Rule Definition
When using compound indexes, avoid having different indexes. Indeed, Mongo DB give the compound index, when we have index on more than two column, it is always good to have the indexes put the same way, it is highly recommended to have the indexes, either both positives(ascending) or either both negatives(descending).

Remediation
Use instead indexes, either both positives(ascending) or either both negatives(descending)

Violation Code Sample

MongoCollection<Bsondoc> collection = db.GetCollection<Bsondoc>("document");

            collection.CreateIndex(new IndexKeysBuilder().Ascending("field_1")
                                                            .Descending("field_2"));

            col = client.GetDatabase("database").GetCollection<Bsondoc>("document_1");
            var keys = Builders<Bsondoc>.IndexKeys.Ascending("field_1").Descending("field_2");
            col.Indexes.CreateOne(keys);

            collection.CreateIndex(new IndexKeysBuilder().Ascending("field_1").Descending("field_2"), IndexOptions.SetBackground(true));

            collection.EnsureIndex(new IndexKeysBuilder().Ascending("field_1")
                                                        .Descending("field_2"));
                                                        
            collection.CreateIndex(new IndexKeysBuilder().Ascending("field_1")
                                        .Descending("field_2")
                                        .Ascending("field_3")
                                        .Descending("id"), IndexOptions.SetBackground(true));

            var total = client.GetDatabase("database").GetCollection<Bsondoc>("document_1").Indexes.CreateOne(Builders<Bsondoc>.IndexKeys.Ascending(x => x.field_1).Descending(y => y.field_2));

Fixed Code Sample

MongoCollection<Bsondoc> collection = db.GetCollection<Bsondoc>("document");

            collection.CreateIndex(new IndexKeysBuilder().Ascending("field_1")
                                                            .Ascending("field_2"));

            col = client.GetDatabase("Database").GetCollection<Bsondoc>("document1");
            var keys = Builders<Bsondoc>.IndexKeys.Ascending("field_1").Ascending("field_2");
            col.Indexes.CreateOne(keys);

            collection.CreateIndex(new IndexKeysBuilder().Descending("field_1").Descending("field_2"), IndexOptions.SetBackground(true));

            collection.EnsureIndex(new IndexKeysBuilder().Descending("field_1")
                                                        .Descending("field_2"));
                                                        
            collection.CreateIndex(new IndexKeysBuilder().Ascending("field_1")
                                        .Ascending("field_2")
                                        .Ascending("field_3")
                                        .Ascending("id"), IndexOptions.SetBackground(true));

            var total = client.GetDatabase("Database").GetCollection<Bsondoc>("document").Indexes.CreateOne(Builders<Bsondoc>.IndexKeys.Ascending(x => x.field_1).Ascending(y => y.field_2));