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Asked: 2026-05-20T14:37:58+00:00

I am using the official 10gen C# driver for mongoDB. I have some code

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I am using the official 10gen C# driver for mongoDB. I have some code that sets up BSON mappings, it’s ran once on application start:

BsonClassMap.RegisterClassMap<Review>(cm =>
{
    cm.AutoMap();
    cm.MapProperty(c => c.Date)
        .SetSerializationOptions(new DateTimeSerializationOptions{DateOnly = true});
    cm.UnmapProperty(c => c.Author);
});

This works fine when a Review is an embedded document and gets saved with its parent. However now I am trying to update a particular review and it involves some manual coding like so (magic strings, ugh):

collection.Update(query,
    Update.Set("Reviews.$.Date", review.Date)
    .Set("Reviews.$.Rating", review.Rating)
    .Set("Reviews.$.Title", review.Title)
    .Set("Reviews.$.Comment", review.Comment)
);

This also works… But it doesn’t honor the serialization settings for DateTime on the Review class and apparently saves the time part as well (even though it’d be midnight, since review.Date == DateTime.Today). Then when I try to retrieve that document I get an exception: [FileFormatException: TimeOfDay component for DateOnly DateTime value is not zero].

How do I fix this?

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1 Answer

  1. Editorial Team

    Wow. This is a tough one. I’ll have to think some how to best solve this. Bryan’s answer is correct that the serialization options are only used when you are serializing the whole class. But his last suggestion to call RegisterClassMap on DateTime is not right.

    But I can offer you a workaround. The reason you are getting the exception is that the DateTime you are using might be midnight in your local timezone, but it is being converted to UTC to store in the database. That’s where the non-zero time component is coming from.

    You can suppress the conversion to UTC by using:

    .Set(“Reviews.$.Date”, DateTime.SpecifyKind(review.Date, DateTimeKind.Utc))

    This should work, but let me know if it doesn’t and I’ll investigate further.

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