public class Author
{
    public string FullName { get; set; }
}

public class AuthorDTO
{
    public string AuthorName { get; set; }
}

ConventionRegistry.Register(
    name: "CustomConventionPack",
    conventions: new ConventionPack { new CamelCaseElementNameConvention() },
    filter: _ => true);

var collection = new MongoClient("mongodb://localhost").GetDatabase("test").GetCollection<Author>("Author");

await collection.Database.DropCollectionAsync("Author");

await collection.InsertOneAsync(new Author { FullName = "author name" });

var projectStage = PipelineStageDefinitionBuilder.Project<Author, AuthorDTO>(
    a => new AuthorDTO
    {
        AuthorName = a.FullName
    });

var resultsFacet = AggregateFacet.Create<Author, AuthorDTO>("_results", new[] { projectStage });

var res = await collection
    .Aggregate()
    //.AppendStage(projectStage)
    .Facet(resultsFacet)
    .ToListAsync();

var authorName = res[0].Facets[0].Output<AuthorDTO>()[0].AuthorName;