How to ignore null values globally by calling obj.ToBsonDocument() using MongoDB C# driver?

Question

Prehistory: I have a class hierarchy (a big one) which was generated from an XML schema (XSD).

MyObj obj;

Nowadays: I need to parse a string value, which is actually an XML to an object using the generated class hierarchy and then I need to put the object into MongoDB. The code is:

private BsonDocument XmlToBson(string content)
{
    MyObj obj;
    using (var reader = new StringReader(content))
    {
        var serializer = new XmlSerializer(typeof(MyObj));
        obj = (MyObj) serializer.Deserialize(reader);
    }

    return obj.ToBsonDocument();
}

The problem: I have a lot of null children objects in the hierarchy, it looks like this in Robomongo:

I want to avoid adding null objects into the database. I know I can ignore them one by one, like described here Ignoring Default Values by using attributes or lambda expressions. But because the hierarchy is big and complex I do not like this approach. I am to lazy to list all the properties which have to be ignored when they have null values. Even more, the class can be regenerated in future and it will be a nightmare to support the list of properties which have to be ignored.

Question: How can I achieve the goal and ignore the null values globally no matter where the object is in hierarchy? I am working with MongoDB C# driver ver. 2.2.

Hey Antipod, ignoring them will be difficult if they exist and you're not removing them by a null value check. What do you mean by "ignore" btw, as in not insert them or insert another object if it's null into the column as a "placeholder"? — Nick Bull
– Nick Bull, Commented Aug 3, 2016 at 15:31
By "ignore" I mean I do not want to see them in the database. Put fake values, i.e. values which will not be used, does not sound as a good idea, I do not want to have trash in the database. And checking if something is null (how?) and remove it sounds like an overhead or probably I do not understand what you mean. — Antipod
– Antipod, Commented Aug 3, 2016 at 15:38

CodeCaster · Accepted Answer · 2021-09-08 09:27:06Z

18

You can apply the effects of most attributes to all properties while serializing by registering convention packs.

Below the IgnoreIfNullConvention is registered, implicitly applying the [IgnoreIfNull] attribute to all properties while serializing and deserializing.

var anon = new
{
    Foo = "bar",
    Baz = (string)null,
};

ConventionRegistry.Register("Ignore", 
                            new ConventionPack 
                            { 
                                new IgnoreIfNullConvention(true) 
                            }, 
                            t => true);

var bsonDocument = anon.ToBsonDocument();

This will yield a document only containing the Foo key.

When desired, you can also Remove() this convention pack by name after serialization.

edited Sep 8, 2021 at 9:27

answered Aug 3, 2016 at 15:37

CodeCaster

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1 Comment

Nick Bull Over a year ago

This is very elegant, wish I'd known about this for a long time!

Anthony Liriano · Accepted Answer · 2019-01-11 14:41:00Z

6

You can also apply the [BsonIgnoreIfNull] attribute from the MongoDB.Bson.Serialization.Attributes
namespace to a class field if you don't what that field with a null value to appear in the BSON Document.

public class Person
{
    public string Name { get; set; }

    [BsonIgnoreIfNull]
    public List<string> Children { get; set; }
}

answered Jan 11, 2019 at 14:41

Anthony Liriano

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Comments

Rafael Jusi · Accepted Answer · 2021-05-19 15:32:12Z

6

Actually, using IgnoreIfDefaultConvention can cause an unintended behavior due the fact it affects all default values. For instance, values as the list above will not be saved:

int = 0
decimal = 0
bool = false

If you want to ignore only null values, the best approach is IgnoreIfNullConvention.

Here is an example of it:

ConventionRegistry.Register("IgnoreIfNullConvention", new ConventionPack { new IgnoreIfNullConvention(true) }, t => true);

answered May 19, 2021 at 15:32

Rafael Jusi

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1 Comment

AliReza Sabouri Over a year ago

this brings another problem, what if we need to ignore empty arrays and null values but not the int decimal bool?

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