Reverse

Elements can be reversed in many ways. One way applies the Reverse extension method for a declarative, clear syntax. The Reverse method acts upon many collection types.

With this extension method, we receive the elements in the opposite order. Reverse can be used on lists, arrays and IEnumerable things. It is useful, but not perfect.

First example

We invoke the Reverse extension method from the System.Linq namespace. The program includes the System.Linq namespace at the top—this is important.

using System;
using System.Linq;

// Create an array.
int[] array = { 1, 2, 4 };
// Call reverse extension method on the array.
var reverse = array.Reverse();
// Write contents of array to screen.
foreach (int value in reverse)
{
    Console.WriteLine(value);
}
4
2
1

Reverse, expression

A key benefit to Reverse() here is that it can be called directly upon a query expression. So we do not need to convert anything to an array first.

using System;
using System.Linq;

int[] array = { 1, 0, 20, 40, -2 };

// Use Reverse as part of a query expression.
var higherNumbers = (from number in array
                     where number >= 10
                     select number).Reverse();

// Display results.
foreach (int value in higherNumbers)
{
    Console.WriteLine("GREATER THAN 10 REVERSED: {0}", value);
}
GREATER THAN 10 REVERSED: 40
GREATER THAN 10 REVERSED: 20

Performance

The Reverse extension method will likely have worse performance in many cases than other methods. This is because there is an iterator used in the LINQ method.

Array.Reverse

To achieve better performance, it would be possible to use the Array.Reverse method, or even use imperative statements and loops.

The Reverse extension method is effective. But it may not be ideal for all programs. The declarative, function-based syntax of Reverse is short but not optimally fast.