MongoDB C# driver deserialization introduce discrepancies

Working with Data Drivers
1

Hi folks,

I’m using c# 2.10.4 driver, and encountered data discrepancy using this code

    public void testDeserialization()
    {
        string jsonString = "{ a: 1, b: 0.5710711036366942 }";
        JsonReader reader = new JsonReader(jsonString);
        var context = BsonDeserializationContext.CreateRoot(reader);

        BsonDocument doc = BsonDocumentSerializer.Instance.Deserialize(context);
        Console.WriteLine(doc.ToString());
    }

This simple test code will output below, there is one more ‘2’ digit in the last!
{ “a” : 1, “b” : 0.57107110363669422 }

Seems this is related to double/decimal deserialization, any idea how to resolve this discrepancy?
Thanks in advance

2

Actually I’m using BsonDocument.ToJson() extension.

Console.WriteLine(doc.ToJson());

This will output the same results with discrepancy

4

Hi @Jiaxing_Song,

Seems this is related to double/decimal deserialization,

I don’t think this is specifically related to the MongoDB C# driver, but instead due to the binary representation of double and float numbers in C#. To elaborate, C# represents double and float numbers in binary, and unable to represent many decimal numbers accurately. Depending on the requested precision of the ToString() conversion, the binary value would be to the closest binary equivalent.

To illustrate, see this simple code:

    var x = 0.5710711036366942D; 
    Console.WriteLine(x.ToString("G17"));
    // outputs 
    // 0.57107110363669422

any idea how to resolve this discrepancy?

If you have control over the input string to be parsed, you could explicitly express the decimal format with NumberDecimal(). For example, you could use

    string jsonString = "{ a: 1, b: NumberDecimal('0.5710711036366942') }";
    ...
    // output
    // { "a" : 1, "b" : NumberDecimal("0.5710711036366942") }

Depending on your use case, you could also try to deserialise into a class and truncate, i.e.

    public class MyClass {
        public int a { get; set; }
        [BsonRepresentation(BsonType.Decimal128, AllowTruncation=true)]
        public decimal b { get; set; }
    }

    ... 
    
    BsonDocument doc = BsonDocumentSerializer.Instance.Deserialize(context);
    MyClass foo = BsonSerializer.Deserialize<MyClass>(doc);
    Console.WriteLine(foo.ToJson());
    // output
    // { "a" : 1, "b" : NumberDecimal("0.5710711036366942") }

Regards,
Wan.

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