Note, MongoDB only serializes properties, no fields.

Program.cs

// reference MongoDB.Driver 2.11.0
using MongoDB.Bson.Serialization.Attributes;
using MongoDB.Bson.Serialization.Conventions;
using MongoDB.Driver;

void Main()
{
	var client = new MongoClient("mongodb://connection");
	var collection = client.GetDatabase("test").GetCollection<Test>("test");
	
	collection.InsertOne(new Test("key", 123, new TestX(123.45m)));
	collection.Find(t => t.Key == "key").Single().Dump();
}

public sealed class Test {
	[BsonId]
	public string Key {get;}
	public long Offset {get;}
	public TestX Data {get;}
	
	public Test(string Key, long Offset, TestX Data) {
		this.Key = Key;
		this.Offset = Offset;
		this.Data = Data;
	}
}

public sealed class TestX {
	public decimal Value { get; }
	
	public TestX(decimal Value) { this.Value = Value; }
}