Fix DateTimeKind collisions in dictionary encoding and mark INT96 DateTimeKind as Unspecified

src/Parquet.Test/Types/EndToEndDateTimeKindTest.cs

@@ -0,0 +1,35 @@
+using System;
+using System.Globalization;
+using System.Linq;
+using System.Threading.Tasks;
+using Parquet.Data;
+using Parquet.Schema;
+using Xunit;
+
+namespace Parquet.Test.Types {
+    public class EndToEndDateTimeKindTest : TestBase {
+        private static DateTime BaseDate => DateTime.Parse("2020-06-10T11:12:13", CultureInfo.InvariantCulture);
+        private static DateTime UtcDate => DateTime.SpecifyKind(BaseDate, DateTimeKind.Utc);
+        private static DateTime UnknownDate => DateTime.SpecifyKind(BaseDate, DateTimeKind.Unspecified);
+        private static DateTime LocalDate => DateTime.SpecifyKind(BaseDate, DateTimeKind.Local);
+
+        private static DateTime ExpectedUnspecified => DateTime.SpecifyKind(BaseDate, DateTimeKind.Unspecified);
+        private static DateTime ExpectedLocal => DateTime.SpecifyKind(LocalDate.ToUniversalTime(), DateTimeKind.Unspecified);
+
+        [Fact]
+        public async Task Int96_dates_round_trip_as_unknown() {
+            var schema = new ParquetSchema(new DataField<DateTime>("datetime"));
+            DataField field = schema.DataFields[0];
+            var column = new DataColumn(field, new[] { UtcDate, UnknownDate, LocalDate });
+            DataColumn? actualColumn = await WriteReadSingleColumn(column);
+            Assert.NotNull(actualColumn);
+
+            DateTime[] actualValues = Assert.IsType<DateTime[]>(actualColumn.Data);
+            Assert.Equal(3, actualValues.Length);
+            Assert.Equal(ExpectedUnspecified, actualValues[0]); // utc will be converted to unspecified in the round trip
+            Assert.Equal(ExpectedUnspecified, actualValues[1]);
+            Assert.Equal(ExpectedLocal, actualValues[2]);
+            Assert.True(actualValues.All(d => d.Kind == DateTimeKind.Unspecified));
+        }
+    }
+}

src/Parquet/Data/NanoTime.cs

@@ -77,7 +77,7 @@ public static implicit operator DateTime(NanoTime nanoTime) {
         long timeOfDayTicks = nanoTime._timeOfDayNanos / 100;
 
         var result = new DateTime(Year, Month, Day,
-           0, 0, 0, DateTimeKind.Utc);
+           0, 0, 0, DateTimeKind.Unspecified);
         result = result.AddTicks(timeOfDayTicks);
 
         return result;