How to: Control Ordering in a PLINQ Query

.NET Framework (current version)

These examples show how to control the ordering in a PLINQ query by using the AsOrdered<TSource> extension method.

System_CAPS_ICON_warning.jpg Warning

These examples are primarily intended to demonstrate usage, and may or may not run faster than the equivalent sequential LINQ to Objects queries.

The following example preserves the ordering of the source sequence. This is sometimes necessary; for example some query operators require an ordered source sequence to produce correct results.

            var source = Enumerable.Range(9, 10000);

            // Source is ordered; let's preserve it.
            var parallelQuery = from num in source.AsParallel().AsOrdered()
                                where num % 3 == 0
                                select num;

            // Use foreach to preserve order at execution time.
            foreach (var v in parallelQuery)
                Console.Write("{0} ", v);

            // Some operators expect an ordered source sequence.
            var lowValues = parallelQuery.Take(10);

The following example shows some query operators whose source sequence is probably expected to be ordered. These operators will work on unordered sequences, but they might produce unexpected results.

        // Paste into PLINQDataSample class.
        static void SimpleOrdering()

            var customers = GetCustomers();

            // Take the first 20, preserving the original order
            var firstTwentyCustomers = customers

            foreach (var c in firstTwentyCustomers)
                Console.Write("{0} ", c.CustomerID);

            // All elements in reverse order.
            var reverseOrder = customers

            foreach (var v in reverseOrder)
                Console.Write("{0} ", v.CustomerID);

            // Get the element at a specified index. 
            var cust = customers.AsParallel()

            Console.WriteLine("Element #48 is: {0}", cust.CustomerID);


To run this method, paste it into the PLINQDataSample class in the PLINQ Data Sample project and press F5.

The following example shows how to preserve ordering for the first part of a query, then remove the ordering to increase the performance of a join clause, and then reapply ordering to the final result sequence.

        // Paste into PLINQDataSample class.
        static void OrderedThenUnordered()

            var orders = GetOrders();
            var orderDetails = GetOrderDetails();

            var q2 = orders.AsParallel()
               .Where(o => o.OrderDate < DateTime.Parse("07/04/1997"))
               .Select(o => o)
               .OrderBy(o => o.CustomerID) // Preserve original ordering for Take operation.
               .AsUnordered()  // Remove ordering constraint to make join faster.
                      ord => ord.OrderID,
                      od => od.OrderID,
                      (ord, od) =>
                          ID = ord.OrderID,
                          Customer = ord.CustomerID,
                          Product = od.ProductID
               .OrderBy(i => i.Product); // Apply new ordering to final result sequence.

            foreach (var v in q2)
                Console.WriteLine("{0} {1} {2}", v.ID, v.Customer, v.Product);


To run this method, paste it into the PLINQDataSample class in the PLINQ Data Sample project and press F5.

Parallel LINQ (PLINQ)