CSHARP-3774: Parsing Enums as Strings is case-insensitive. (#621)

CSHARP-3774: Parsing Enums as Strings is case-insensitive.

Author: DmitryLukyanov

src/MongoDB.Bson/Serialization/Serializers/EnumSerializer.cs

@@ -243,7 +243,15 @@ private TEnum ConvertInt64ToEnum(long value)
             }
         }
 
-        private TEnum ConvertStringToEnum(string value) =>
-            (TEnum)Enum.Parse(typeof(TEnum), value, true);
+        private TEnum ConvertStringToEnum(string value)
+        {
+            if (Enum.TryParse<TEnum>(value, ignoreCase: false, out var result))
+            {
+                return result;
+            }
+
+            // fall back to case-insensitive parse
+            return (TEnum)Enum.Parse(typeof(TEnum), value, ignoreCase: true);
+        }
     }
 }

tests/MongoDB.Bson.Tests/Serialization/Serializers/EnumSerializerTests.cs

@@ -27,6 +27,12 @@ namespace MongoDB.Bson.Tests.Serialization.Serializers
 {
     public class EnumSerializerTests
     {
+        public enum CaseSensitiveEnum
+        {
+            AnEnumValue,
+            anenumvalue
+        }
+
         public enum EnumByte : byte
         {
             Min = byte.MinValue,
@@ -257,6 +263,30 @@ public void Deserialize_should_throw_on_overflow<TEnum>(TEnum _, string json)
             exception.Should().BeOfType<OverflowException>();
         }
 
+        [Theory]
+        [InlineData("{ x : 'AnEnumValue' }", CaseSensitiveEnum.AnEnumValue)]
+        [InlineData("{ x : 'anenumvalue' }", CaseSensitiveEnum.anenumvalue)]
+        [InlineData("{ x : 'ANENUMVALUE' }", CaseSensitiveEnum.AnEnumValue)]
+        public void Deserialize_string_should_be_caseinsensitive(string json, CaseSensitiveEnum result)
+        {
+            var subject = new EnumSerializer<CaseSensitiveEnum>(BsonType.String);
+
+            var deserialized = Deserialize(subject, ToBson(json));
+
+            deserialized.Should().Be(result);
+        }
+
+        [Fact]
+        public void Deserialize_string_should_throw_when_enum_field_is_not_found()
+        {
+            var subject = new EnumSerializer<CaseSensitiveEnum>(BsonType.String);
+
+            var json = "{ x : 'NotEnumField' }";
+            var exception = Record.Exception(() => Deserialize(subject, ToBson(json)));
+
+            exception.Should().BeOfType<ArgumentException>();
+        }
+
         [Theory]
         // EnumByte test cases
         [InlineData(EnumByte.Min, (BsonType).0, "{ x : { $numberInt : '0' } }")]