from the documentation:

References and namespaces

To use the C# Driver you must add references to the following DLLs:

1. MongoDB.Bson.dll
2. MongoDB.Driver.dll

As a minimum add the following using statements to your source files:

using MongoDB.Bson;
using MongoDB.Driver;

Additionally you will frequently add some of the following using statements:

using MongoDB.Driver.Builders;
using MongoDB.Driver.GridFS;
using MongoDB.Driver.Linq;

In some cases you might add some of the following using statements if you are using some of the optional parts of the C# Driver:

using MongoDB.Bson.IO;
using MongoDB.Bson.Serialization;
using MongoDB.Bson.Serialization.Attributes;
using MongoDB.Bson.Serialization.Conventions;
using MongoDB.Bson.Serialization.IdGenerators;
using MongoDB.Bson.Serialization.Options;
using MongoDB.Bson.Serialization.Serializers;
using MongoDB.Driver.Wrappers;

There are more namespaces than strictly necessary. We can compress some of these and reorganize.