diff --git a/src/libraries/System.Text.Json/gen/JsonSourceGenerator.Emitter.cs b/src/libraries/System.Text.Json/gen/JsonSourceGenerator.Emitter.cs
index 2526edd227055..75d8cdd978171 100644
--- a/src/libraries/System.Text.Json/gen/JsonSourceGenerator.Emitter.cs
+++ b/src/libraries/System.Text.Json/gen/JsonSourceGenerator.Emitter.cs
@@ -125,7 +125,8 @@ private void AddSource(string fileName, string source, bool isRootContextDef = f
                 string @namespace = _currentContext.ContextType.Namespace;
                 bool isInGlobalNamespace = @namespace == JsonConstants.GlobalNamespaceValue;
 
-                StringBuilder sb = new("// <auto-generated/>");
+                StringBuilder sb = new(@"// <auto-generated/>
+#nullable enable");
 
                 if (!isInGlobalNamespace)
                 {
@@ -283,10 +284,10 @@ private string GenerateForTypeWithUnknownConverter(TypeGenerationSpec typeMetada
                     {TypeTypeRef} typeToConvert = typeof({typeCompilableName});
                     if (!converter.CanConvert(typeToConvert))
                     {{
-                        {TypeTypeRef} underlyingType = {NullableTypeRef}.GetUnderlyingType(typeToConvert);
+                        {TypeTypeRef}? underlyingType = {NullableTypeRef}.GetUnderlyingType(typeToConvert);
                         if (underlyingType != null && converter.CanConvert(underlyingType))
                         {{
-                            {JsonConverterTypeRef} actualConverter = converter;
+                            {JsonConverterTypeRef}? actualConverter = converter;
 
                             if (converter is {JsonConverterFactoryTypeRef} converterFactory)
                             {{
@@ -670,8 +671,8 @@ private string GeneratePropMetadataInitFunc(TypeGenerationSpec typeGenerationSpe
                     if (memberMetadata.CanUseSetter)
                     {
                         string propMutation = typeGenerationSpec.IsValueType
-                            ? @$"{UnsafeTypeRef}.Unbox<{declaringTypeCompilableName}>(obj).{clrPropertyName} = value"
-                            : $@"(({declaringTypeCompilableName})obj).{clrPropertyName} = value";
+                            ? @$"{UnsafeTypeRef}.Unbox<{declaringTypeCompilableName}>(obj).{clrPropertyName} = value!"
+                            : $@"(({declaringTypeCompilableName})obj).{clrPropertyName} = value!";
 
                         setterNamedArg = $"setter: static (obj, value) => {propMutation}";
                     }
@@ -1036,7 +1037,7 @@ private string GenerateForType(TypeGenerationSpec typeMetadata, string metadataI
                 string typeFriendlyName = typeMetadata.TypeInfoPropertyName;
                 string typeInfoPropertyTypeRef = $"{JsonTypeInfoTypeRef}<{typeCompilableName}>";
 
-                return @$"private {typeInfoPropertyTypeRef} _{typeFriendlyName};
+                return @$"private {typeInfoPropertyTypeRef}? _{typeFriendlyName};
 public {typeInfoPropertyTypeRef} {typeFriendlyName}
 {{
     get
@@ -1058,7 +1059,7 @@ private string WrapWithCheckForCustomConverterIfRequired(string source, string t
                     return source;
                 }
 
-                return @$"{JsonConverterTypeRef} customConverter;
+                return @$"{JsonConverterTypeRef}? customConverter;
                 if ({OptionsInstanceVariableName}.Converters.Count > 0 && (customConverter = {RuntimeCustomConverterFetchingMethodName}(typeof({typeCompilableName}))) != null)
                 {{
                     _{typeFriendlyName} = {JsonMetadataServicesTypeRef}.{GetCreateValueInfoMethodRef(typeCompilableName)}({OptionsInstanceVariableName}, customConverter);
@@ -1078,7 +1079,7 @@ private string GetRootJsonContextImplementation()
 
                 sb.Append(@$"{GetLogicForDefaultSerializerOptionsInit()}
 
-private static {contextTypeRef} {DefaultContextBackingStaticVarName};
+private static {contextTypeRef}? {DefaultContextBackingStaticVarName};
 public static {contextTypeRef} Default => {DefaultContextBackingStaticVarName} ??= new {contextTypeRef}(new {JsonSerializerOptionsTypeRef}({DefaultOptionsStaticVarName}));
 
 public {contextTypeName}() : base(null, {DefaultOptionsStaticVarName})
@@ -1122,13 +1123,13 @@ private string GetFetchLogicForRuntimeSpecifiedCustomConverter()
                 }
 
                 // TODO (https://github.com/dotnet/runtime/issues/52218): use a dictionary if count > ~15.
-                return @$"private {JsonConverterTypeRef} {RuntimeCustomConverterFetchingMethodName}({TypeTypeRef} type)
+                return @$"private {JsonConverterTypeRef}? {RuntimeCustomConverterFetchingMethodName}({TypeTypeRef} type)
 {{
     {IListTypeRef}<{JsonConverterTypeRef}> converters = {OptionsInstanceVariableName}.Converters;
 
     for (int i = 0; i < converters.Count; i++)
     {{
-        {JsonConverterTypeRef} converter = converters[i];
+        {JsonConverterTypeRef}? converter = converters[i];
 
         if (converter.CanConvert(type))
         {{
diff --git a/src/libraries/System.Text.Json/ref/System.Text.Json.cs b/src/libraries/System.Text.Json/ref/System.Text.Json.cs
index 97da0d7f7faa2..3cd4bb4959d28 100644
--- a/src/libraries/System.Text.Json/ref/System.Text.Json.cs
+++ b/src/libraries/System.Text.Json/ref/System.Text.Json.cs
@@ -978,21 +978,21 @@ public static partial class JsonMetadataServices
         public static System.Text.Json.Serialization.Metadata.JsonTypeInfo<TElement[]> CreateArrayInfo<TElement>(System.Text.Json.JsonSerializerOptions options, System.Text.Json.Serialization.Metadata.JsonTypeInfo elementInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<System.Text.Json.Utf8JsonWriter, TElement[]>? serializeFunc) { throw null; }
         public static System.Text.Json.Serialization.Metadata.JsonTypeInfo<TCollection> CreateConcurrentQueueInfo<TCollection, TElement>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection>? createObjectFunc, System.Text.Json.Serialization.Metadata.JsonTypeInfo elementInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<Utf8JsonWriter, TCollection>? serializeFunc) where TCollection : System.Collections.Concurrent.ConcurrentQueue<TElement> { throw null; }
         public static System.Text.Json.Serialization.Metadata.JsonTypeInfo<TCollection> CreateConcurrentStackInfo<TCollection, TElement>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection>? createObjectFunc, System.Text.Json.Serialization.Metadata.JsonTypeInfo elementInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<Utf8JsonWriter, TCollection>? serializeFunc) where TCollection : System.Collections.Concurrent.ConcurrentStack<TElement> { throw null; }
-        public static System.Text.Json.Serialization.Metadata.JsonTypeInfo<TCollection> CreateDictionaryInfo<TCollection, TKey, TValue>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection> createObjectFunc, System.Text.Json.Serialization.Metadata.JsonTypeInfo keyInfo, System.Text.Json.Serialization.Metadata.JsonTypeInfo valueInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<System.Text.Json.Utf8JsonWriter, TCollection>? serializeFunc) where TCollection : System.Collections.Generic.Dictionary<TKey, TValue> where TKey : notnull { throw null; }
+        public static System.Text.Json.Serialization.Metadata.JsonTypeInfo<TCollection> CreateDictionaryInfo<TCollection, TKey, TValue>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection>? createObjectFunc, System.Text.Json.Serialization.Metadata.JsonTypeInfo keyInfo, System.Text.Json.Serialization.Metadata.JsonTypeInfo valueInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<System.Text.Json.Utf8JsonWriter, TCollection>? serializeFunc) where TCollection : System.Collections.Generic.Dictionary<TKey, TValue> where TKey : notnull { throw null; }
         public static JsonTypeInfo<TCollection> CreateICollectionInfo<TCollection, TElement>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection>? createObjectFunc, System.Text.Json.Serialization.Metadata.JsonTypeInfo elementInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<Utf8JsonWriter, TCollection>? serializeFunc) where TCollection : System.Collections.Generic.ICollection<TElement> { throw null; }
         public static JsonTypeInfo<TCollection> CreateIDictionaryInfo<TCollection>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection>? createObjectFunc, JsonTypeInfo stringInfo, JsonTypeInfo objectInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<Utf8JsonWriter, TCollection>? serializeFunc) where TCollection : System.Collections.IDictionary { throw null; }
-        public static JsonTypeInfo<TCollection> CreateIDictionaryInfo<TCollection, TKey, TValue>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection> createObjectFunc, JsonTypeInfo keyInfo, JsonTypeInfo valueInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<Utf8JsonWriter, TCollection>? serializeFunc) where TCollection : System.Collections.Generic.IDictionary<TKey, TValue> where TKey : notnull { throw null; }
+        public static JsonTypeInfo<TCollection> CreateIDictionaryInfo<TCollection, TKey, TValue>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection>? createObjectFunc, JsonTypeInfo keyInfo, JsonTypeInfo valueInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<Utf8JsonWriter, TCollection>? serializeFunc) where TCollection : System.Collections.Generic.IDictionary<TKey, TValue> where TKey : notnull { throw null; }
         public static JsonTypeInfo<TCollection> CreateIEnumerableInfo<TCollection>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection>? createObjectFunc, System.Text.Json.Serialization.Metadata.JsonTypeInfo elementInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<Utf8JsonWriter, TCollection>? serializeFunc) where TCollection : System.Collections.IEnumerable { throw null; }
         public static JsonTypeInfo<TCollection> CreateIEnumerableInfo<TCollection, TElement>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection>? createObjectFunc, System.Text.Json.Serialization.Metadata.JsonTypeInfo elementInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<Utf8JsonWriter, TCollection>? serializeFunc) where TCollection : System.Collections.Generic.IEnumerable<TElement> { throw null; }
         public static JsonTypeInfo<TCollection> CreateIListInfo<TCollection>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection>? createObjectFunc, System.Text.Json.Serialization.Metadata.JsonTypeInfo objectInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<Utf8JsonWriter, TCollection>? serializeFunc) where TCollection : System.Collections.IList { throw null; }
         public static JsonTypeInfo<TCollection> CreateIListInfo<TCollection, TElement>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection>? createObjectFunc, System.Text.Json.Serialization.Metadata.JsonTypeInfo elementInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<Utf8JsonWriter, TCollection>? serializeFunc) where TCollection : System.Collections.Generic.IList<TElement> { throw null; }
-        public static JsonTypeInfo<TCollection> CreateImmutableDictionaryInfo<TCollection, TKey, TValue>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection> createObjectFunc, JsonTypeInfo keyInfo, JsonTypeInfo valueInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<Utf8JsonWriter, TCollection>? serializeFunc, System.Func<System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<TKey, TValue>>, TCollection> createRangeFunc) where TCollection : System.Collections.Generic.IReadOnlyDictionary<TKey, TValue> where TKey : notnull { throw null; }
+        public static JsonTypeInfo<TCollection> CreateImmutableDictionaryInfo<TCollection, TKey, TValue>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection>? createObjectFunc, JsonTypeInfo keyInfo, JsonTypeInfo valueInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<Utf8JsonWriter, TCollection>? serializeFunc, System.Func<System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<TKey, TValue>>, TCollection> createRangeFunc) where TCollection : System.Collections.Generic.IReadOnlyDictionary<TKey, TValue> where TKey : notnull { throw null; }
         public static JsonTypeInfo<TCollection> CreateImmutableEnumerableInfo<TCollection, TElement>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection>? createObjectFunc, System.Text.Json.Serialization.Metadata.JsonTypeInfo elementInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<Utf8JsonWriter, TCollection>? serializeFunc, System.Func<System.Collections.Generic.IEnumerable<TElement>, TCollection> createRangeFunc) where TCollection : System.Collections.Generic.IEnumerable<TElement> { throw null; }
-        public static JsonTypeInfo<TCollection> CreateIReadOnlyDictionaryInfo<TCollection, TKey, TValue>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection> createObjectFunc, JsonTypeInfo keyInfo, JsonTypeInfo valueInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<Utf8JsonWriter, TCollection>? serializeFunc) where TCollection : System.Collections.Generic.IReadOnlyDictionary<TKey, TValue> where TKey : notnull { throw null; }
+        public static JsonTypeInfo<TCollection> CreateIReadOnlyDictionaryInfo<TCollection, TKey, TValue>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection>? createObjectFunc, JsonTypeInfo keyInfo, JsonTypeInfo valueInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<Utf8JsonWriter, TCollection>? serializeFunc) where TCollection : System.Collections.Generic.IReadOnlyDictionary<TKey, TValue> where TKey : notnull { throw null; }
         public static JsonTypeInfo<TCollection> CreateISetInfo<TCollection, TElement>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection>? createObjectFunc, System.Text.Json.Serialization.Metadata.JsonTypeInfo elementInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<Utf8JsonWriter, TCollection>? serializeFunc) where TCollection : System.Collections.Generic.ISet<TElement> { throw null; }
         public static System.Text.Json.Serialization.Metadata.JsonTypeInfo<TCollection> CreateListInfo<TCollection, TElement>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection>? createObjectFunc, System.Text.Json.Serialization.Metadata.JsonTypeInfo elementInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<System.Text.Json.Utf8JsonWriter, TCollection>? serializeFunc) where TCollection : System.Collections.Generic.List<TElement> { throw null; }
         public static System.Text.Json.Serialization.Metadata.JsonTypeInfo<T> CreateObjectInfo<T>(System.Text.Json.JsonSerializerOptions options, System.Text.Json.Serialization.Metadata.JsonObjectInfoValues<T> objectInfo) where T : notnull { throw null; }
-        public static System.Text.Json.Serialization.Metadata.JsonPropertyInfo CreatePropertyInfo<T>(System.Text.Json.JsonSerializerOptions options, bool isProperty, bool isPublic, bool isVirtual, System.Type declaringType, System.Text.Json.Serialization.Metadata.JsonTypeInfo propertyTypeInfo, System.Text.Json.Serialization.JsonConverter<T>? converter, System.Func<object, T>? getter, System.Action<object, T>? setter, System.Text.Json.Serialization.JsonIgnoreCondition? ignoreCondition, bool hasJsonInclude, System.Text.Json.Serialization.JsonNumberHandling? numberHandling, string propertyName, string? jsonPropertyName) { throw null; }
+        public static System.Text.Json.Serialization.Metadata.JsonPropertyInfo CreatePropertyInfo<T>(System.Text.Json.JsonSerializerOptions options, bool isProperty, bool isPublic, bool isVirtual, System.Type declaringType, System.Text.Json.Serialization.Metadata.JsonTypeInfo propertyTypeInfo, System.Text.Json.Serialization.JsonConverter<T>? converter, System.Func<object, T?>? getter, System.Action<object, T?>? setter, System.Text.Json.Serialization.JsonIgnoreCondition? ignoreCondition, bool hasJsonInclude, System.Text.Json.Serialization.JsonNumberHandling? numberHandling, string propertyName, string? jsonPropertyName) { throw null; }
         public static JsonTypeInfo<TCollection> CreateQueueInfo<TCollection, TElement>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection>? createObjectFunc, System.Text.Json.Serialization.Metadata.JsonTypeInfo elementInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<Utf8JsonWriter, TCollection>? serializeFunc) where TCollection : System.Collections.Generic.Queue<TElement> { throw null; }
         public static JsonTypeInfo<TCollection> CreateStackInfo<TCollection, TElement>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection>? createObjectFunc, System.Text.Json.Serialization.Metadata.JsonTypeInfo elementInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<Utf8JsonWriter, TCollection>? serializeFunc) where TCollection : System.Collections.Generic.Stack<TElement> { throw null; }
         public static JsonTypeInfo<TCollection> CreateStackOrQueueInfo<TCollection>(System.Text.Json.JsonSerializerOptions options, System.Func<TCollection>? createObjectFunc, System.Text.Json.Serialization.Metadata.JsonTypeInfo elementInfo, System.Text.Json.Serialization.JsonNumberHandling numberHandling, System.Action<Utf8JsonWriter, TCollection>? serializeFunc, System.Action<TCollection, object?> addFunc) where TCollection : System.Collections.IEnumerable { throw null; }
diff --git a/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonMetadataServices.Collections.cs b/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonMetadataServices.Collections.cs
index ceb1fca9ad19f..71f48ad41b0ef 100644
--- a/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonMetadataServices.Collections.cs
+++ b/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonMetadataServices.Collections.cs
@@ -75,7 +75,7 @@ public static partial class JsonMetadataServices
         /// <returns></returns>
         public static JsonTypeInfo<TCollection> CreateDictionaryInfo<TCollection, TKey, TValue>(
             JsonSerializerOptions options,
-            Func<TCollection> createObjectFunc,
+            Func<TCollection>? createObjectFunc,
             JsonTypeInfo keyInfo,
             JsonTypeInfo valueInfo,
             JsonNumberHandling numberHandling,
@@ -113,7 +113,7 @@ public static partial class JsonMetadataServices
 #pragma warning restore CS1574 // XML comment has cref attribute that could not be resolved
         public static JsonTypeInfo<TCollection> CreateImmutableDictionaryInfo<TCollection, TKey, TValue>(
             JsonSerializerOptions options,
-            Func<TCollection> createObjectFunc,
+            Func<TCollection>? createObjectFunc,
             JsonTypeInfo keyInfo,
             JsonTypeInfo valueInfo,
             JsonNumberHandling numberHandling,
@@ -148,7 +148,7 @@ public static partial class JsonMetadataServices
         /// <returns></returns>
         public static JsonTypeInfo<TCollection> CreateIDictionaryInfo<TCollection, TKey, TValue>(
             JsonSerializerOptions options,
-            Func<TCollection> createObjectFunc,
+            Func<TCollection>? createObjectFunc,
             JsonTypeInfo keyInfo,
             JsonTypeInfo valueInfo,
             JsonNumberHandling numberHandling,
@@ -181,7 +181,7 @@ public static partial class JsonMetadataServices
         /// <returns></returns>
         public static JsonTypeInfo<TCollection> CreateIReadOnlyDictionaryInfo<TCollection, TKey, TValue>(
             JsonSerializerOptions options,
-            Func<TCollection> createObjectFunc,
+            Func<TCollection>? createObjectFunc,
             JsonTypeInfo keyInfo,
             JsonTypeInfo valueInfo,
             JsonNumberHandling numberHandling,
diff --git a/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonMetadataServices.cs b/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonMetadataServices.cs
index d9552c2ea1f8c..20b33ac2e2876 100644
--- a/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonMetadataServices.cs
+++ b/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonMetadataServices.cs
@@ -39,8 +39,8 @@ public static partial class JsonMetadataServices
             Type declaringType,
             JsonTypeInfo propertyTypeInfo,
             JsonConverter<T>? converter,
-            Func<object, T>? getter,
-            Action<object, T>? setter,
+            Func<object, T?>? getter,
+            Action<object, T?>? setter,
             JsonIgnoreCondition? ignoreCondition,
             bool hasJsonInclude,
             JsonNumberHandling? numberHandling,
diff --git a/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonPropertyInfoOfT.cs b/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonPropertyInfoOfT.cs
index c7a3585c5fc6c..6111f46117c34 100644
--- a/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonPropertyInfoOfT.cs
+++ b/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonPropertyInfoOfT.cs
@@ -123,8 +123,8 @@ internal sealed class JsonPropertyInfo<T> : JsonPropertyInfo
             Type declaringType,
             JsonTypeInfo typeInfo,
             JsonConverter<T> converter,
-            Func<object, T>? getter,
-            Action<object, T>? setter,
+            Func<object, T?>? getter,
+            Action<object, T?>? setter,
             JsonIgnoreCondition? ignoreCondition,
             bool hasJsonInclude,
             JsonNumberHandling? numberHandling,
@@ -168,7 +168,7 @@ internal sealed class JsonPropertyInfo<T> : JsonPropertyInfo
             }
             else
             {
-                Get = getter;
+                Get = getter!;
                 Set = setter;
                 HasGetter = Get != null;
                 HasSetter = Set != null;