BSON serializer not handling DateTimeOffsets properly

[remids]

It appears that BSON serialization deserializes DateTimeOffset instances as DateTime instances. I looked into it, and there does not appear to be any tests using DateTimeOfset in BsonReaderTests.cs.

Used the following to determine what's currently written out:

MemoryStream ms = new MemoryStream();
BsonWriter writer = new BsonWriter(ms);
writer.WriteStartArray();
writer.WriteValue(new DateTimeOffset(2000, 12, 29, 12, 30, 0, new TimeSpan(-5, 0, 0)));
writer.WriteEnd();
string bson = BytesToHex(ms.ToArray());

This gave me the following strings, for testing the reader:

Offset of 00:00: "10-00-00-00-09-30-00-40-C5-E2-BA-E3-00-00-00-00"
Offset of 05:00: "10-00-00-00-09-30-00-C0-1C-D0-B9-E3-00-00-00-00"
Offset of -05:00: "10-00-00-00-09-30-00-C0-6D-F5-BB-E3-00-00-00-00"

Using the existing BSON tests as a guide, I then tried this:

[Test]
public void ReadDateTimeOffset()
{
    // In all cases, looking at 2000-12-29 12:30:00 but with differing offset
    var dt = new DateTimeOffset(2000, 12, 29, 12, 30, 0, TimeSpan.Zero);
    byte[] data = HexToBytes("10-00-00-00-09-30-00-40-C5-E2-BA-E3-00-00-00-00"); // Timespan +00:00
    //byte[] data = HexToBytes("10-00-00-00-09-30-00-C0-1C-D0-B9-E3-00-00-00-00"); // Timespan +05:00
    //byte[] data = HexToBytes("10-00-00-00-09-30-00-C0-6D-F5-BB-E3-00-00-00-00"); // Timespan -05:00

    MemoryStream ms = new MemoryStream(data);
    BsonReader reader = new BsonReader(ms);
#pragma warning disable 612, 618
    reader.JsonNet35BinaryCompatibility = true;
#pragma warning restore 612, 618
    reader.ReadRootValueAsArray = true;
    reader.DateTimeKindHandling = DateTimeKind.Utc;

    Assert.IsTrue(reader.Read());
    Assert.AreEqual(JsonToken.StartArray, reader.TokenType);

    Assert.IsTrue(reader.Read());
    Assert.AreEqual(JsonToken.Date, reader.TokenType);
    Assert.AreEqual(dt, reader.Value);
    Assert.AreEqual(typeof(DateTimeOffset), reader.ValueType);

    Assert.IsTrue(reader.Read());
    Assert.AreEqual(JsonToken.EndArray, reader.TokenType);

    Assert.IsFalse(reader.Read());
    Assert.AreEqual(JsonToken.None, reader.TokenType);
}

The Assert comparing the value fails, it has a DateTime - not a DateTimeOffset:

Test 'Newtonsoft.Json.Tests.Bson.BsonReaderTests.ReadDateTimeOffset' failed:
Expected: 12/29/2000 12:30:00 +00:00
But was: 2000-12-29 12:30:00.000

This is also confirmed by reader.ValueType, which is System.DateTime.

I was originally trying to get to the bottom of this:
https://stackoverflow.com/questions/36944671/json-net-bson-serialization-incorrectly-handling-datetimeoffset

It works fine de/serializing to JSON; it's just BSON that appears to be a problem.

[JamesNK]

The BSON spec doesn't support dates with timezones - just UTC ticks. It's a known limitation.

[remids]

Apologies, I didn't realize and hadn't thought to look at the spec.