Implied resolver implementation

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerContext.cs

@@ -106,6 +106,7 @@ protected JsonSerializerContext(JsonSerializerOptions? options, bool bindOptions
                 else
                 {
                     _options = options;
+                    options.SetImpliedResolver(this);
                 }
             }
         }

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.Caching.cs

@@ -286,7 +286,7 @@ public bool Equals(JsonSerializerOptions? left, JsonSerializerOptions? right)
                     left._includeFields == right._includeFields &&
                     left._propertyNameCaseInsensitive == right._propertyNameCaseInsensitive &&
                     left._writeIndented == right._writeIndented &&
-                    NormalizeResolver(left._typeInfoResolver) == NormalizeResolver(right._typeInfoResolver) &&
+                    left.NormalizedResolver == right.NormalizedResolver &&
                     CompareLists(left._converters, right._converters) &&
                     CompareLists(left._polymorphicTypeConfigurations, right._polymorphicTypeConfigurations);
 
@@ -331,7 +331,7 @@ public int GetHashCode(JsonSerializerOptions options)
                 hc.Add(options._includeFields);
                 hc.Add(options._propertyNameCaseInsensitive);
                 hc.Add(options._writeIndented);
-                hc.Add(NormalizeResolver(options._typeInfoResolver));
+                hc.Add(options.NormalizedResolver);
                 GetHashCode(ref hc, options._converters);
                 GetHashCode(ref hc, options._polymorphicTypeConfigurations);
 
@@ -346,10 +346,6 @@ static void GetHashCode<TValue>(ref HashCode hc, ConfigurationList<TValue> list)
                 return hc.ToHashCode();
             }
 
-            // An options instance might be locked but not initialized for reflection serialization yet.
-            private static IJsonTypeInfoResolver? NormalizeResolver(IJsonTypeInfoResolver? resolver)
-                => resolver ?? DefaultJsonTypeInfoResolver.DefaultInstance;
-
 #if !NETCOREAPP
             /// <summary>
             /// Polyfill for System.HashCode.

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.cs

@@ -36,6 +36,12 @@ public sealed partial class JsonSerializerOptions
 
         // For any new option added, adding it to the options copied in the copy constructor below must be considered.
         private IJsonTypeInfoResolver? _typeInfoResolver;
+        // If _typeInfoResolver is not set, we try to use _impliedTypeInfoResolver, if that is not set we use DefaultJsonTypeInfoResolver
+        private IJsonTypeInfoResolver? _impliedTypeInfoResolver;
+
+        // Effective resolver which doesn't root and can be used for GetHashCode or Equals
+        internal IJsonTypeInfoResolver? NormalizedResolver => _typeInfoResolver ?? _impliedTypeInfoResolver ?? DefaultJsonTypeInfoResolver.DefaultInstance;
+
         private MemberAccessor? _memberAccessorStrategy;
         private JsonNamingPolicy? _dictionaryKeyPolicy;
         private JsonNamingPolicy? _jsonPropertyNamingPolicy;
@@ -173,7 +179,7 @@ public IJsonTypeInfoResolver TypeInfoResolver
             [RequiresDynamicCode(JsonSerializer.SerializationRequiresDynamicCodeMessage)]
             get
             {
-                return _typeInfoResolver ?? DefaultJsonTypeInfoResolver.RootDefaultInstance();
+                return NormalizedResolver ?? DefaultJsonTypeInfoResolver.RootDefaultInstance();
             }
             set
             {
@@ -619,6 +625,18 @@ internal MemberAccessor MemberAccessorStrategy
             }
         }
 
+        private bool _moreThanOneImpliedResolverSet;
+
+        // This is used only by JsonSerializerContext ctor which takes options.
+        // For backward compatibility we need to use context as our resolver
+        internal void SetImpliedResolver(IJsonTypeInfoResolver resolver)
+        {
+            VerifyMutable();
+
+            _moreThanOneImpliedResolverSet = _impliedTypeInfoResolver != null;
+            _impliedTypeInfoResolver = resolver;
+        }
+
         internal bool IsInitializedForReflectionSerializer { get; private set; }
         // Effective resolver, populated when enacting reflection-based fallback
         // Should not be taken into account when calculating options equality.
@@ -631,6 +649,13 @@ internal MemberAccessor MemberAccessorStrategy
         [RequiresDynamicCode(JsonSerializer.SerializationRequiresDynamicCodeMessage)]
         internal void InitializeForReflectionSerializer()
         {
+            if (_typeInfoResolver == null && _moreThanOneImpliedResolverSet)
+            {
+                ThrowHelper.ThrowInvalidOperationException_SerializerContextOptionsImmutable();
+            }
+
+            _typeInfoResolver ??= _impliedTypeInfoResolver;
+
             if (_typeInfoResolver is JsonSerializerContext ctx)
             {
                 // .NET 6 backward compatibility; use fallback to reflection serialization