public class Thing
{
  [BsonRepresentation(BsonType.ObjectId)]
  [BsonIgnoreIfDefault]
  public string Id { get; set; }
  public string Name { get; set; }
  public Dictionary<string,string> Attributes { get; set; }
}

// Given the following

protected static IndexKeysDefinitionBuilder<TEntity> Index => Builders<TEntity>.IndexKeys;

// Some data layer code that builds indexes

var collection = database.GetCollection<Thing>("things");

await collection.Indexes.CreateOneAsync(
  new CreateIndexModel<Thing>(
    Index.Combine(
      Index.Ascending(o => o.Name),
      Index.Ascending(o => o.Attributes.Keys))));