diff --git a/src/libraries/System.Resources.Extensions/ref/System.Resources.Extensions.cs b/src/libraries/System.Resources.Extensions/ref/System.Resources.Extensions.cs
index 4c08022721854a..f4e0695166247d 100644
--- a/src/libraries/System.Resources.Extensions/ref/System.Resources.Extensions.cs
+++ b/src/libraries/System.Resources.Extensions/ref/System.Resources.Extensions.cs
@@ -21,11 +21,11 @@ public sealed partial class PreserializedResourceWriter : System.IDisposable, Sy
         public PreserializedResourceWriter(System.IO.Stream stream) { }
         public PreserializedResourceWriter(string fileName) { }
         public void AddActivatorResource(string name, System.IO.Stream value, string typeName, bool closeAfterWrite = false) { }
-        public void AddBinaryFormattedResource(string name, byte[] value, string typeName = null) { }
-        public void AddResource(string name, byte[] value) { }
-        public void AddResource(string name, System.IO.Stream value, bool closeAfterWrite = false) { }
-        public void AddResource(string name, object value) { }
-        public void AddResource(string name, string value) { }
+        public void AddBinaryFormattedResource(string name, byte[] value, string? typeName = null) { }
+        public void AddResource(string name, byte[]? value) { }
+        public void AddResource(string name, System.IO.Stream? value, bool closeAfterWrite = false) { }
+        public void AddResource(string name, object? value) { }
+        public void AddResource(string name, string? value) { }
         public void AddResource(string name, string value, string typeName) { }
         public void AddTypeConverterResource(string name, byte[] value, string typeName) { }
         public void Close() { }
diff --git a/src/libraries/System.Resources.Extensions/src/System/Resources/Extensions/DeserializingResourceReader.cs b/src/libraries/System.Resources.Extensions/src/System/Resources/Extensions/DeserializingResourceReader.cs
index 972e9ccfeb8170..972f20378b1b5e 100644
--- a/src/libraries/System.Resources.Extensions/src/System/Resources/Extensions/DeserializingResourceReader.cs
+++ b/src/libraries/System.Resources.Extensions/src/System/Resources/Extensions/DeserializingResourceReader.cs
@@ -51,9 +51,9 @@ private object ReadBinaryFormattedObject()
 
         internal class UndoTruncatedTypeNameSerializationBinder : SerializationBinder
         {
-            public override Type BindToType(string assemblyName, string typeName)
+            public override Type? BindToType(string assemblyName, string typeName)
             {
-                Type type = null;
+                Type? type = null;
 
                 // determine if we have a mangled generic type name
                 if (typeName != null && assemblyName != null && !AreBracketsBalanced(typeName))
@@ -212,7 +212,7 @@ private object DeserializeObject(int typeIndex)
                             stream = new MemoryStream(bytes, false);
                         }
 
-                        value = Activator.CreateInstance(type, new object[] { stream });
+                        value = Activator.CreateInstance(type, new object[] { stream })!;
                         break;
                     }
                 default:
diff --git a/src/libraries/System.Resources.Extensions/src/System/Resources/Extensions/PreserializedResourceWriter.cs b/src/libraries/System.Resources.Extensions/src/System/Resources/Extensions/PreserializedResourceWriter.cs
index 5eba6fb9faa22f..c1017736d01d23 100644
--- a/src/libraries/System.Resources.Extensions/src/System/Resources/Extensions/PreserializedResourceWriter.cs
+++ b/src/libraries/System.Resources.Extensions/src/System/Resources/Extensions/PreserializedResourceWriter.cs
@@ -25,7 +25,7 @@ public partial class PreserializedResourceWriter
         // an internal type name used to represent an unknown resource type, explicitly omit version to save
         // on size and avoid changes in user resources.  This works since we only ever load this type name
         // from calls to GetType from this assembly.
-        private static readonly string UnknownObjectTypeName = typeof(UnknownType).FullName;
+        private static readonly string UnknownObjectTypeName = typeof(UnknownType).FullName!;
 
         private string ResourceReaderTypeName => _requiresDeserializingResourceReader ?
             DeserializingResourceReaderFullyQualifiedName :
@@ -40,22 +40,22 @@ public partial class PreserializedResourceWriter
         // is done by reflection
         private static readonly IReadOnlyDictionary<string, Type> s_primitiveTypes = new Dictionary<string, Type>(16, TypeNameComparer.Instance)
         {
-            { typeof(string).FullName, typeof(string) },
-            { typeof(int).FullName, typeof(int) },
-            { typeof(bool).FullName, typeof(bool) },
-            { typeof(char).FullName, typeof(char) },
-            { typeof(byte).FullName, typeof(byte) },
-            { typeof(sbyte).FullName, typeof(sbyte) },
-            { typeof(short).FullName, typeof(short) },
-            { typeof(long).FullName, typeof(long) },
-            { typeof(ushort).FullName, typeof(ushort) },
-            { typeof(uint).FullName, typeof(uint) },
-            { typeof(ulong).FullName, typeof(ulong) },
-            { typeof(float).FullName, typeof(float) },
-            { typeof(double).FullName, typeof(double) },
-            { typeof(decimal).FullName, typeof(decimal) },
-            { typeof(DateTime).FullName, typeof(DateTime) },
-            { typeof(TimeSpan).FullName, typeof(TimeSpan) }
+            { typeof(string).FullName!, typeof(string) },
+            { typeof(int).FullName!, typeof(int) },
+            { typeof(bool).FullName!, typeof(bool) },
+            { typeof(char).FullName!, typeof(char) },
+            { typeof(byte).FullName!, typeof(byte) },
+            { typeof(sbyte).FullName!, typeof(sbyte) },
+            { typeof(short).FullName!, typeof(short) },
+            { typeof(long).FullName!, typeof(long) },
+            { typeof(ushort).FullName!, typeof(ushort) },
+            { typeof(uint).FullName!, typeof(uint) },
+            { typeof(ulong).FullName!, typeof(ulong) },
+            { typeof(float).FullName!, typeof(float) },
+            { typeof(double).FullName!, typeof(double) },
+            { typeof(decimal).FullName!, typeof(decimal) },
+            { typeof(DateTime).FullName!, typeof(DateTime) },
+            { typeof(TimeSpan).FullName!, typeof(TimeSpan) }
             // byte[] and Stream are primitive types but do not define a conversion from string
         };
 
@@ -80,7 +80,7 @@ public void AddResource(string name, string value, string typeName)
                 throw new ArgumentNullException(nameof(typeName));
 
             // determine if the type is a primitive type
-            if (s_primitiveTypes.TryGetValue(typeName, out Type primitiveType))
+            if (s_primitiveTypes.TryGetValue(typeName, out Type? primitiveType))
             {
                 // directly add strings
                 if (primitiveType == typeof(string))