Fix 3.0->3.1 regression in JSON serializing nested concurrent dictionaries

src/System.Text.Json/src/System/Text/Json/Serialization/JsonPropertyInfoNotNullable.cs

@@ -130,7 +130,7 @@ public override Type GetDictionaryConcreteType()
 
         public override void GetDictionaryKeyAndValueFromGenericDictionary(ref WriteStackFrame writeStackFrame, out string key, out object value)
         {
-            if (writeStackFrame.CollectionEnumerator is IEnumerator<KeyValuePair<string, TRuntimeProperty>> genericEnumerator)
+            if (writeStackFrame.CollectionEnumerator is IEnumerator<KeyValuePair<string, TDeclaredProperty>> genericEnumerator)
             {
                 key = genericEnumerator.Current.Key;
                 value = genericEnumerator.Current.Value;

src/System.Text.Json/src/System/Text/Json/Serialization/JsonPropertyInfoNotNullableContravariant.cs

@@ -132,7 +132,7 @@ public override Type GetDictionaryConcreteType()
 
         public override void GetDictionaryKeyAndValueFromGenericDictionary(ref WriteStackFrame writeStackFrame, out string key, out object value)
         {
-            if (writeStackFrame.CollectionEnumerator is IEnumerator<KeyValuePair<string, TRuntimeProperty>> genericEnumerator)
+            if (writeStackFrame.CollectionEnumerator is IEnumerator<KeyValuePair<string, TDeclaredProperty>> genericEnumerator)
             {
                 key = genericEnumerator.Current.Key;
                 value = genericEnumerator.Current.Value;

src/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Read.HandleArray.cs

@@ -254,11 +254,26 @@ internal static void ApplyValueToEnumerable<TProperty>(
             else if (state.Current.IsProcessingDictionary())
             {
                 Debug.Assert(state.Current.ReturnValue != null);
-                IDictionary<string, TProperty> dictionary = (IDictionary<string, TProperty>)state.Current.JsonPropertyInfo.GetValueAsObject(state.Current.ReturnValue);
+
+                object currentDictionary = state.Current.JsonPropertyInfo.GetValueAsObject(state.Current.ReturnValue);
 
                 string key = state.Current.KeyName;
                 Debug.Assert(!string.IsNullOrEmpty(key));
-                dictionary[key] = value;
+
+                if (currentDictionary is IDictionary<string, TProperty> genericDict)
+                {
+                    Debug.Assert(!genericDict.IsReadOnly);
+                    genericDict[key] = value;
+                }
+                else if (currentDictionary is IDictionary dict)
+                {
+                    Debug.Assert(!dict.IsReadOnly);
+                    dict[key] = value;
+                }
+                else
+                {
+                    throw ThrowHelper.GetNotSupportedException_SerializationNotSupportedCollection(currentDictionary.GetType(), parentType: null, memberInfo: null);
+                }
             }
             else if (state.Current.IsProcessingIDictionaryConstructible())
             {

src/System.Text.Json/tests/Serialization/DictionaryTests.cs

@@ -3,8 +3,10 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Collections;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Collections.Immutable;
+using System.Collections.Specialized;
 using System.Reflection;
 using System.Text.Encodings.Web;
 using Xunit;
@@ -932,6 +934,112 @@ public static void DictionaryOfArrayOfDictionary()
             Assert.Equal(JsonString, json);
         }
 
+        private interface IClass { }
+
+        private class MyClass : IClass { }
+
+        private class MyNonGenericDictionary : Dictionary<string, int> { }
+
+        private class MyFactory : JsonConverterFactory
+        {
+            public override bool CanConvert(Type typeToConvert)
+            {
+                return typeToConvert == typeof(IClass) || typeToConvert == typeof(MyClass);
+            }
+
+            public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options)
+            {
+                return new MyStuffConverter();
+            }
+        }
+
+        private class MyStuffConverter : JsonConverter<IClass>
+        {
+            public override IClass Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+            {
+                return new MyClass();
+            }
+
+            public override void Write(Utf8JsonWriter writer, IClass value, JsonSerializerOptions options)
+            {
+                writer.WriteNumberValue(1);
+            }
+        }
+
+        // This method generates 316 unique test cases for nested dictionaries up to 4
+        // levels deep, along with matching JSON, encompassing the various planes of
+        // dictionaries that can be combined: generic, non-generic, BCL, user-derived,
+        // immutable, mutable, readonly, concurrent, specialized.
+        private static IEnumerable<(Type, string)> NestedDictionaryTypeData()
+        {
+            string testJson = @"{""Key"":1}";
+
+            List<Type> genericDictTypes = new List<Type>()
+            {
+                typeof(IDictionary<,>),
+                typeof(ConcurrentDictionary<,>),
+                typeof(GenericIDictionaryWrapper<,>),
+            };
+
+            List<Type> nonGenericDictTypes = new List<Type>()
+            {
+                typeof(Hashtable),
+                typeof(OrderedDictionary),
+            };
+
+            List<Type> baseDictionaryTypes = new List<Type>
+            {
+                typeof(MyNonGenericDictionary),
+                typeof(IReadOnlyDictionary<string, MyClass>),
+                typeof(ConcurrentDictionary<string, int>),
+                typeof(ImmutableDictionary<string, IClass>),
+                typeof(GenericIDictionaryWrapper<string, int?>),
+            };
+            baseDictionaryTypes.AddRange(nonGenericDictTypes);
+
+            // This method has exponential behavior which this depth value significantly impacts.
+            // Don't change this value without checking how many test cases are generated and
+            // how long the tests run for.
+            int maxTestDepth = 4;
+
+            HashSet<(Type, string)> tests = new HashSet<(Type, string)>();
+
+            for (int i = 0; i < maxTestDepth; i++)
+            {
+                List<Type> newBaseTypes = new List<Type>();
+
+                foreach (Type testType in baseDictionaryTypes)
+                {
+                    tests.Add((testType, testJson));
+
+                    foreach (Type genericType in genericDictTypes)
+                    {
+                        newBaseTypes.Add(genericType.MakeGenericType(typeof(string), testType));
+                    }
+
+                    newBaseTypes.AddRange(nonGenericDictTypes);
+                }
+
+                baseDictionaryTypes = newBaseTypes;
+                testJson = @"{""Key"":" + testJson + "}";
+            }
+
+            return tests;
+        }
+
+        [Fact]
+        public static void NestedDictionariesRoundtrip()
+        {
+            JsonSerializerOptions options = new JsonSerializerOptions();
+            options.Converters.Add(new MyFactory());
+
+            foreach ((Type dictionaryType, string testJson) in NestedDictionaryTypeData())
+            {
+                object dict = JsonSerializer.Deserialize(testJson, dictionaryType, options);
+                Assert.Equal(testJson, JsonSerializer.Serialize(dict, options));
+            }
+        }
+
         [Fact]
         public static void DictionaryOfClasses()
         {

src/System.Text.Json/tests/Serialization/TestClasses.GenericCollections.cs

@@ -936,6 +936,76 @@ IEnumerator IEnumerable.GetEnumerator()
         }
     }
 
+    public class GenericIDictionaryWrapper<TKey, TValue> : IDictionary<TKey, TValue>
+    {
+        private Dictionary<TKey, TValue> _dict = new Dictionary<TKey, TValue>();
+
+        public TValue this[TKey key] { get => ((IDictionary<TKey, TValue>)_dict)[key]; set => ((IDictionary<TKey, TValue>)_dict)[key] = value; }
+
+        public ICollection<TKey> Keys => ((IDictionary<TKey, TValue>)_dict).Keys;
+
+        public ICollection<TValue> Values => ((IDictionary<TKey, TValue>)_dict).Values;
+
+        public int Count => ((IDictionary<TKey, TValue>)_dict).Count;
+
+        public bool IsReadOnly => ((IDictionary<TKey, TValue>)_dict).IsReadOnly;
+
+        public void Add(TKey key, TValue value)
+        {
+            ((IDictionary<TKey, TValue>)_dict).Add(key, value);
+        }
+
+        public void Add(KeyValuePair<TKey, TValue> item)
+        {
+            ((IDictionary<TKey, TValue>)_dict).Add(item);
+        }
+
+        public void Clear()
+        {
+            ((IDictionary<TKey, TValue>)_dict).Clear();
+        }
+
+        public bool Contains(KeyValuePair<TKey, TValue> item)
+        {
+            return ((IDictionary<TKey, TValue>)_dict).Contains(item);
+        }
+
+        public bool ContainsKey(TKey key)
+        {
+            return ((IDictionary<TKey, TValue>)_dict).ContainsKey(key);
+        }
+
+        public void CopyTo(KeyValuePair<TKey, TValue>[] array, int arrayIndex)
+        {
+            ((IDictionary<TKey, TValue>)_dict).CopyTo(array, arrayIndex);
+        }
+
+        public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
+        {
+            return ((IDictionary<TKey, TValue>)_dict).GetEnumerator();
+        }
+
+        public bool Remove(TKey key)
+        {
+            return ((IDictionary<TKey, TValue>)_dict).Remove(key);
+        }
+
+        public bool Remove(KeyValuePair<TKey, TValue> item)
+        {
+            return ((IDictionary<TKey, TValue>)_dict).Remove(item);
+        }
+
+        public bool TryGetValue(TKey key, out TValue value)
+        {
+            return ((IDictionary<TKey, TValue>)_dict).TryGetValue(key, out value);
+        }
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return ((IDictionary<TKey, TValue>)_dict).GetEnumerator();
+        }
+    }
+
     public class StringToStringIReadOnlyDictionaryWrapper : IReadOnlyDictionary<string, string>
     {
         private Dictionary<string, string> _dictionary = new Dictionary<string, string>();