{
  "_id": { "$oid": "000000000000000000000001" },
  "Name": "Example Name",
  "SecretValue": "Some Secret Value"
}

public class MainClass
{
    [BsonId]
    public ObjectId Id { get; set; }
    public string Name { get; set; }
    public string SecretValue { get; set; }
    public OptionalClass OptionalData { get; set; }
}

public class OptionalClass
{
   public string PublicData { get; set; }
   public string PrivateData { get; set; }
}

public class ProjectedMainClassWithoutSecretData
{
    [BsonId]
    public ObjectId Id { get; set; }
    public string Name { get; set; }
    public ProjectedOptionalClassWithoutSecretData OptionalData { get; set }
    public bool HasOptionalPrivateData { get; set; }
}

public class ProjectedOptionalClassWithoutSecretData
{
    public string PublicData { get; set; }
}

async Task<ProjectedMainClassWithoutSecretData> GetPublicData(IMongoCollection<MainClass> collection, ObjectId itemId)
{
    var filter = Builders<MainClass>.Filter.Eq(x => x.Id, itemId);
    var projection = Builders<MainClass>.Projection.Expression(main => new ProjectedMainClassWithoutSecretData() {
      Id = main.Id,
      Name = main.Name,
      OptionalData = main.OptionalData != null ? new ProjectedOptionalClassWithoutSecretData() {
         PublicData = main.OptionalData.PublicData
      } : null,
      HasOptionalPrivateData = !string.IsNullOrEmpty(main.OptionalData.PrivateData)
    });
    var result = await collection.Find(filter).Project(projection).FirstOrDefaultAsync();
    return result;
}