How to create a composite key when merging tables?

Question

ASP.NET

I have a method that fills the database with values ​​from a table.

static void PopulateDatabaseStock(OPCDBEntities context, List<StockOnHand> entries,
        out TimeSpan time)
        {
            Stopwatch timer = new Stopwatch();
            timer.Start();
            IEnumerable<StockOnHand> entriesToAdd = EFUtils.MergeEntities(context.StockOnHands, entries, x => x.StockId);

            context.StockOnHands.AddRange(entriesToAdd);
            context.SaveChanges();
            timer.Stop();
            time = timer.Elapsed;
        }

And there is a method that either adds new entries or updates existing ones

/// <summary>
        /// Helps to merge two entity lists together. 
        /// Updates entities in first list matched by provided ID in second list
        /// </summary>
        /// <param name="persistedEntities">Entities in database</param>
        /// <param name="toMergeEntities">Entities not managed by database</param>
        /// <param name="idFn">Identity function. Must return object with valid GetHashCode() and Equals()</param>
        /// <typeparam name="T">Entity class</typeparam>
        /// <typeparam name="ID">Identity class</typeparam>
        /// <returns>list of new entities which were not found in database</returns>
        public static IEnumerable<T> MergeEntities<T, ID>(IEnumerable<T> persistedEntities, 
            IEnumerable<T> toMergeEntities, Func<T, ID> idFn) where T : IUpdatableFrom<T>
        {
            Dictionary<ID, T> persistedDict = persistedEntities.ToDictionary(idFn);

            List<T> toAddEntities = new List<T>();

            foreach (var entity in toMergeEntities)
            {
                ID id = idFn.Invoke(entity);

                if (!persistedDict.ContainsKey(id))
                {
                    toAddEntities.Add(entity);
                }
                else
                {
                    persistedDict[id].TryUpdateFrom(entity);
                }
            }

            return toAddEntities;
        }

Now it checks against the primary key StockId (Guid), but I need it to check against two keys StoreId, PartNumber (because otherwise it just adds the entry again).
Please help me to solve this problem.