add flag converters nullable serialization (#857)

test/Microsoft.SqlTools.Hosting.UnitTests/Contracts/Utilities/FlagsStringConverterTests.cs

@@ -0,0 +1,65 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using System;
+using Microsoft.SqlTools.DataProtocol.Contracts.Utilities;
+using Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
+using Xunit;
+
+namespace Microsoft.SqlTools.Hosting.UnitTests.Contracts.Utilities
+{
+    public class FlagsStringConverterTests
+    {
+        [Fact]
+        public void NullableValueCanBeDeserialized()
+        {
+            var jsonObject = JObject.Parse("{\"optionalValue\": [\"First\", \"Second\"]}");
+            var contract = jsonObject.ToObject<DataContract>();
+            Assert.NotNull(contract);
+            Assert.NotNull(contract.OptionalValue);
+            Assert.Equal(TestFlags.FirstItem | TestFlags.SecondItem, contract.OptionalValue);
+        }
+
+        [Fact]
+        public void RegularValueCanBeDeserialized()
+        {
+            var jsonObject = JObject.Parse("{\"Value\": [\"First\", \"Third\"]}");
+            var contract = jsonObject.ToObject<DataContract>();
+            Assert.NotNull(contract);
+            Assert.Equal(TestFlags.FirstItem | TestFlags.ThirdItem, contract.Value);
+        }
+
+        [Fact]
+        public void ExplicitNullCanBeDeserialized()
+        {
+            var jsonObject = JObject.Parse("{\"optionalValue\": null}");
+            var contract = jsonObject.ToObject<DataContract>();
+            Assert.NotNull(contract);
+            Assert.Null(contract.OptionalValue);
+        }
+
+        [Flags]
+        [JsonConverter(typeof(FlagsStringConverter))]
+        private enum TestFlags
+        {
+            [FlagsStringConverter.SerializeValue("First")]
+            FirstItem = 1 << 0,
+
+            [FlagsStringConverter.SerializeValue("Second")]
+            SecondItem = 1 << 1,
+
+            [FlagsStringConverter.SerializeValue("Third")]
+            ThirdItem = 1 << 2,
+        }
+
+        private class DataContract
+        {
+            public TestFlags? OptionalValue { get; set; }
+
+            public TestFlags Value { get; set; }
+        }
+    }
+}