Issue #728 - JSON string-to-enum converters improvements:

- Added internal helper class for creating/storing string name (from attribute) to value mapping for enums.
- Added internal helper extension method for converting a string into an enum value with fallback handling.
- The new helper method is used for all custom JSON-to-enum converters now.

(cherry picked from commit 00d8a6eefe6045997df8e199b9ba46770bf875f4)

Alpaca.Markets/Helpers/AssetAttributesEnumConverter.cs

@@ -5,15 +5,15 @@
     Justification = "Object instances of this class will be created by Newtonsoft.JSON library.")]
 internal sealed class AssetAttributesEnumConverter : StringEnumConverter
 {
-    public override Object? ReadJson(
+    public override Object ReadJson(
         JsonReader reader,
         Type objectType,
         Object? existingValue,
         JsonSerializer serializer)
     {
         try
         {
-            return base.ReadJson(reader, objectType, existingValue, serializer);
+            return AssetAttributes.Unknown.FromEnumString(reader);
         }
         catch (JsonSerializationException)
         {

Alpaca.Markets/Helpers/CryptoExchangeEnumConverter.cs

@@ -5,15 +5,15 @@
     Justification = "Object instances of this class will be created by Newtonsoft.JSON library.")]
 internal sealed class CryptoExchangeEnumConverter : StringEnumConverter
 {
-    public override Object? ReadJson(
+    public override Object ReadJson(
         JsonReader reader,
         Type objectType,
         Object? existingValue,
         JsonSerializer serializer)
     {
         try
         {
-            return base.ReadJson(reader, objectType, existingValue, serializer);
+            return CryptoExchange.Unknown.FromEnumString(reader);
         }
         catch (JsonSerializationException)
         {

Alpaca.Markets/Helpers/DictionaryExtensions.cs

@@ -10,4 +10,11 @@ public static IReadOnlyDictionary<String, IReadOnlyList<TInto>> SetSymbol<TInto,
             pair => pair.Value.SetSymbol(pair.Key).EmptyIfNull<TInto, TFrom>(),
             StringComparer.Ordinal)
         ?? new Dictionary<String, IReadOnlyList<TInto>>(StringComparer.Ordinal);
+
+#if NETFRAMEWORK || NETSTANDARD2_0
+    public static TValue GetValueOrDefault<TKey, TValue>(
+        this IReadOnlyDictionary<TKey, TValue> dictionary,
+        TKey key, TValue defaultValue) =>
+        dictionary.TryGetValue(key, out var value) ? value : defaultValue;
+#endif
 }

Alpaca.Markets/Helpers/EnumExtensions.cs

@@ -1,11 +1,42 @@
-namespace Alpaca.Markets;
+using System.Reflection;
+
+namespace Alpaca.Markets;
 
 internal static class EnumExtensions
 {
+    private static class NamesHelper<
+#if NET6_0_OR_GREATER
+        [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicFields)]
+#endif
+        T> where T : struct, Enum
+    {
+        public static readonly IReadOnlyDictionary<String, T> ValuesByNames =
+            Enum.GetValues(typeof(T)).OfType<T>()
+                .ToDictionary(getJsonName, value => value);
+
+        private static string getJsonName(
+            T value) =>
+            typeof(T).GetField(Enum.GetName(typeof(T), value) ?? value.ToString())?
+                .GetCustomAttribute<EnumMemberAttribute>()?.Value ?? value.ToString();
+    }
+
     private static readonly Char[] _doubleQuotes = [ '"' ];
 
     public static String ToEnumString<T>(
         this T enumValue)
         where T : struct, Enum =>
         JsonConvert.SerializeObject(enumValue).Trim(_doubleQuotes);
+
+    public static T FromEnumString<
+#if NET6_0_OR_GREATER
+        [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicFields)]
+#endif
+        T>(
+        this T fallbackEnumValue,
+        JsonReader reader)
+        where T : struct, Enum =>
+        reader.TokenType == JsonToken.String
+            ? NamesHelper<T>.ValuesByNames.GetValueOrDefault(
+                reader.Value as String ?? String.Empty, fallbackEnumValue)
+            : fallbackEnumValue;
 }

Alpaca.Markets/Helpers/ExchangeEnumConverter.cs

@@ -5,15 +5,15 @@
     Justification = "Object instances of this class will be created by Newtonsoft.JSON library.")]
 internal sealed class ExchangeEnumConverter : StringEnumConverter
 {
-    public override Object? ReadJson(
+    public override Object ReadJson(
         JsonReader reader,
         Type objectType,
         Object? existingValue,
         JsonSerializer serializer)
     {
         try
         {
-            return base.ReadJson(reader, objectType, existingValue, serializer);
+            return Exchange.Unknown.FromEnumString(reader);
         }
         catch (JsonSerializationException)
         {

Alpaca.Markets/Helpers/OrderSideEnumConverter.cs

@@ -5,15 +5,15 @@
     Justification = "Object instances of this class will be created by Newtonsoft.JSON library.")]
 internal sealed class OrderSideEnumConverter : StringEnumConverter
 {
-    public override Object? ReadJson(
+    public override Object ReadJson(
         JsonReader reader,
         Type objectType,
         Object? existingValue,
         JsonSerializer serializer)
     {
         try
         {
-            return base.ReadJson(reader, objectType, existingValue, serializer);
+            return OrderSide.Sell.FromEnumString(reader);
         }
         catch (JsonSerializationException)
         {