Question

we are having DateTime field and we are reading from MongoDB collection and deserialising into that class. Lets say if it has null in DateTime DB field and MongoDriver is trying to set null to that datetime field which is non nullable type . It s throwing error.

.FindAll().ToList() => error in here.

Any help to overcome this problem ?

Note that : We can use nullable Datetime (DateTime?) . but we need Non nullable type only in domain model. So i just want to use non nullable DateTime while serializing

Answer 1

There are two possibilities for null in this case. You could have stored an actual null in your database:

{
    _id:ObjectId(),
    MyDateTime:null
}

Or you are not storing the field at all:

{
    _id:ObjectId()
}

---

In the first case, you could handle this by creating your own serializer:

public class DateTimeSerializer : BsonBaseSerializer
{
    public override object Deserialize(BsonReader bsonReader, Type nominalType, Type actualType, IBsonSerializationOptions options)
    {
        var bsonType = bsonReader.CurrentBsonType;
        switch (bsonType)
        {
        case BsonType.Null:
            bsonReader.ReadNull();
            return new DateTime();
        case BsonType.DateTime:
            return bsonReader.ReadDateTime();
        default:
            var message = string.Format("DateTimeSerializer needs a DateTime not {0}.", bsonType);
            throw new BsonSerializationException(message);
        }
    }

    public override void Serialize(BsonWriter bsonWriter, Type nominalType, object value, IBsonSerializationOptions options)
    {
        if (value == null)
        {
            TimeSpan nowMs = DateTime.Now-new DateTime(1970,1,1);
            bsonWriter.WriteDateTime((long)nowMs.TotalMilliseconds);
        }
        else
        {
            bsonWriter.WriteString((string)value);
        }
    }
}

(In this case, giving the current date whenever a null is serialized or de-serialized)

You would then need to register this as the serializer for the DateTime type:

BsonClassMap.RegisterClassMap<MyClass>(cm =>
{
    cm.AutoMap();
    cm.GetMemberMap(mc => mc.MyDateTime).SetSerializer(new DateTimeSerializer());
});

It has to be said that it would be easier to sanitize the data at the source, so that it didn’t have nulls in the first place.

---

In the second case, this has been handled by the C# driver for MongoDB since 1.5, which version are you using? You could set the default value by registering your own class map, like the following, but as mentioned above it should no longer be required.

BsonClassMap.RegisterClassMap<MyClass>(cm =>
{
   cm.AutoMap();
   cm.GetMemberMap(mc => mc.MyDateTime).SetDefaultValue(new DateTime());
});