Fix enum parsing

Signed-off-by: Thomas Farr <tsfarr@amazon.com>

src/OpenSearch.Net/Api/Enums.cs

@@ -113,7 +113,7 @@ private static Func<string, Enum> GetEnumStringParser(Type type)
             var isFlag = type.GetCustomAttributes(typeof(FlagsAttribute), false).Length > 0;
 
             return isFlag
-                ? s => (Enum) Convert.ChangeType(s.ToLowerInvariant().Split(',').Aggregate(0ul, (acc, value) => acc | Convert.ToUInt64(dictionary[value])), type)
+                ? s => (Enum)Enum.ToObject(type, s.ToLowerInvariant().Split(',').Aggregate(0, (acc, value) => acc | Convert.ToInt32(dictionary[value])))
                 : s => dictionary[s.ToLowerInvariant()];
         }
 	}

tests/Tests/ClientConcepts/Enums/EnumParsingTests.cs

@@ -0,0 +1,62 @@
+/* SPDX-License-Identifier: Apache-2.0
+*
+* The OpenSearch Contributors require contributions made to
+* this file be licensed under the Apache-2.0 license or a
+* compatible open source license.
+*/
+
+using System;
+using System.Runtime.Serialization;
+using FluentAssertions;
+using OpenSearch.Net;
+using OpenSearch.OpenSearch.Xunit.XunitPlumbing;
+
+namespace Tests.ClientConcepts.Enums;
+
+public class EnumParsingTests
+{
+    [U]
+    public void CanParseRegularEnum() => KnownEnums.Parse<RegularEnum>("first").Should().Be(RegularEnum.First);
+
+    [U]
+    public void CanParseEnumWithAttribute() => KnownEnums.Parse<EnumWithAttribute>("second_value").Should().Be(EnumWithAttribute.Second);
+
+    [U]
+    public void CanParseFlagsEnum() => KnownEnums.Parse<FlagsEnum>("first,third").Should().Be(FlagsEnum.First | FlagsEnum.Third);
+
+    [U]
+    public void CanParseFlagsEnumWithAttribute() => KnownEnums.Parse<FlagsEnumWithAttribute>("first_value,third_value").Should().Be(FlagsEnumWithAttribute.First | FlagsEnumWithAttribute.Third);
+
+private enum RegularEnum
+    {
+        First,
+        Second
+    }
+
+    private enum EnumWithAttribute
+    {
+        [EnumMember(Value = "first_value")]
+        First,
+        [EnumMember(Value = "second_value")]
+        Second
+    }
+
+    [Flags]
+    private enum FlagsEnum
+    {
+        First = 1 << 0,
+        Second = 1 << 1,
+        Third = 1 << 2
+    }
+
+    [Flags]
+    private enum FlagsEnumWithAttribute
+    {
+        [EnumMember(Value = "first_value")]
+        First = 1 << 0,
+        [EnumMember(Value = "second_value")]
+        Second = 1 << 1,
+        [EnumMember(Value = "third_value")]
+        Third = 1 << 2
+    }
+}