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

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

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

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

ConventionRegistry.Register("IgnoreIfDefault", 
                            new ConventionPack { new IgnoreIfDefaultConvention(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.

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; }
}