Sorting Operator: Reverse

In this tutorial, we will learn about sorting operator - Reverse in LINQ

Reverse Operator

In LINQ, the Reverse operator is used to reverse the order of the items in the sequence or collection. This operator simply inverted the sequence of items without requiring loops or additional logic. This operator does not modify the existing collection instead of this, it returns the new collection.

For instance, you have an integer array in ascending order, by using the Reverse() method, it simply reverses the order of the array in descending order.

Reverse Method in C#

The Reverse() method exists in two different namespaces:

System.Linq Namespace:

Implemented as an extension method on the IEnumerable<TSource> interface.
Returns an IEnumerable<TSource> type.
Reverses the order of elements in the collection.

System.Collections.Generic Namespace:

Not returning any value (void return type).
Useful for reversing the elements in-place within a collection.

Example of Reverse Operator

Example 1: Reverse() Method of System.Linq Namespace

Here is an example of reversing an array which contains the following elements: 60, 80, 50, 90, 10, 30, 70, 40, 20, 100, 110 and 120. We’ll apply the Reverse() method to this array and observe the results:

Example 1: Reverse() Method of System.Linq Namespace

using System;
using System.Linq;

namespace LINQDemo
{
    public class LinqPrograms
    {
        public static void Main(string[] args)
        {
            int[] intNumbers = new int[] { 60, 80, 50, 90, 10, 30, 70, 40, 20, 100, 110, 120 };

			 //Original Array
			Console.WriteLine("Original Array:");
			foreach(int item in intNumbers){
				Console.Write(item + " ");
			}
			
            //Reverse Using Method Syntax
            var MSTotalCount = intNumbers.Reverse();
			Console.WriteLine("\n\nReverse using Method Syntax:");
			foreach(int item in MSTotalCount){
				Console.Write(item + " ");
			}
			
			
            //Reverse Using Mixed Syntax
            var QSTotalCount = (from num in intNumbers
                                  select num).Reverse();
            Console.WriteLine("\n\nReverse using using Mixed Syntax: ");
			foreach(int item in QSTotalCount){
				Console.Write(item + " ");
			}
        }
    }
}

Output

Original Array:
60 80 50 90 10 30 70 40 20 100 110 120

Reverse using Method Syntax:
120 110 100 20 40 70 30 10 90 50 80 60

Reverse using using Mixed Syntax:
120 110 100 20 40 70 30 10 90 50 80 60

Example 2: Reverse() Method of System.Collections.Generic Namespace

The System.Collections.Generic namespace provides an in-place reversal method. Although it doesn’t return a new collection, it modifies the existing collection. Here’s is an example of Reverse() method of System.Collections.Generic namespace:

Example 2: Reverse() Method of System.Collections.Generic Namespace

using System;
using System.Collections.Generic;

namespace LINQDemo
{
    public class LinqPrograms
    {
        public static void Main(string[] args)
        {
            List<int> intNumbers = new List<int> { 60, 80, 50, 90, 10, 30, 70, 40, 20, 100, 110, 120 };

			 //Original Array
			Console.WriteLine("Original Array:");
			foreach(int item in intNumbers){
				Console.Write(item + " ");
			}
			
			//Reverse using Reverse Method of System.Collections.Generic
            intNumbers.Reverse();
            Console.WriteLine("\n\nAfter Reverse (In-Place):");
            foreach (var item in intNumbers)
            {
                Console.Write(item + " ");
            }
        }
    }
}

Output

Original Array:
60 80 50 90 10 30 70 40 20 100 110 120

After Reverse (In-Place):
120 110 100 20 40 70 30 10 90 50 80 60

Key Points to Remember

Reverse Is Not a Sorting Operator: Unlike OrderBy and OrderByDescending, the Reverse method in LINQ does not sort the elements of a collection. Instead, it simply reverses the order of the elements based on their current sequence. This means it doesn’t alter the natural or specified sorting order, but flips the existing order.
Deferred Execution Applies: Like many LINQ methods, Reverse() follows deferred execution.
No Effect on Empty or Single-Element Collections: If the collection contains no elements or only one element, Reverse() has no effect, because there’s nothing to reverse. This is important to remember in scenarios where your collection might be dynamic or filtered down to very few elements.
Method works on Arrays, Lists, and Other Enumerables: The Reverse method works on any collection that implements IEnumerable<T>. Whether you are working with arrays, lists, or any enumerable collection, you can use Reverse to flip the order of elements.
Does Not Modify the Original Collection: It’s important to note that Reverse() returns a new sequence with the elements in reverse order. The original collection remains unchanged.
Reverse() Is Part of LINQ to Objects: The Reverse() method is specifically part of LINQ to Objects. This means it operates on in-memory collections like lists and arrays. It does not translate into SQL when using LINQ to SQL or Entity Framework, so it's limited to client-side operations on data that has already been loaded into memory.

Learn Grouping Operators - GroupBy & ToLookup in the next tutorial.