Addressed some nits and PR feedback on API.

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/ClassDataContract.cs

@@ -11,13 +11,13 @@
 using System.Threading;
 using System.Xml;
 
-using DataContractDictionary = System.Collections.Generic.Dictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
+using DataContractDictionary = System.Collections.Generic.IDictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
 
 namespace System.Runtime.Serialization
 {
     internal sealed class ClassDataContract : DataContract
     {
-        internal const string ContractTypeString = "ClassDataContract";
+        internal const string ContractTypeString = nameof(ClassDataContract);
         public override string? ContractType => ContractTypeString;
 
         public XmlDictionaryString[]? ContractNamespaces;
@@ -121,7 +121,7 @@ public override bool IsISerializable
 
         internal bool HasExtensionData => _helper.HasExtensionData;
 
-        internal string? SerialiazationExceptionMessage => _helper.SerializationExceptionMessage;
+        internal string? SerializationExceptionMessage => _helper.SerializationExceptionMessage;
         internal string? DeserializationExceptionMessage => _helper.DeserializationExceptionMessage;
 
         internal bool IsReadOnlyContract => DeserializationExceptionMessage != null;
@@ -599,10 +599,6 @@ private sealed class ClassDataContractCriticalHelper : DataContract.DataContract
             private bool _hasDataContract;
             private bool _hasExtensionData;
 
-            private XmlDictionaryString?[]? _childElementNamespaces;
-            private XmlFormatClassReaderDelegate? _xmlFormatReaderDelegate;
-            private XmlFormatClassWriterDelegate? _xmlFormatWriterDelegate;
-
             public XmlDictionaryString[]? ContractNamespaces;
             public XmlDictionaryString[]? MemberNames;
             public XmlDictionaryString[]? MemberNamespaces;
@@ -688,7 +684,7 @@ internal ClassDataContractCriticalHelper([DynamicallyAccessedMembers(DataContrac
                     {
                         if (BaseClassContract.IsReadOnlyContract)
                         {
-                            _serializationExceptionMessage = BaseClassContract.SerialiazationExceptionMessage;
+                            _serializationExceptionMessage = BaseClassContract.SerializationExceptionMessage;
                         }
                         baseMemberCount = BaseClassContract.MemberNames!.Length;
                         MemberNames = new XmlDictionaryString[Members.Count + baseMemberCount];
@@ -1193,14 +1189,6 @@ internal override DataContractDictionary? KnownDataContracts
                 [RequiresUnreferencedCode(DataContract.SerializerTrimmerWarning)]
                 get
                 {
-                    // NOTE TODO smolloy - Oddly, this shortcut was not here in NetFx.  I'm not sure this was added strictly as a perf-optimization though.
-                    // See https://github.com/dotnet/runtime/commit/5d2dba7ffcc4ad6a11a570f5bd8d1964000099ea
-                    // This shouldn't change between invocations though, right? So seems like a good thing to leave in.
-                    if (_knownDataContracts != null)
-                    {
-                        return _knownDataContracts;
-                    }
-
                     if (!_isKnownTypeAttributeChecked && UnderlyingType != null)
                     {
                         lock (this)
@@ -1264,23 +1252,11 @@ internal override bool IsISerializable
                 return ctor;
             }
 
-            internal XmlFormatClassWriterDelegate? XmlFormatWriterDelegate
-            {
-                get => _xmlFormatWriterDelegate;
-                set => _xmlFormatWriterDelegate = value;
-            }
+            internal XmlFormatClassWriterDelegate? XmlFormatWriterDelegate { get; set; }
 
-            internal XmlFormatClassReaderDelegate? XmlFormatReaderDelegate
-            {
-                get => _xmlFormatReaderDelegate;
-                set => _xmlFormatReaderDelegate = value;
-            }
+            internal XmlFormatClassReaderDelegate? XmlFormatReaderDelegate { get; set; }
 
-            public XmlDictionaryString?[]? ChildElementNamespaces
-            {
-                get => _childElementNamespaces;
-                set => _childElementNamespaces = value;
-            }
+            public XmlDictionaryString?[]? ChildElementNamespaces { get; set; }
 
             private static Type[] SerInfoCtorArgs => s_serInfoCtorArgs ??= new Type[] { typeof(SerializationInfo), typeof(StreamingContext) };
 
@@ -1317,7 +1293,7 @@ public int Compare(Member x, Member y)
         }
 
         [RequiresUnreferencedCode(DataContract.SerializerTrimmerWarning)]
-        public override DataContract BindGenericParameters(DataContract[] paramContracts, Dictionary<DataContract, DataContract> boundContracts)
+        public override DataContract BindGenericParameters(DataContract[] paramContracts, IDictionary<DataContract, DataContract> boundContracts)
         {
             Type type = UnderlyingType;
             if (!type.IsGenericType || !type.ContainsGenericParameters)

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/CollectionDataContract.cs

@@ -12,7 +12,7 @@
 using System.Threading;
 using System.Xml;
 
-using DataContractDictionary = System.Collections.Generic.Dictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
+using DataContractDictionary = System.Collections.Generic.IDictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
 
 namespace System.Runtime.Serialization
 {
@@ -77,7 +77,7 @@ internal enum CollectionKind : byte
 
     internal sealed class CollectionDataContract : DataContract
     {
-        internal const string ContractTypeString = "CollectionDataContract";
+        internal const string ContractTypeString = nameof(CollectionDataContract);
         public override string? ContractType => ContractTypeString;
 
         private XmlDictionaryString _collectionItemName;
@@ -1339,7 +1339,7 @@ private static bool IsKnownInterface(Type type)
         }
 
         [RequiresUnreferencedCode(DataContract.SerializerTrimmerWarning)]
-        public override DataContract BindGenericParameters(DataContract[] paramContracts, Dictionary<DataContract, DataContract> boundContracts)
+        public override DataContract BindGenericParameters(DataContract[] paramContracts, IDictionary<DataContract, DataContract> boundContracts)
         {
             DataContract boundContract;
             if (boundContracts.TryGetValue(this, out boundContract!))

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/DataContract.cs

@@ -14,7 +14,7 @@
 using System.Text;
 using System.Xml;
 
-using DataContractDictionary = System.Collections.Generic.Dictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
+using DataContractDictionary = System.Collections.Generic.IDictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
 
 namespace System.Runtime.Serialization
 {
@@ -274,7 +274,7 @@ internal virtual void WriteRootElement(XmlWriterDelegator writer, XmlDictionaryS
         }
 
         [RequiresUnreferencedCode(DataContract.SerializerTrimmerWarning)]
-        public virtual DataContract BindGenericParameters(DataContract[] paramContracts, Dictionary<DataContract, DataContract> boundContracts)
+        public virtual DataContract BindGenericParameters(DataContract[] paramContracts, IDictionary<DataContract, DataContract> boundContracts)
         {
             return this;
         }
@@ -2026,7 +2026,7 @@ private static void ImportKnownTypeAttributes(Type? type, Dictionary<Type, Type>
                         collectionDataContract.ItemType.GetGenericTypeDefinition() == Globals.TypeOfKeyValue)
                     {
                         DataContract itemDataContract = DataContract.GetDataContract(Globals.TypeOfKeyValuePair.MakeGenericType(collectionDataContract.ItemType.GetGenericArguments()));
-                        knownDataContracts ??= new DataContractDictionary();
+                        knownDataContracts ??= new Dictionary<XmlQualifiedName, DataContract>();
 
                         knownDataContracts.TryAdd(itemDataContract.StableName, itemDataContract);
                     }
@@ -2050,7 +2050,7 @@ internal static void CheckAndAdd(Type type, Dictionary<Type, Type> typesChecked,
             DataContract dataContract = DataContract.GetDataContract(type);
             if (nameToDataContractTable == null)
             {
-                nameToDataContractTable = new DataContractDictionary();
+                nameToDataContractTable = new Dictionary<XmlQualifiedName, DataContract>();
             }
             else if (nameToDataContractTable.TryGetValue(dataContract.StableName, out DataContract? alreadyExistingContract))
             {

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/DataContractSerializer.cs

@@ -8,7 +8,7 @@
 using System.Runtime.CompilerServices;
 using System.Xml;
 
-using DataContractDictionary = System.Collections.Generic.Dictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
+using DataContractDictionary = System.Collections.Generic.IDictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
 
 namespace System.Runtime.Serialization
 {

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/DataContractSet.cs

@@ -8,7 +8,7 @@
 using System.Text;
 using System.Xml.Schema;
 
-using DataContractDictionary = System.Collections.Generic.Dictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
+using DataContractDictionary = System.Collections.Generic.IDictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
 
 namespace System.Runtime.Serialization
 {
@@ -54,10 +54,10 @@ public DataContractSet(DataContractSet dataContractSet)
             }
         }
 
-        internal DataContractDictionary Contracts =>
-            _contracts ??= new DataContractDictionary();
+        public DataContractDictionary Contracts =>
+            _contracts ??= new Dictionary<XmlQualifiedName, DataContract>();
 
-        public Dictionary<DataContract, object> ProcessedContracts =>
+        public IDictionary<DataContract, object> ProcessedContracts =>
             _processedContracts ??= new Dictionary<DataContract, object>();
 
         private Hashtable SurrogateDataTable => _surrogateDataTable ??= new Hashtable();
@@ -515,7 +515,7 @@ internal void SetContractProcessed(DataContract dataContract)
             ProcessedContracts.Add(dataContract, dataContract);
         }
 
-        public IEnumerator<KeyValuePair<XmlQualifiedName, DataContract>> GetEnumerator()
+        internal IEnumerator<KeyValuePair<XmlQualifiedName, DataContract>> GetEnumerator()
         {
             return Contracts.GetEnumerator();
         }

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/DataMember.cs

@@ -283,7 +283,7 @@ internal bool RequiresMemberAccessForSet()
         }
 
         [RequiresUnreferencedCode(DataContract.SerializerTrimmerWarning)]
-        internal DataMember BindGenericParameters(DataContract[] paramContracts, Dictionary<DataContract, DataContract> boundContracts)
+        internal DataMember BindGenericParameters(DataContract[] paramContracts, IDictionary<DataContract, DataContract> boundContracts)
         {
             DataContract memberTypeContract = MemberTypeContract.BindGenericParameters(paramContracts, boundContracts);
             DataMember boundDataMember = new DataMember(memberTypeContract,

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/EnumDataContract.cs

@@ -14,7 +14,7 @@ namespace System.Runtime.Serialization
 {
     internal sealed class EnumDataContract : DataContract
     {
-        internal const string ContractTypeString = "EnumDataContract";
+        internal const string ContractTypeString = nameof(EnumDataContract);
         public override string? ContractType => ContractTypeString;
 
         private readonly EnumDataContractCriticalHelper _helper;

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/GenericParameterDataContract.cs

@@ -40,7 +40,7 @@ internal GenericParameterDataContractCriticalHelper(
         }
 
         [RequiresUnreferencedCode(DataContract.SerializerTrimmerWarning)]
-        public override DataContract BindGenericParameters(DataContract[] paramContracts, Dictionary<DataContract, DataContract> boundContracts)
+        public override DataContract BindGenericParameters(DataContract[] paramContracts, IDictionary<DataContract, DataContract> boundContracts)
         {
             return paramContracts[ParameterPosition];
         }

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/Json/DataContractJsonSerializer.cs

@@ -13,7 +13,8 @@
 using System.Security;
 using System.Text;
 using System.Xml;
-using DataContractDictionary = System.Collections.Generic.Dictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
+
+using DataContractDictionary = System.Collections.Generic.IDictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
 
 namespace System.Runtime.Serialization.Json
 {

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/Json/JsonXmlDataContract.cs

@@ -7,6 +7,8 @@
 using System.Text;
 using System.Xml;
 
+using DataContractDictionary = System.Collections.Generic.IDictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
+
 namespace System.Runtime.Serialization.Json
 {
     internal sealed class JsonXmlDataContract : JsonDataContract
@@ -62,12 +64,12 @@ private static List<Type> GetKnownTypesFromContext(XmlObjectSerializerContext? c
             if (context != null)
             {
                 List<XmlQualifiedName> stableNames = new List<XmlQualifiedName>();
-                Dictionary<XmlQualifiedName, DataContract>[] entries = context.scopedKnownTypes.dataContractDictionaries;
+                DataContractDictionary[] entries = context.scopedKnownTypes.dataContractDictionaries;
                 if (entries != null)
                 {
                     for (int i = 0; i < entries.Length; i++)
                     {
-                        Dictionary<XmlQualifiedName, DataContract> entry = entries[i];
+                        DataContractDictionary entry = entries[i];
                         if (entry != null)
                         {
                             foreach (KeyValuePair<XmlQualifiedName, DataContract> pair in entry)

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/PrimitiveDataContract.cs

@@ -13,7 +13,7 @@ namespace System.Runtime.Serialization
 {
     internal abstract class PrimitiveDataContract : DataContract
     {
-        internal const string ContractTypeString = "PrimitiveDataContract";
+        internal const string ContractTypeString = nameof(PrimitiveDataContract);
         public override string? ContractType => ContractTypeString;
 
         internal static readonly PrimitiveDataContract NullContract = new NullPrimitiveDataContract();

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/SchemaHelper.cs

@@ -7,7 +7,7 @@
 using System.Collections;
 using System.Collections.Generic;
 
-using SchemaObjectDictionary = System.Collections.Generic.Dictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.SchemaObjectInfo>;
+using SchemaObjectDictionary = System.Collections.Generic.IDictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.SchemaObjectInfo>;
 
 namespace System.Runtime.Serialization
 {

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/SchemaImporter.cs

@@ -12,8 +12,8 @@
 using System.Xml;
 using System.Xml.Schema;
 
-using DataContractDictionary = System.Collections.Generic.Dictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
-using SchemaObjectDictionary = System.Collections.Generic.Dictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.SchemaObjectInfo>;
+using DataContractDictionary = System.Collections.Generic.IDictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
+using SchemaObjectDictionary = System.Collections.Generic.IDictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.SchemaObjectInfo>;
 
 namespace System.Runtime.Serialization
 {
@@ -236,7 +236,7 @@ private void ImportKnownTypesForObject()
 
         internal SchemaObjectDictionary CreateSchemaObjects()
         {
-            SchemaObjectDictionary schemaObjects = new SchemaObjectDictionary();
+            SchemaObjectDictionary schemaObjects = new Dictionary<XmlQualifiedName, SchemaObjectInfo>();
             ICollection schemaList = _schemaSet.Schemas();
             List<XmlSchemaType> knownTypesForObject = new List<XmlSchemaType>();
             schemaObjects.Add(SchemaExporter.AnytypeQualifiedName, new SchemaObjectInfo(null, null, null, knownTypesForObject));
@@ -907,7 +907,7 @@ private void ImportBaseContract(XmlQualifiedName baseTypeName, ClassDataContract
                 DataContractDictionary? knownDataContracts = ancestorDataContract.KnownDataContracts;
                 if (knownDataContracts == null)
                 {
-                    knownDataContracts = new DataContractDictionary();
+                    knownDataContracts = new Dictionary<XmlQualifiedName, DataContract>();
                     ancestorDataContract.KnownDataContracts = knownDataContracts;
                 }
                 knownDataContracts.Add(dataContract.StableName, dataContract);

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/ScopedKnownTypes.cs

@@ -3,7 +3,7 @@
 
 using System;
 using System.Xml;
-using DataContractDictionary = System.Collections.Generic.Dictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
+using DataContractDictionary = System.Collections.Generic.IDictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
 
 namespace System.Runtime.Serialization
 {

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/XmlDataContract.cs

@@ -12,14 +12,14 @@
 using System.Xml;
 using System.Xml.Schema;
 using System.Xml.Serialization;
-using DataContractDictionary = System.Collections.Generic.Dictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
+using DataContractDictionary = System.Collections.Generic.IDictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
 
 namespace System.Runtime.Serialization
 {
     internal delegate IXmlSerializable CreateXmlSerializableDelegate();
     public sealed class XmlDataContract : DataContract
     {
-        internal const string ContractTypeString = "XmlDataContract";
+        internal const string ContractTypeString = nameof(XmlDataContract);
         public override string? ContractType => ContractTypeString;
 
         private readonly XmlDataContractCriticalHelper _helper;
@@ -48,7 +48,11 @@ public bool IsAnonymous
             get => _helper.IsAnonymous;
         }
 
-        public void SetIsValueType(bool isValueType) => IsValueType = isValueType;
+        public new bool IsValueType
+        {
+            get => _helper.IsValueType;
+            set => _helper.IsValueType = value;
+        }
 
         public override bool HasRoot
         {

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/XmlObjectSerializer.cs

@@ -10,7 +10,7 @@
 using System.Security;
 using System.Text;
 using System.Xml;
-using DataContractDictionary = System.Collections.Generic.Dictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
+using DataContractDictionary = System.Collections.Generic.IDictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
 
 namespace System.Runtime.Serialization
 {

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/XmlObjectSerializerContext.cs

@@ -8,7 +8,7 @@
 using System.Reflection;
 using System.Security;
 using System.Xml;
-using DataContractDictionary = System.Collections.Generic.Dictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
+using DataContractDictionary = System.Collections.Generic.IDictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
 
 namespace System.Runtime.Serialization
 {
@@ -177,7 +177,7 @@ internal virtual DataContractDictionary? SerializerKnownDataContracts
         internal static DataContractDictionary? GetDataContractsForKnownTypes(IList<Type> knownTypeList)
         {
             if (knownTypeList == null) return null;
-            DataContractDictionary dataContracts = new DataContractDictionary();
+            DataContractDictionary dataContracts = new Dictionary<XmlQualifiedName, DataContract>();
             Dictionary<Type, Type> typesChecked = new Dictionary<Type, Type>();
             for (int i = 0; i < knownTypeList.Count; i++)
             {

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/XmlObjectSerializerReadContext.cs

@@ -10,7 +10,7 @@
 using System.Text;
 using System.Xml;
 using System.Xml.Serialization;
-using DataContractDictionary = System.Collections.Generic.Dictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
+using DataContractDictionary = System.Collections.Generic.IDictionary<System.Xml.XmlQualifiedName, System.Runtime.Serialization.DataContract>;
 
 namespace System.Runtime.Serialization
 {