Calculate field in a projection mongodb c#

I have the following query in mongoDB

pdb.getCollection('articulo').aggregate(
   {
        $match: { $text: { $search: "monitor" } } ,
   },
   {
        $project: {
             _id: 1,
            titulo :1,               
            ranking : {
                $avg: "$valoracion.valor"
            }
        }
    },
    {
       $sort : { 
                ranking : -1 , fechaAlta : -1               
           }         
    });

And I'm trying to convert it to a C # query (MongoDB.Driver), but I have a problem with the calculated field (average) in the projection.

My question is: How can I get a calculated field ("ranking") in the projection?

This is what I have:

 public async Task<List<T>> All<T>(string searchBy, int page, int size) where T : class, new()
    {
        try
        {
            var projectionSelect = Builders<T>.Projection.Include("_id")
                                                                    .Include("titulo")
                                                                    //.Include(new BsonDocument { { "rating", new BsonDocument { { "$avg", "$valoracion.valor" } } } }); that didn´t wokt for me


            return await _dataBase.GetCollection<T>(GetName<T>()).Aggregate().Project<T>(projectionSelect)                    
             .Sort(Builders<T>.Sort.Descending("ranking")).Skip(page).Limit(size).ToListAsync();


        }
        catch (Exception ex)
        {
            Log.Instance.LogErrors(ex);
            throw;
        }
    }

I appreciate the help in advance

Copyright Notice：Content Author:「M. Saponara」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/42769971/calculate-field-in-a-projection-mongodb-c-sharp