Program.cs

using MongoDB.Bson.Serialization;
using MongoDB.Bson.Serialization.Attributes;
using System;

namespace TestConsole
{
    class Program
    {
        static void Main(string[] args)
        {
            string json = "{\"Child\":{\"Height\":0.5,\"Weight\":2.9}}";

            try
            {
                var model = BsonSerializer.Deserialize<SampleClass>(json);
                Console.WriteLine("Success");
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.ToString());
            }

            Console.ReadKey();
        }
    }

    [BsonIgnoreExtraElements]
    public class SampleClass
    {
        public SubClass Child { get; set; }

        public SampleClass()
        {
            this.Child = new SubClass();
        }
    }

    [BsonIgnoreExtraElements]
    public class SubClass
    {
        public float Height { get; set; }
        public float Weight { get; set; }
    }
}

System.FormatException: An error occurred while deserializing the Child property of class TestConsole.SampleClass: An error occurred while deserializing the Weight property of class TestConsole.SubClass: Truncation resulted in data loss. ---> System.FormatException: An error occurred while deserializing the Weight property of class TestConsole.SubClass: Truncation resulted in data loss. ---> MongoDB.Bson.TruncationException: Truncation resulted in data loss.
   at MongoDB.Bson.Serialization.Options.RepresentationConverter.ToSingle(Double value)
   at MongoDB.Bson.Serialization.Serializers.SingleSerializer.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
   at MongoDB.Bson.Serialization.Serializers.SerializerBase`1.MongoDB.Bson.Serialization.IBsonSerializer.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
   at MongoDB.Bson.Serialization.IBsonSerializerExtensions.Deserialize(IBsonSerializer serializer, BsonDeserializationContext context)
   at MongoDB.Bson.Serialization.BsonClassMapSerializer`1.DeserializeMemberValue(BsonDeserializationContext context, BsonMemberMap memberMap)
   --- End of inner exception stack trace ---
   at MongoDB.Bson.Serialization.BsonClassMapSerializer`1.DeserializeMemberValue(BsonDeserializationContext context, BsonMemberMap memberMap)
   at MongoDB.Bson.Serialization.BsonClassMapSerializer`1.DeserializeClass(BsonDeserializationContext context)
   at MongoDB.Bson.Serialization.BsonClassMapSerializer`1.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
   at MongoDB.Bson.Serialization.Serializers.SerializerBase`1.MongoDB.Bson.Serialization.IBsonSerializer.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
   at MongoDB.Bson.Serialization.IBsonSerializerExtensions.Deserialize(IBsonSerializer serializer, BsonDeserializationContext context)
   at MongoDB.Bson.Serialization.BsonClassMapSerializer`1.DeserializeMemberValue(BsonDeserializationContext context, BsonMemberMap memberMap)
   --- End of inner exception stack trace ---
   at MongoDB.Bson.Serialization.BsonClassMapSerializer`1.DeserializeMemberValue(BsonDeserializationContext context, BsonMemberMap memberMap)
   at MongoDB.Bson.Serialization.BsonClassMapSerializer`1.DeserializeClass(BsonDeserializationContext context)
   at MongoDB.Bson.Serialization.BsonClassMapSerializer`1.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
   at MongoDB.Bson.Serialization.IBsonSerializerExtensions.Deserialize[TValue](IBsonSerializer`1 serializer, BsonDeserializationContext context)
   at MongoDB.Bson.Serialization.BsonSerializer.Deserialize[TNominalType](String json, Action`1 configurator)
   at TestConsole.Program.Main(String[] args) in D:\DEV\TestConsole\TestConsole\Program.cs:line 15