-Refactored enum name writing

-Improved performance of reading and writing enum names
-Tests

Src/Newtonsoft.Json.Tests/JsonTextWriterAsyncTests.cs

@@ -201,6 +201,32 @@ namespace Newtonsoft.Json.Tests
     ]
 }", sw.ToString());
         }
+
+        [Test]
+        public async Task WriteLazy_Property()
+        {
+            LazyStringWriter sw = new LazyStringWriter(CultureInfo.InvariantCulture);
+
+            using (JsonTextWriter writer = new JsonTextWriter(sw))
+            {
+                writer.Indentation = 4;
+                writer.Formatting = Formatting.Indented;
+
+                await writer.WriteStartArrayAsync();
+
+                await writer.WriteStartObjectAsync();
+
+                await writer.WritePropertyNameAsync("IncompleteProp");
+
+                await writer.WriteEndArrayAsync();
+            }
+
+            StringAssert.AreEqual(@"[
+    {
+        ""IncompleteProp"": null
+    }
+]", sw.ToString());
+        }
 #endif
 
         [Test]

Src/Newtonsoft.Json.Tests/Utilities/EnumUtilsTests.cs

@@ -0,0 +1,311 @@
+#region License
+// Copyright (c) 2007 James Newton-King
+//
+// Permission is hereby granted, free of charge, to any person
+// obtaining a copy of this software and associated documentation
+// files (the "Software"), to deal in the Software without
+// restriction, including without limitation the rights to use,
+// copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the
+// Software is furnished to do so, subject to the following
+// conditions:
+//
+// The above copyright notice and this permission notice shall be
+// included in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+// OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+// HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+// WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+// OTHER DEALINGS IN THE SOFTWARE.
+#endregion
+
+#if !(NET20 || NET35)
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reflection;
+using System.Text;
+using Newtonsoft.Json.Utilities;
+#if DNXCORE50
+using Xunit;
+using Test = Xunit.FactAttribute;
+using Assert = Newtonsoft.Json.Tests.XUnitAssert;
+using TestCaseSource = Xunit.MemberDataAttribute;
+#else
+using NUnit.Framework;
+#endif
+
+namespace Newtonsoft.Json.Tests.Utilities
+{
+    public class EnumUtilsTests : TestFixtureBase
+    {
+#if DNXCORE50
+        [Theory]
+#endif
+        [TestCaseSource(nameof(Parse_TestData))]
+        public void Parse(string value, object expected)
+        {
+            Type enumType = expected.GetType();
+
+            Enum result = (Enum)EnumUtils.ParseEnumName(value, false, false, enumType);
+            Assert.AreEqual(expected, result);
+        }
+
+#if DNXCORE50
+        [Theory]
+#endif
+        [TestCaseSource(nameof(Parse_Invalid_TestData))]
+        public void Parse_Invalid(Type enumType, string value, Type exceptionType)
+        {
+            try
+            {
+                EnumUtils.ParseEnumName(value, false, false, enumType);
+            }
+            catch (Exception ex) when (ex.GetType() == exceptionType)
+            {
+                // nom nom nom
+                return;
+            }
+
+            Assert.Fail($"Expected {exceptionType.FullName} exception.");
+        }
+
+#if DNXCORE50
+        [Theory]
+#endif
+        [TestCaseSource(nameof(ToString_Format_TestData))]
+        public static void ToString_Format(Enum e, string expected)
+        {
+            EnumUtils.TryToString(e.GetType(), e, false, out string result);
+
+            Assert.AreEqual(expected, result);
+        }
+
+        #region Test data
+        // test data from https://github.com/dotnet/corefx/blob/master/src/System.Runtime/tests/System/EnumTests.cs
+        public static IEnumerable<object[]> Parse_TestData()
+        {
+            // SByte
+            yield return new object[] { "Min", SByteEnum.Min };
+            yield return new object[] { "mAx", SByteEnum.Max };
+            yield return new object[] { "1", SByteEnum.One };
+            yield return new object[] { "5", (SByteEnum)5 };
+
+            // Byte
+            yield return new object[] { "Min", ByteEnum.Min };
+            yield return new object[] { "mAx", ByteEnum.Max };
+            yield return new object[] { "1", ByteEnum.One };
+            yield return new object[] { "5", (ByteEnum)5 };
+
+            // Int16
+            yield return new object[] { "Min", Int16Enum.Min };
+            yield return new object[] { "mAx", Int16Enum.Max };
+            yield return new object[] { "1", Int16Enum.One };
+            yield return new object[] { "5", (Int16Enum)5 };
+
+            // UInt16
+            yield return new object[] { "Min", UInt16Enum.Min };
+            yield return new object[] { "mAx", UInt16Enum.Max };
+            yield return new object[] { "1", UInt16Enum.One };
+            yield return new object[] { "5", (UInt16Enum)5 };
+
+            // Int32
+            yield return new object[] { "Min", Int32Enum.Min };
+            yield return new object[] { "mAx", Int32Enum.Max };
+            yield return new object[] { "1", Int32Enum.One };
+            yield return new object[] { "5", (Int32Enum)5 };
+
+            // UInt32
+            yield return new object[] { "Min", UInt32Enum.Min };
+            yield return new object[] { "mAx", UInt32Enum.Max };
+            yield return new object[] { "1", UInt32Enum.One };
+            yield return new object[] { "5", (UInt32Enum)5 };
+
+            // Int64
+            yield return new object[] { "Min", Int64Enum.Min };
+            yield return new object[] { "mAx", Int64Enum.Max };
+            yield return new object[] { "1", Int64Enum.One };
+            yield return new object[] { "5", (Int64Enum)5 };
+
+            // UInt64
+            yield return new object[] { "Min", UInt64Enum.Min };
+            yield return new object[] { "mAx", UInt64Enum.Max };
+            yield return new object[] { "1", UInt64Enum.One };
+            yield return new object[] { "5", (UInt64Enum)5 };
+
+            // SimpleEnum
+            yield return new object[] { "Red", SimpleEnum.Red };
+            yield return new object[] { " Red", SimpleEnum.Red };
+            yield return new object[] { "Red ", SimpleEnum.Red };
+            yield return new object[] { " red ", SimpleEnum.Red };
+            yield return new object[] { "B", SimpleEnum.B };
+            yield return new object[] { "B,B", SimpleEnum.B };
+            yield return new object[] { " Red , Blue ", SimpleEnum.Red | SimpleEnum.Blue };
+            yield return new object[] { "Blue,Red,Green", SimpleEnum.Red | SimpleEnum.Blue | SimpleEnum.Green };
+            yield return new object[] { "Blue,Red,Red,Red,Green", SimpleEnum.Red | SimpleEnum.Blue | SimpleEnum.Green };
+            yield return new object[] { "Red,Blue,   Green", SimpleEnum.Red | SimpleEnum.Blue | SimpleEnum.Green };
+            yield return new object[] { "1", SimpleEnum.Red };
+            yield return new object[] { " 1 ", SimpleEnum.Red };
+            yield return new object[] { "2", SimpleEnum.Blue };
+            yield return new object[] { "99", (SimpleEnum)99 };
+            yield return new object[] { "-42", (SimpleEnum)(-42) };
+            yield return new object[] { "   -42", (SimpleEnum)(-42) };
+            yield return new object[] { "   -42 ", (SimpleEnum)(-42) };
+        }
+
+        // test data from https://github.com/dotnet/corefx/blob/master/src/System.Runtime/tests/System/EnumTests.cs
+        public static IEnumerable<object[]> Parse_Invalid_TestData()
+        {
+            // SimpleEnum
+            yield return new object[] { null, "", typeof(ArgumentNullException) };
+            yield return new object[] { typeof(SimpleEnum), null, typeof(ArgumentNullException) };
+            yield return new object[] { typeof(object), "", typeof(ArgumentException) };
+            yield return new object[] { typeof(SimpleEnum), "", typeof(ArgumentException) };
+            yield return new object[] { typeof(SimpleEnum), "    \t", typeof(ArgumentException) };
+            yield return new object[] { typeof(SimpleEnum), "Purple", typeof(ArgumentException) };
+            yield return new object[] { typeof(SimpleEnum), ",Red", typeof(ArgumentException) };
+            yield return new object[] { typeof(SimpleEnum), "Red,", typeof(ArgumentException) };
+            yield return new object[] { typeof(SimpleEnum), "B,", typeof(ArgumentException) };
+            yield return new object[] { typeof(SimpleEnum), " , , ,", typeof(ArgumentException) };
+            yield return new object[] { typeof(SimpleEnum), "Red,Blue,", typeof(ArgumentException) };
+            yield return new object[] { typeof(SimpleEnum), "Red,,Blue", typeof(ArgumentException) };
+            yield return new object[] { typeof(SimpleEnum), "Red,Blue, ", typeof(ArgumentException) };
+            yield return new object[] { typeof(SimpleEnum), "Red Blue", typeof(ArgumentException) };
+            yield return new object[] { typeof(SimpleEnum), "1,Blue", typeof(ArgumentException) };
+            yield return new object[] { typeof(SimpleEnum), "Blue,1", typeof(ArgumentException) };
+            yield return new object[] { typeof(SimpleEnum), "Blue, 1", typeof(ArgumentException) };
+            yield return new object[] { typeof(SimpleEnum), "2147483649", typeof(OverflowException) };
+            yield return new object[] { typeof(SimpleEnum), "2147483648", typeof(OverflowException) };
+        }
+
+        // test data from https://github.com/dotnet/corefx/blob/master/src/System.Runtime/tests/System/EnumTests.cs
+        public static IEnumerable<object[]> ToString_Format_TestData()
+        {
+            yield return new object[] { SByteEnum.Min, "Min" };
+            yield return new object[] { (SByteEnum)5, null };
+            yield return new object[] { SByteEnum.Max, "Max" };
+
+            yield return new object[] { ByteEnum.Min, "Min" };
+            yield return new object[] { (ByteEnum)5, null };
+            yield return new object[] { (ByteEnum)0xff, "Max" };
+            yield return new object[] { (ByteEnum)3, null };
+
+            yield return new object[] { Int16Enum.Min, "Min" };
+            yield return new object[] { (Int16Enum)5, null };
+            yield return new object[] { Int16Enum.Max, "Max" };
+            yield return new object[] { (Int16Enum)3, null };
+
+            yield return new object[] { UInt16Enum.Min, "Min" };
+            yield return new object[] { (UInt16Enum)5, null };
+            yield return new object[] { UInt16Enum.Max, "Max" };
+            yield return new object[] { (UInt16Enum)3, null };
+
+            yield return new object[] { Int32Enum.Min, "Min" };
+            yield return new object[] { (Int32Enum)5, null };
+            yield return new object[] { Int32Enum.Max, "Max" };
+            yield return new object[] { (Int32Enum)3, null };
+
+            yield return new object[] { UInt32Enum.Min, "Min" };
+            yield return new object[] { (UInt32Enum)5, null };
+            yield return new object[] { UInt32Enum.Max, "Max" };
+            yield return new object[] { (UInt32Enum)3, null };
+
+            yield return new object[] { Int64Enum.Min, "Min" };
+            yield return new object[] { (Int64Enum)5, null };
+            yield return new object[] { Int64Enum.Max, "Max" };
+            yield return new object[] { (Int64Enum)3, null };
+
+            yield return new object[] { UInt64Enum.Min, "Min" };
+            yield return new object[] { (UInt64Enum)5, null };
+            yield return new object[] { UInt64Enum.Max, "Max" };
+            yield return new object[] { (UInt64Enum)3, null };
+
+            yield return new object[] { SimpleEnum.Red, "Red" };
+            yield return new object[] { SimpleEnum.Blue, "Blue" };
+            yield return new object[] { (SimpleEnum)99, null };
+            yield return new object[] { (SimpleEnum)0, null };
+
+            yield return new object[] { AttributeTargets.Class | AttributeTargets.Delegate, "Class, Delegate" };
+        }
+        #endregion
+    }
+
+    public enum SimpleEnum
+    {
+        Red = 1,
+        Blue = 2,
+        Green = 3,
+        Green_a = 3,
+        Green_b = 3,
+        B = 4
+    }
+
+    public enum ByteEnum : byte
+    {
+        Min = byte.MinValue,
+        One = 1,
+        Two = 2,
+        Max = byte.MaxValue,
+    }
+
+    public enum SByteEnum : sbyte
+    {
+        Min = sbyte.MinValue,
+        One = 1,
+        Two = 2,
+        Max = sbyte.MaxValue,
+    }
+
+    public enum UInt16Enum : ushort
+    {
+        Min = ushort.MinValue,
+        One = 1,
+        Two = 2,
+        Max = ushort.MaxValue,
+    }
+
+    public enum Int16Enum : short
+    {
+        Min = short.MinValue,
+        One = 1,
+        Two = 2,
+        Max = short.MaxValue,
+    }
+
+    public enum UInt32Enum : uint
+    {
+        Min = uint.MinValue,
+        One = 1,
+        Two = 2,
+        Max = uint.MaxValue,
+    }
+
+    public enum Int32Enum : int
+    {
+        Min = int.MinValue,
+        One = 1,
+        Two = 2,
+        Max = int.MaxValue,
+    }
+
+    public enum UInt64Enum : ulong
+    {
+        Min = ulong.MinValue,
+        One = 1,
+        Two = 2,
+        Max = ulong.MaxValue,
+    }
+
+    public enum Int64Enum : long
+    {
+        Min = long.MinValue,
+        One = 1,
+        Two = 2,
+        Max = long.MaxValue,
+    }
+}
+#endif

Src/Newtonsoft.Json/Converters/StringEnumConverter.cs

@@ -93,13 +93,11 @@ namespace Newtonsoft.Json.Converters
 
             Enum e = (Enum)value;
 
-            string enumName = e.ToString("G");
-
-            if (char.IsNumber(enumName[0]) || enumName[0] == '-')
+            if (!EnumUtils.TryToString(e.GetType(), value, CamelCaseText, out string enumName))
             {
                 if (!AllowIntegerValues)
                 {
-                    throw JsonSerializationException.Create(null, writer.ContainerPath, "Integer value {0} is not allowed.".FormatWith(CultureInfo.InvariantCulture, enumName), null);
+                    throw JsonSerializationException.Create(null, writer.ContainerPath, "Integer value {0} is not allowed.".FormatWith(CultureInfo.InvariantCulture, e.ToString("D")), null);
                 }
 
                 // enum value has no name so write number
@@ -107,11 +105,7 @@ namespace Newtonsoft.Json.Converters
             }
             else
             {
-                Type enumType = e.GetType();
-
-                string finalName = EnumUtils.ToEnumName(enumType, enumName, CamelCaseText);
-
-                writer.WriteValue(finalName);
+                writer.WriteValue(enumName);
             }
         }
 

Src/Newtonsoft.Json/JsonWriter.Async.cs

@@ -293,7 +293,7 @@ namespace Newtonsoft.Json
                     t = WriteNullAsync(cancellationToken);
                     if (!t.IsCompletedSucessfully())
                     {
-                        return AwaitProperty(t, levelsToComplete, token, type, cancellationToken);
+                        return AwaitProperty(t, levelsToComplete, token, cancellationToken);
                     }
                 }
 
@@ -304,7 +304,7 @@ namespace Newtonsoft.Json
                         t = WriteIndentAsync(cancellationToken);
                         if (!t.IsCompletedSucessfully())
                         {
-                            return AwaitIndent(t, levelsToComplete, token, type, cancellationToken);
+                            return AwaitIndent(t, levelsToComplete, token, cancellationToken);
                         }
                     }
                 }
@@ -312,7 +312,7 @@ namespace Newtonsoft.Json
                 t = WriteEndAsync(token, cancellationToken);
                 if (!t.IsCompletedSucessfully())
                 {
-                    return AwaitEnd(t, levelsToComplete, type, cancellationToken);
+                    return AwaitEnd(t, levelsToComplete, cancellationToken);
                 }
 
                 UpdateCurrentState();
@@ -321,7 +321,7 @@ namespace Newtonsoft.Json
             return AsyncUtils.CompletedTask;
 
             // Local functions, params renamed (capitalized) so as not to capture and allocate when calling async
-            async Task AwaitProperty(Task task, int LevelsToComplete, JsonToken token, JsonContainerType Type, CancellationToken CancellationToken)
+            async Task AwaitProperty(Task task, int LevelsToComplete, JsonToken token, CancellationToken CancellationToken)
             {
                 await task.ConfigureAwait(false);
 
@@ -338,10 +338,10 @@ namespace Newtonsoft.Json
 
                 UpdateCurrentState();
 
-                await AwaitRemaining(LevelsToComplete, Type, CancellationToken).ConfigureAwait(false);
+                await AwaitRemaining(LevelsToComplete, CancellationToken).ConfigureAwait(false);
             }
 
-            async Task AwaitIndent(Task task, int LevelsToComplete, JsonToken token, JsonContainerType Type, CancellationToken CancellationToken)
+            async Task AwaitIndent(Task task, int LevelsToComplete, JsonToken token, CancellationToken CancellationToken)
             {
                 await task.ConfigureAwait(false);
 
@@ -351,10 +351,10 @@ namespace Newtonsoft.Json
 
                 UpdateCurrentState();
 
-                await AwaitRemaining(LevelsToComplete, Type, CancellationToken).ConfigureAwait(false);
+                await AwaitRemaining(LevelsToComplete, CancellationToken).ConfigureAwait(false);
             }
 
-            async Task AwaitEnd(Task task, int LevelsToComplete, JsonContainerType Type, CancellationToken CancellationToken)
+            async Task AwaitEnd(Task task, int LevelsToComplete, CancellationToken CancellationToken)
             {
                 await task.ConfigureAwait(false);
 
@@ -362,10 +362,10 @@ namespace Newtonsoft.Json
 
                 UpdateCurrentState();
 
-                await AwaitRemaining(LevelsToComplete, Type, CancellationToken).ConfigureAwait(false);
+                await AwaitRemaining(LevelsToComplete, CancellationToken).ConfigureAwait(false);
             }
 
-            async Task AwaitRemaining(int LevelsToComplete, JsonContainerType Type, CancellationToken CancellationToken)
+            async Task AwaitRemaining(int LevelsToComplete, CancellationToken CancellationToken)
             {
                 while (LevelsToComplete-- > 0)
                 {

Src/Newtonsoft.Json/JsonWriter.cs

@@ -82,7 +82,7 @@ namespace Newtonsoft.Json
             State[] errorStates = StateArrayTempate[0];
             State[] valueStates = StateArrayTempate[7];
 
-            TypeValuesAndNames enumValuesAndNames = EnumUtils.GetEnumValuesAndNames(typeof(JsonToken));
+            EnumInfo enumValuesAndNames = EnumUtils.GetEnumValuesAndNames(typeof(JsonToken));
 
             foreach (ulong valueToken in enumValuesAndNames.Values)
             {

Src/Newtonsoft.Json/Schema/JsonSchemaGenerator.cs

@@ -309,7 +309,7 @@ namespace Newtonsoft.Json.Schema
                         {
                             CurrentSchema.Enum = new List<JToken>();
 
-                            TypeValuesAndNames enumValues = EnumUtils.GetEnumValuesAndNames(type);
+                            EnumInfo enumValues = EnumUtils.GetEnumValuesAndNames(type);
                             for (int i = 0; i < enumValues.Names.Length; i++)
                             {
                                 ulong v = enumValues.Values[i];

Src/Newtonsoft.Json/Utilities/EnumUtils.cs

@@ -25,6 +25,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Globalization;
 using System.Runtime.Serialization;
 #if !HAVE_LINQ
@@ -33,16 +34,18 @@ using Newtonsoft.Json.Utilities.LinqBridge;
 using System.Linq;
 #endif
 using System.Reflection;
+using System.Text;
 
 namespace Newtonsoft.Json.Utilities
 {
     internal static class EnumUtils
     {
         private const char EnumSeparatorChar = ',';
+        private const string EnumSeparatorString = ", ";
 
-        private static readonly ThreadSafeStore<Type, TypeValuesAndNames> ValuesAndNamesPerEnum = new ThreadSafeStore<Type, TypeValuesAndNames>(InitializeValuesAndNames);
+        private static readonly ThreadSafeStore<Type, EnumInfo> ValuesAndNamesPerEnum = new ThreadSafeStore<Type, EnumInfo>(InitializeValuesAndNames);
 
-        private static TypeValuesAndNames InitializeValuesAndNames(Type enumType)
+        private static EnumInfo InitializeValuesAndNames(Type enumType)
         {
             string[] names = Enum.GetNames(enumType);
             string[] resolvedNames = new string[names.Length];
@@ -72,7 +75,9 @@ namespace Newtonsoft.Json.Utilities
                 resolvedNames[i] = resolvedName;
             }
 
-            return new TypeValuesAndNames(values, names, resolvedNames);
+            bool isFlags = enumType.IsDefined(typeof(FlagsAttribute), false);
+
+            return new EnumInfo(isFlags, values, names, resolvedNames);
         }
 
         public static IList<T> GetFlagsValues<T>(T value) where T : struct
@@ -87,7 +92,7 @@ namespace Newtonsoft.Json.Utilities
             Type underlyingType = Enum.GetUnderlyingType(value.GetType());
 
             ulong num = ToUInt64(value);
-            TypeValuesAndNames enumNameValues = GetEnumValuesAndNames(enumType);
+            EnumInfo enumNameValues = GetEnumValuesAndNames(enumType);
             IList<T> selectedFlagsValues = new List<T>();
 
             for (int i = 0; i < enumNameValues.Values.Length; i++)
@@ -118,34 +123,103 @@ namespace Newtonsoft.Json.Utilities
             return ParseEnum(t, enumText, disallowValue);
         }
 
-        public static string ToEnumName(Type enumType, string enumText, bool camelCaseText)
+        public static bool TryToString(Type enumType, object value, bool camelCaseText, out string name)
         {
-            TypeValuesAndNames enumValuesAndNames = ValuesAndNamesPerEnum.Get(enumType);
+            EnumInfo enumInfo = ValuesAndNamesPerEnum.Get(enumType);
+            ulong v = ToUInt64(value);
 
-            string[] names = enumText.Split(',');
-            for (int i = 0; i < names.Length; i++)
+            if (!enumInfo.IsFlags)
             {
-                string name = names[i].Trim();
-
-                int? matchingIndex = FindIndexByName(enumValuesAndNames.Names, name, 0, name.Length, StringComparison.Ordinal);
-                string resolvedEnumName = matchingIndex != null
-                    ? enumValuesAndNames.ResolvedNames[matchingIndex.Value]
-                    : name;
-
-                if (camelCaseText)
+                int index = Array.BinarySearch(enumInfo.Values, v);
+                if (index >= 0)
                 {
-                    resolvedEnumName = StringUtils.ToCamelCase(resolvedEnumName);
+                    name = enumInfo.ResolvedNames[index];
+                    if (camelCaseText)
+                    {
+                        name = StringUtils.ToCamelCase(name);
+                    }
+
+                    return true;
                 }
 
-                names[i] = resolvedEnumName;
+                // is number value
+                name = null;
+                return false;
+            }
+            else // These are flags OR'ed together (We treat everything as unsigned types)
+            {
+                name = InternalFlagsFormat(enumInfo, v, camelCaseText);
+                return name != null;
             }
-
-            string finalName = string.Join(", ", names);
-
-            return finalName;
         }
 
-        public static TypeValuesAndNames GetEnumValuesAndNames(Type enumType)
+        private static String InternalFlagsFormat(EnumInfo entry, ulong result, bool camelCaseText)
+        {
+            string[] resolvedNames = entry.ResolvedNames;
+            ulong[] values = entry.Values;
+
+            int index = values.Length - 1;
+            StringBuilder sb = new StringBuilder();
+            bool firstTime = true;
+            ulong saveResult = result;
+
+            // We will not optimize this code further to keep it maintainable. There are some boundary checks that can be applied
+            // to minimize the comparsions required. This code works the same for the best/worst case. In general the number of
+            // items in an enum are sufficiently small and not worth the optimization.
+            while (index >= 0)
+            {
+                if (index == 0 && values[index] == 0)
+                {
+                    break;
+                }
+
+                if ((result & values[index]) == values[index])
+                {
+                    result -= values[index];
+                    if (!firstTime)
+                    {
+                        sb.Insert(0, EnumSeparatorString);
+                    }
+
+                    string resolvedName = resolvedNames[index];
+                    sb.Insert(0, camelCaseText ? StringUtils.ToCamelCase(resolvedName) : resolvedName);
+                    firstTime = false;
+                }
+
+                index--;
+            }
+
+            string returnString;
+            if (result != 0)
+            {
+                // We were unable to represent this number as a bitwise or of valid flags
+                returnString = null; // return null so the caller knows to .ToString() the input
+            }
+            else if (saveResult == 0)
+            {
+                // For the cases when we have zero
+                if (values.Length > 0 && values[0] == 0)
+                {
+                    returnString = resolvedNames[0]; // Zero was one of the enum values.
+                    if (camelCaseText)
+                    {
+                        returnString = StringUtils.ToCamelCase(returnString);
+                    }
+                }
+                else
+                {
+                    returnString = null;
+                }
+            }
+            else
+            {
+                returnString = sb.ToString(); // Return the string representation
+            }
+
+            return returnString;
+        }
+
+        public static EnumInfo GetEnumValuesAndNames(Type enumType)
         {
             return ValuesAndNamesPerEnum.Get(enumType);
         }
@@ -193,7 +267,7 @@ namespace Newtonsoft.Json.Utilities
                 throw new ArgumentException("Type provided must be an Enum.", nameof(enumType));
             }
 
-            TypeValuesAndNames entry = ValuesAndNamesPerEnum.Get(enumType);
+            EnumInfo entry = ValuesAndNamesPerEnum.Get(enumType);
             string[] enumNames = entry.Names;
             string[] resolvedNames = entry.ResolvedNames;
             ulong[] enumValues = entry.Values;
@@ -333,15 +407,17 @@ namespace Newtonsoft.Json.Utilities
         }
     }
 
-    internal class TypeValuesAndNames
+    internal class EnumInfo
     {
-        public TypeValuesAndNames(ulong[] values, string[] names, string[] resolvedNames)
+        public EnumInfo(bool isFlags, ulong[] values, string[] names, string[] resolvedNames)
         {
+            IsFlags = isFlags;
             Values = values;
             Names = names;
             ResolvedNames = resolvedNames;
         }
 
+        public readonly bool IsFlags;
         public readonly ulong[] Values;
         public readonly string[] Names;
         public readonly string[] ResolvedNames;