C# and MongoDB Update Operation

Program.cs

using System;
using MongoDB.Bson;
using MongoDB.Driver;

namespace MongoDBCRUDExample
{

    class Program
    {
        static void Main(string[] args)
        {
            MongoClient dbClient = new MongoClient("mongodb+srv://test_user:MongoDBR0cks@demodata-uymyo.mongodb.net/test?retryWrites=true&w=majority");

            var dbList = dbClient.ListDatabases().ToList();

            Console.WriteLine("The list of databases on this server is: ");
            foreach (var db in dbList)
            {
                Console.WriteLine(db);
            }

            Console.WriteLine("Connecting to sample_training.grades");

            var database = dbClient.GetDatabase("sample_training");
            var collection = database.GetCollection<BsonDocument>("grades");

            // Define a new student for the grade book.

            var document = new BsonDocument
            {
                { "student_id", 10000 },
                { "scores", new BsonArray
                    {
                    new BsonDocument{ {"type", "exam"}, {"score", 88.12334193287023 } },
                    new BsonDocument{ {"type", "quiz"}, {"score", 74.92381029342834 } },
                    new BsonDocument{ {"type", "homework"}, {"score", 89.97929384290324 } },
                    new BsonDocument{ {"type", "homework"}, {"score", 82.12931030513218 } }
                    }
                },
                { "class_id", 480}
            };

            // *********************************
            // Create Operations
            // *********************************

            // Insert the new student grade records into the database.

            Console.WriteLine("Inserting Document");
            collection.InsertOne(document);
            Console.WriteLine("Document Inserted.\n");

            // *********************************
            // Read Operations
            // *********************************

            // Find first record in the database
            var firstDocument = collection.Find(new BsonDocument()).FirstOrDefault();
            Console.WriteLine("\n**********\n");
            Console.WriteLine(firstDocument.ToString());

            // Find a specific document with a filter
            var filter = Builders<BsonDocument>.Filter.Eq("student_id", 10000);
            var studentDocument = collection.Find(filter).FirstOrDefault();
            Console.WriteLine("\n**********\n");
            Console.WriteLine(studentDocument.ToString());

            // Find all documents with an exam score equal or above 95 as a list
            //var highExamScoreFilter = Builders<BsonDocument>
            //    .Filter.Eq("scores.type", "exam") & Builders<BsonDocument>.Filter.Gte("score", 95);
            var highExamScoreFilter = Builders<BsonDocument>.Filter
                .ElemMatch<BsonValue>("scores",
                new BsonDocument { { "type", "exam" },
                    { "score", new BsonDocument { { "$gte", 95 } } }
                });
            var highExamScores = collection.Find(highExamScoreFilter).ToList();
            Console.WriteLine("\n**********\n");
            Console.WriteLine(highExamScores);

            // Find all documents with an exam score equal
            // or above 95 as an iterable

            var cursor = collection.Find(highExamScoreFilter).ToCursor();
            Console.WriteLine("\n**********\n");
            Console.WriteLine("\nHigh Scores Iterable\n");
            Console.WriteLine("\n**********\n");
            foreach (var cursorDocument in cursor.ToEnumerable())
            {
                Console.WriteLine(cursorDocument);
            }

            // Sort the exam scores by student_id
            var sort = Builders<BsonDocument>.Sort.Descending("student_id");
            var highestScore = collection.Find(highExamScoreFilter).Sort(sort).First();
            Console.WriteLine("\n**********\n");
            Console.WriteLine("\nHigh Score\n");
            Console.WriteLine("\n**********\n");
            Console.WriteLine(highestScore);


            // *********************************
            // Update Operations
            // *********************************

            // Update quiz score.
            Console.WriteLine("\n**********\n");
            Console.WriteLine("Update class_id");
            var quizUpdateFilter = Builders<BsonDocument>.Filter.Eq("student_id", 10000);

            var update = Builders<BsonDocument>.Update.Set("class_id", 483);
            var result = collection.UpdateOne(quizUpdateFilter, update);
            Console.WriteLine(result);

            // Array Updates
            Console.WriteLine("\n**********\n");
            Console.WriteLine("Update score.type.quiz array value");
            var arrayFilter = Builders<BsonDocument>.Filter
                .Eq("student_id", 10000)
                & Builders<BsonDocument>.Filter.Eq("scores.type", "quiz");
            var arrayUpdate = Builders<BsonDocument>.Update
                .Set("scores.$.score", 84.92381029342834);

            var arrayUpdateResult = collection.UpdateOne(arrayFilter, arrayUpdate);
            Console.WriteLine(arrayUpdateResult);

        }
    }
}

Hi
Thank you for this gist, I have a question though,
if a document which is included in the "filter" list doesn't have the field which is going to be updated, how can we insert that field in the document?
for example, in the line of "var update = Builders.Update.Set("class_id", 483);", if a document (or all) didn't have the field of class_id, how can we add that field with the updated value?

thank you