LINQ Reverse Method in C#

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LINQ Reverse Method in C# with Examples

In this article, I am going to discuss the LINQ Reverse Method in C# with Examples. Please read our previous article before proceeding to this article where we discussed LINQ ThenBy and ThenByDescending Method in C# with Examples. As part of this article, we are going to discuss the following pointers in detail.

1. What is Reverse Method in C#?
2. Different Types of Examples using both Method and Query Syntax.
3. We will discuss the Reverse Method available in System.LInq and System.Collections.Generic namespaces.
4. Finally, we will discuss how to apply the LINQ Reverse method (the Reverse Method which is available in System.LInq namespace) on a collection of List<T> type.

What is Reverse Method in C#?

The LINQ Reverse method is used to reverse the data stored in a data source. That means this method will not change the data rather it simply reverses the data stored in the data source. As a result, we will get the output in reverse order.

Reverse Method in C#:

The Reverse Method in C# is implemented in two different namespaces. This method is implemented in System.LInq namespace as well as in System.Collections.Generic namespaces. Let us have a look at the signature or definition of the Reverse Method which is shown in the below image.

As you can see the Reverse Method which belongs to System.Linq namespace is implemented as an extension method on IEnumerable<TSource> interface and more importantly, this method also returns an IEnumerable<TSource> type. On the other hand, the Reverse method belongs to the System.Collections.Generic namespace is not returning any value as the return type is void. With this kept in mind, let us see some examples for a better understanding of the Reverse method in C#.

Example to Understand System.Linq Reverse Method in C#

Let us see an example to Understand the System.Linq namespace Reverse Method in C#. Please have a look at the below example, In the below example, we created one integer array to store integer numbers. And then we are calling the Reverse method on the intArray which will return a new collection of int type with data in reverse order. And then printing the data using a for each loop. In the below example, it is going to use the Reverse method which is defined inside the System.Linq namespace. Here, we are using the Method Syntax.

using System.Collections.Generic;
using System;
using System.Linq;
namespace LINQDemo
{
    class Program
    {
        static void Main(string[] args)
        {
            int[] intArray = new int[] { 10, 30, 50, 40,60,20,70,100 };
            Console.WriteLine("Before Reverse the Data");
            foreach (var number in intArray)
            {
                Console.Write(number + " ");
            }
            Console.WriteLine();

            IEnumerable<int> ArrayReversedData = intArray.Reverse();
            Console.WriteLine("After Reverse the Data");
            foreach (var number in ArrayReversedData)
            {
                Console.Write(number + " ");
            }
            
            Console.ReadKey();
        }
    }
}

Output:

Note: In the above example, if you go to the definition of the Reverse method then you will see that this Reverse method belongs to the System.Linq namespace and hence we can able to store the data in a variable of type IEnumerable<int> as the source contains integer data.

Example to Understand System.Linq Reverse Method with Query Syntax:

Let us see an example to Understand the System.Linq Reverse Method with Query Syntax. The most important point that you need to remember is that we do not have any such operator called Reverse available in Linq to write Query Syntax. So, here we need to use Mixed syntax. Let us see rewrite the previous example using Mixed Syntax.

using System.Collections.Generic;
using System;
using System.Linq;
namespace LINQDemo
{
    class Program
    {
        static void Main(string[] args)
        {
            int[] intArray = new int[] { 10, 30, 50, 40,60,20,70,100 };
            Console.WriteLine("Before Reverse the Data");
            foreach (var number in intArray)
            {
                Console.Write(number + " ");
            }
            Console.WriteLine();

            IEnumerable<int> ArrayReversedData = (from num in intArray
                                                  select num).Reverse();

            Console.WriteLine("After Reverse the Data");
            foreach (var number in ArrayReversedData)
            {
                Console.Write(number + " ");
            }
            
            Console.ReadKey();
        }
    }
}

Output:

Example to Understand System.Collections.Generic Reverse Method in C#

Let us see an example to Understand the System.Collections.Generic namespace Reverse Method in C#. In order to understand the System.Collections.Generic Reverse method, we need to create a collection of List<T> types. Please have a look at the below example, In the below example, we created one list collection to store string values. And then we are calling the Reverse method on the stringList collection which will reverse data in the original collection. And then printing the original collection data using a for each loop. In the below example, it is going to use the Reverse method which is defined inside the System.Collections.Generic namespace. As this method return type is void, so you can store the result in a variable as we did in our previous example.

using System.Collections.Generic;
using System;
using System.Linq;
namespace LINQDemo
{
    class Program
    {
        static void Main(string[] args)
        {
            List<string> stringList = new List<string>() { "Preety", "Tiwary", "Agrawal", "Priyanka", "Dewangan"};
            
            Console.WriteLine("Before Reverse the Data");
            foreach (var name in stringList)
            {
                Console.Write(name + " ");
            }
            Console.WriteLine();

            //You cannot store the data like below as this method belongs to
            //System.Collections.Generic namespace whose return type is void
            //IEnumerable<int> ArrayReversedData = stringList.Reverse();

            stringList.Reverse();
            Console.WriteLine("After Reverse the Data");
            foreach (var name in stringList)
            {
                Console.Write(name + " ");
            }

            Console.ReadKey();
        }
    }
}

If you go to the definition of the above Reverse method, then you can see this method belongs to the System.Collections.Generic namespace and the return type is Void. Now run the application and you will get the output as expected as shown in the below image.

How to Apply the LINQ Reverse Method on a Collection of List<T> Type in C#?

If you want to apply the Reverse method which belongs to System.Linq namespace on a collection of type List<T>, then first you need to convert the List<T> collection to IEnumerable or IQueryable by using the AsEnumerable() or AsQueryable() method on the data source.

If you use the AsEnumerable() method then it will convert the collection to IEnumerable whereas if you use AsQueryable() method then it will convert the collection to IQueryable. The following program shows how to apply the LINQ Reverse method on a collection of type List<T>.

using System.Collections.Generic;
using System;
using System.Linq;
namespace LINQDemo
{
    class Program
    {
        static void Main(string[] args)
        {
            List<string> stringList = new List<string>() { "Preety", "Tiwary", "Agrawal", "Priyanka", "Dewangan" };

            Console.WriteLine("Before Reverse the Data");
            foreach (var name in stringList)
            {
                Console.Write(name + " ");
            }
            Console.WriteLine();

            IEnumerable<string> ReverseData1 = stringList.AsEnumerable().Reverse();
            IQueryable<string> ReverseData2 = stringList.AsQueryable().Reverse();

            Console.WriteLine("After Reverse the Data");
            foreach (var name in ReverseData1)
            {
                Console.Write(name + " ");
            }

            Console.ReadKey();
        }
    }
}

Output:

Note: The LINQ Reverse Method in C# will return the data as IEnumerable<TSource> or IQuereable<TSource> based on how we use the LINQ method.

In the next article, I am going to discuss the Linq Aggregate Methods in C# with Examples. I hope this article gives you a very good understanding of how to use the Reverse Method in C# with Examples.