Workaround for updating nested objects using the MongoDB C# driver. The driver does not support multiple positional operators in one statement, making multi-level schema tedious to use.

positionalOperator.cs

public static void EditNested(string collectionName, string primaryKey, string secondaryKey, string tertiaryKey,
            ArrayList parameters, PropertyInfo[] fields)
        {
            var collection = Mongo.Connect(out _client, out _database, collectionName);
            var filter = Builders<BsonDocument>.Filter.Eq(someField, primaryKey);
            var result = BsonSerializer.Deserialize<SomeType>(collection.Find(filter).First());
            int index1 = 0, index2 = 0;

            for (int i = 0; i < result.EmbeddedArray.Length; i++)
            {
                if (result.EmbeddedArray[i].SomeProperty == secondaryKey)
                {
                    index1 = i;
                    break;
                }
            }

            for (int j = 0; j < result.EmbeddedArray[index1].DeeplyEmbeddedArray.Length; j++)
            {
                if (result.EmbeddedArray[index1].DeeplyEmbeddedArray[j].SomeOtherProperty == tertiaryKey)
                {
                    index2 = j;
                    break;
                }
            }

            var changeList = new List<UpdateDefinition<BsonDocument>>();
            for (int i = 0; i < fields.Length; i++)
            {
                changeList.Add(Builders<BsonDocument>.Update.Set("EmbeddedArray." + index1 +
                    ".DeeplyEmbeddedArray." + index2 + "." + fields[i].Name, parameters[i]));
            }

            var update = Builders<BsonDocument>.Update.Combine(changeList);
            collection.UpdateOne(filter, update);
        }