The current implementation of BsonClassMap (used by BsonClassMapSerializer) in certain scenarios can fall back to creating uninitialized objects using FormatterServices.GetUninitializedObject:

https://github.com/mongodb/mongo-csharp-driver/blob/v2.11.3/src/MongoDB.Bson/Serialization/BsonClassMap.cs#L1294

Since this bypasses calling the constructors it means any code in the constructors will not be executed, which means there is a high risk that the instantiated object is not properly initialized.

During deserialization we already try to call the constructor that takes the most arguments . If no matching constructor taking arguments is found we then call the no-argument constructor (if it exists).

There doesn't seem to really be a use case for using GetUninitializedObject. The risk of creating improperly initialized objects is just too great.

Even in the probably extremely rare case where a user has a class that can only be deserialized using GetUninitializedObject, the workaround if we stop using GetUninitializedObject is as simple as adding a no-argument constructor. If a no-argument constructor already exists we wouldn't be calling GetUninitializedObject anyway.