guess)
             {
-                string gname = GenericUtil.GenericNameForBaseName(this._namespace, name);
+                string? gname = GenericUtil.GenericNameForBaseName(this._namespace, name);
                 if (gname != null)
                 {
                     var o = this.GetAttribute(gname, false);
@@ -343,7 +343,7 @@ public static int tp_traverse(BorrowedReference ob, IntPtr visit, IntPtr arg)
         public new static int tp_setattro(BorrowedReference ob, BorrowedReference key, BorrowedReference val)
         {
             var managedKey = Runtime.GetManagedString(key);
-            if ((settableAttributes.Contains(managedKey)) || 
+            if ((settableAttributes.Contains(managedKey)) ||
                 (ManagedType.GetManagedObject(val) is ModuleObject) )
             {
                 var self = (ModuleObject)ManagedType.GetManagedObject(ob)!;
@@ -376,7 +376,7 @@ protected override void OnSave(BorrowedReference ob, InterDomainContext context)
             cache.Clear();
         }
 
-        protected override void OnLoad(BorrowedReference ob, InterDomainContext context)
+        protected override void OnLoad(BorrowedReference ob, InterDomainContext? context)
         {
             base.OnLoad(ob, context);
             SetObjectDict(ob, new NewReference(dict).Steal());
diff --git a/src/runtime/native/TypeOffset.cs b/src/runtime/native/TypeOffset.cs
index c880db842..3f97b0f45 100644
--- a/src/runtime/native/TypeOffset.cs
+++ b/src/runtime/native/TypeOffset.cs
@@ -99,7 +99,9 @@ internal static void Use(ITypeOffsets offsets, int extraHeadOffset)
 
         static readonly BindingFlags FieldFlags = BindingFlags.NonPublic | BindingFlags.Static;
 
+#pragma warning disable CS8618 // Non-nullable field must contain a non-null value when exiting constructor. Initialized in ABI.cs
         static Dictionary<string, int> SlotOffsets;
+#pragma warning restore CS8618
         internal static Dictionary<string, int> GetOffsets()
         {
             var properties = typeof(TypeOffset).GetProperties(FieldFlags);
diff --git a/src/runtime/operatormethod.cs b/src/runtime/operatormethod.cs
index a807f59f3..aad3f013f 100644
--- a/src/runtime/operatormethod.cs
+++ b/src/runtime/operatormethod.cs
@@ -27,7 +27,7 @@ public SlotDefinition(string methodName, int typeOffset)
             public int TypeOffset { get; }
 
         }
-        private static PyObject _opType;
+        private static PyObject? _opType;
 
         static OperatorMethod()
         {
@@ -115,7 +115,7 @@ public static void FixupSlots(BorrowedReference pyType, Type clrType)
                 int offset = OpMethodMap[method.Name].TypeOffset;
                 // Copy the default implementation of e.g. the nb_add slot,
                 // which simply calls __add__ on the type.
-                IntPtr func = Util.ReadIntPtr(_opType, offset);
+                IntPtr func = Util.ReadIntPtr(_opType!, offset);
                 // Write the slot definition of the target Python type, so
                 // that we can later modify __add___ and it will be called
                 // when used with a Python operator.
diff --git a/src/runtime/platform/LibDL.cs b/src/runtime/platform/LibDL.cs
index 3bf2a2057..3dad73735 100644
--- a/src/runtime/platform/LibDL.cs
+++ b/src/runtime/platform/LibDL.cs
@@ -6,7 +6,7 @@ namespace Python.Runtime.Platform
 {
     interface ILibDL
     {
-        IntPtr dlopen(string fileName, int flags);
+        IntPtr dlopen(string? fileName, int flags);
         IntPtr dlsym(IntPtr handle, string symbol);
         int dlclose(IntPtr handle);
         IntPtr dlerror();
@@ -38,13 +38,13 @@ public static ILibDL GetInstance()
             }
         }
 
-        IntPtr ILibDL.dlopen(string fileName, int flags) => dlopen(fileName, flags);
+        IntPtr ILibDL.dlopen(string? fileName, int flags) => dlopen(fileName, flags);
         IntPtr ILibDL.dlsym(IntPtr handle, string symbol) => dlsym(handle, symbol);
         int ILibDL.dlclose(IntPtr handle) => dlclose(handle);
         IntPtr ILibDL.dlerror() => dlerror();
 
         [DllImport(NativeDll, CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
-        private static extern IntPtr dlopen(string fileName, int flags);
+        private static extern IntPtr dlopen(string? fileName, int flags);
 
         [DllImport(NativeDll, CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
         private static extern IntPtr dlsym(IntPtr handle, string symbol);
@@ -64,13 +64,13 @@ class LinuxLibDL2 : ILibDL
         public int RTLD_GLOBAL => 0x100;
         public IntPtr RTLD_DEFAULT => IntPtr.Zero;
 
-        IntPtr ILibDL.dlopen(string fileName, int flags) => dlopen(fileName, flags);
+        IntPtr ILibDL.dlopen(string? fileName, int flags) => dlopen(fileName, flags);
         IntPtr ILibDL.dlsym(IntPtr handle, string symbol) => dlsym(handle, symbol);
         int ILibDL.dlclose(IntPtr handle) => dlclose(handle);
         IntPtr ILibDL.dlerror() => dlerror();
 
         [DllImport(NativeDll, CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
-        private static extern IntPtr dlopen(string fileName, int flags);
+        private static extern IntPtr dlopen(string? fileName, int flags);
 
         [DllImport(NativeDll, CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
         private static extern IntPtr dlsym(IntPtr handle, string symbol);
@@ -89,13 +89,13 @@ class MacLibDL : ILibDL
         const string NativeDll = "/usr/lib/libSystem.dylib";
         public IntPtr RTLD_DEFAULT => new(-2);
 
-        IntPtr ILibDL.dlopen(string fileName, int flags) => dlopen(fileName, flags);
+        IntPtr ILibDL.dlopen(string? fileName, int flags) => dlopen(fileName, flags);
         IntPtr ILibDL.dlsym(IntPtr handle, string symbol) => dlsym(handle, symbol);
         int ILibDL.dlclose(IntPtr handle) => dlclose(handle);
         IntPtr ILibDL.dlerror() => dlerror();
 
         [DllImport(NativeDll, CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
-        private static extern IntPtr dlopen(string fileName, int flags);
+        private static extern IntPtr dlopen(string? fileName, int flags);
 
         [DllImport(NativeDll, CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
         private static extern IntPtr dlsym(IntPtr handle, string symbol);
diff --git a/src/runtime/platform/LibraryLoader.cs b/src/runtime/platform/LibraryLoader.cs
index 4148f2391..6d67bea61 100644
--- a/src/runtime/platform/LibraryLoader.cs
+++ b/src/runtime/platform/LibraryLoader.cs
@@ -8,7 +8,7 @@ namespace Python.Runtime.Platform
 {
     interface ILibraryLoader
     {
-        IntPtr Load(string dllToLoad);
+        IntPtr Load(string? dllToLoad);
 
         IntPtr GetFunction(IntPtr hModule, string procedureName);
 
@@ -17,7 +17,7 @@ interface ILibraryLoader
 
     static class LibraryLoader
     {
-        static ILibraryLoader _instance = null;
+        static ILibraryLoader? _instance = null;
 
         public static ILibraryLoader Instance
         {
@@ -51,7 +51,7 @@ public PosixLoader(ILibDL libDL)
             this.libDL = libDL ?? throw new ArgumentNullException(nameof(libDL));
         }
 
-        public IntPtr Load(string dllToLoad)
+        public IntPtr Load(string? dllToLoad)
         {
             ClearError();
             var res = libDL.dlopen(dllToLoad, libDL.RTLD_NOW | libDL.RTLD_GLOBAL);
@@ -107,8 +107,9 @@ class WindowsLoader : ILibraryLoader
         private const string NativeDll = "kernel32.dll";
 
 
-        public IntPtr Load(string dllToLoad)
+        public IntPtr Load(string? dllToLoad)
         {
+            if (dllToLoad is null) return IntPtr.Zero;
             var res = WindowsLoader.LoadLibrary(dllToLoad);
             if (res == IntPtr.Zero)
                 throw new DllNotFoundException($"Could not load {dllToLoad}.", new Win32Exception());
diff --git a/src/runtime/propertyobject.cs b/src/runtime/propertyobject.cs
index f9ae2585d..140bd47b5 100644
--- a/src/runtime/propertyobject.cs
+++ b/src/runtime/propertyobject.cs
@@ -18,7 +18,7 @@ public PropertyObject(PropertyInfo md)
         {
             getter = md.GetGetMethod(true);
             setter = md.GetSetMethod(true);
-            info = md;
+            info = new MaybeMemberInfo<PropertyInfo>(md);
         }
 
 
diff --git a/src/runtime/pyobject.cs b/src/runtime/pyobject.cs
index 894fff329..709bc11c4 100644
--- a/src/runtime/pyobject.cs
+++ b/src/runtime/pyobject.cs
@@ -175,7 +175,7 @@ public static PyObject FromManagedObject(object ob)
         /// Return a managed object of the given type, based on the
         /// value of the Python object.
         /// </summary>
-        public T As<T>() => (T)this.AsManagedObject(typeof(T));
+        public T As<T>() => (T)this.AsManagedObject(typeof(T))!;
 
         internal bool IsDisposed => rawPtr == IntPtr.Zero;
 
diff --git a/src/runtime/runtime_data.cs b/src/runtime/runtime_data.cs
index 4d49255e2..f30a54dbe 100644
--- a/src/runtime/runtime_data.cs
+++ b/src/runtime/runtime_data.cs
@@ -17,8 +17,8 @@ namespace Python.Runtime
 {
     public static class RuntimeData
     {
-        private static Type _formatterType;
-        public static Type FormatterType
+        private static Type? _formatterType;
+        public static Type? FormatterType
         {
             get => _formatterType;
             set
@@ -31,7 +31,7 @@ public static Type FormatterType
             }
         }
 
-        public static ICLRObjectStorer WrappersStorer { get; set; }
+        public static ICLRObjectStorer? WrappersStorer { get; set; }
 
         /// <summary>
         /// Clears the old "clr_data" entry if a previous one is present.
@@ -57,7 +57,7 @@ internal static void Stash()
                 Classes = ClassManager.SaveRuntimeData(),
                 SharedObjects = SaveRuntimeDataObjects(),
             };
-            
+
             IFormatter formatter = CreateFormatter();
             var ms = new MemoryStream();
             formatter.Serialize(ms, runtimeStorage);
@@ -171,9 +171,8 @@ private static SharedObjectsState SaveRuntimeDataObjects()
                 // Wrapper must be the CLRObject
                 var clrObj = (CLRObject)ManagedType.GetManagedObject(pyObj)!;
                 object inst = clrObj.inst;
-                CLRMappedItem item;
                 List<CLRObject> mappedObjs;
-                if (!userObjects.TryGetValue(inst, out item))
+                if (!userObjects.TryGetValue(inst, out var item))
                 {
                     item = new CLRMappedItem(inst);
                     userObjects.Add(item);
@@ -260,63 +259,28 @@ private static IFormatter CreateFormatter()
     [Serializable]
     public class RuntimeDataStorage
     {
-        private Stack _stack;
-        private Dictionary<string, object> _namedValues;
+        private Dictionary<string, object?>? _namedValues;
 
         public T AddValue<T>(string name, T value)
         {
             if (_namedValues == null)
             {
-                _namedValues = new Dictionary<string, object>();
+                _namedValues = new Dictionary<string, object?>();
             }
             _namedValues.Add(name, value);
             return value;
         }
 
-        public object GetValue(string name)
-        {
-            return _namedValues[name];
-        }
-
-        public T GetValue<T>(string name)
-        {
-            return (T)GetValue(name);
-        }
-
-        public T GetValue<T>(string name, out T value)
-        {
-            value = GetValue<T>(name);
-            return value;
-        }
-
-        public RuntimeDataStorage GetStorage(string name)
-        {
-            return GetValue<RuntimeDataStorage>(name);
-        }
-
-        public T PushValue<T>(T value)
-        {
-            if (_stack == null)
-            {
-                _stack = new Stack();
-            }
-            _stack.Push(value);
-            return value;
-        }
-
-        public object PopValue()
-        {
-            return _stack.Pop();
-        }
-
-        public T PopValue<T>()
+        public object? GetValue(string name)
         {
-            return (T)PopValue();
+            return _namedValues is null
+                ? throw new KeyNotFoundException()
+                : _namedValues[name];
         }
 
-        public T PopValue<T>(out T value)
+        public T? GetValue<T>(string name)
         {
-            return value = (T)PopValue();
+            return (T?)GetValue(name);
         }
     }
 
@@ -324,7 +288,7 @@ public T PopValue<T>(out T value)
     [Serializable]
     class InterDomainContext
     {
-        private RuntimeDataStorage _storage;
+        private RuntimeDataStorage? _storage;
         public RuntimeDataStorage Storage => _storage ?? (_storage = new RuntimeDataStorage());
     }
 }
diff --git a/src/runtime/tricks/InitOnly.cs b/src/runtime/tricks/InitOnly.cs
new file mode 100644
index 000000000..5ead4123a
--- /dev/null
+++ b/src/runtime/tricks/InitOnly.cs
@@ -0,0 +1,4 @@
+namespace System.Runtime.CompilerServices
+{
+    internal static class IsExternalInit { }
+}
diff --git a/src/runtime/typemanager.cs b/src/runtime/typemanager.cs
index 834703e80..c0a835943 100644
--- a/src/runtime/typemanager.cs
+++ b/src/runtime/typemanager.cs
@@ -19,7 +19,11 @@ internal class TypeManager
     {
         internal static IntPtr subtype_traverse;
         internal static IntPtr subtype_clear;
+#pragma warning disable CS8618 // Non-nullable field must contain a non-null value when exiting constructor. Consider declaring as nullable.
+        /// <remarks>initialized in <see cref="Initialize"/> rather than in constructor</remarks>
         internal static IPythonBaseTypeProvider pythonBaseTypeProvider;
+#pragma warning restore CS8618 // Non-nullable field must contain a non-null value when exiting constructor. Consider declaring as nullable.
+
 
         private const BindingFlags tbFlags = BindingFlags.Public | BindingFlags.Static;
         private static Dictionary<MaybeType, PyType> cache = new();
@@ -78,7 +82,7 @@ internal static void RestoreRuntimeData(TypeManagerState storage)
             foreach (var entry in typeCache)
             {
                 Type type = entry.Key.Value;;
-                cache[type] = entry.Value;
+                cache![type] = entry.Value;
                 SlotsHolder holder = CreateSlotsHolder(entry.Value);
                 InitializeSlots(entry.Value, type, holder);
                 Runtime.PyType_Modified(entry.Value);
@@ -624,7 +628,7 @@ static void InheritSubstructs(IntPtr type)
         /// provides the implementation for the type, connect the type slots of
         /// the Python object to the managed methods of the implementing Type.
         /// </summary>
-        internal static void InitializeSlots(PyType type, Type impl, SlotsHolder slotsHolder = null)
+        internal static void InitializeSlots(PyType type, Type impl, SlotsHolder? slotsHolder = null)
         {
             // We work from the most-derived class up; make sure to get
             // the most-derived slot and not to override it with a base
@@ -654,7 +658,7 @@ internal static void InitializeSlots(PyType type, Type impl, SlotsHolder slotsHo
                 }
 
                 var initSlot = impl.GetMethod("InitializeSlots", BindingFlags.Static | BindingFlags.Public);
-                initSlot?.Invoke(null, parameters: new object[] { type, seen, slotsHolder });
+                initSlot?.Invoke(null, parameters: new object?[] { type, seen, slotsHolder });
 
                 impl = impl.BaseType;
             }
@@ -675,7 +679,7 @@ private static void SetRequiredSlots(PyType type, HashSet<string> seen)
             }
         }
 
-        static void InitializeSlot(BorrowedReference type, ThunkInfo thunk, string name, SlotsHolder slotsHolder)
+        static void InitializeSlot(BorrowedReference type, ThunkInfo thunk, string name, SlotsHolder? slotsHolder)
         {
             if (!Enum.TryParse<TypeSlotID>(name, out var id))
             {
@@ -703,7 +707,7 @@ internal static void InitializeSlotIfEmpty(BorrowedReference type, int slotOffse
             InitializeSlot(type, slotOffset, impl, slotsHolder);
         }
 
-        static void InitializeSlot(BorrowedReference type, int slotOffset, ThunkInfo thunk, SlotsHolder slotsHolder)
+        static void InitializeSlot(BorrowedReference type, int slotOffset, ThunkInfo thunk, SlotsHolder? slotsHolder)
         {
             Util.WriteIntPtr(type, slotOffset, thunk.Address);
             if (slotsHolder != null)

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