C# | Reverse the order of the elements in the entire List or in the specified range

List<T>.Reverse Method is used to reverse the order of the elements in the List<T> or a portion of it. There are two methods in the overload list of List<T>.Reverse Method as follows:

• Reverse()
• Reverse(Int32, Int32)

Reverse() Method

This method is used to reverse the order of the elements in the entire List<T>.

Syntax:

public void Reverse ();

Below programs illustrate the use of above discussed method:

Example 1:

// C# Program to reverse the order of
// the elements in the entire List<T>
using System;
using System.Collections;
using System.Collections.Generic;

class Geeks {

    // Main Method
    public static void Main()
    {

        // Creating an List<T> of Integers
        List<int> firstlist = new List<int>();

        // Adding elements to List
        for (int i = 1; i <= 5; i++) {
            firstlist.Add(i);
        }

        Console.WriteLine("Elements Present in List:");

        // Displaying the elements of List
        foreach(int k in firstlist)
        {
            Console.WriteLine(k);
        }

        Console.WriteLine(" ");

        Console.WriteLine("After Reversing: ");

        // using method Reverse()
        firstlist.Reverse();

        // Displaying the elements of List
        foreach(int k in firstlist)
        {
            Console.WriteLine(k);
        }
    }
}

Output:

Elements Present in List:
1
2
3
4
5
 
After Reversing: 
5
4
3
2
1

Example 2:

// C# Program to reverse the order of
// the elements in the entire List<T>
using System;
using System.Collections;
using System.Collections.Generic;

class Geeks {

    // Main Method
    public static void Main()
    {

        // Creating an List<T> of Integers
        List<string> firstlist = new List<string>();

        // Adding elements to List
        firstlist.Add("Geeks");
        firstlist.Add("C#");
        firstlist.Add("Java");
        firstlist.Add("C++");

        Console.WriteLine("Elements Present in List:");

        // Displaying the elements of List
        foreach(string k in firstlist)
        {
            Console.WriteLine(k);
        }

        Console.WriteLine(" ");

        Console.WriteLine("After Reversing: ");

        // using method Reverse()
        firstlist.Reverse();

        // Displaying the elements of List
        foreach(string k in firstlist)
        {
            Console.WriteLine(k);
        }
    }
}

Output:

Elements Present in List:
Geeks
C#
Java
C++
 
After Reversing: 
C++
Java
C#
Geeks

Reverse(Int32, Int32) Method

This method is used to reverse the order of the elements in the specified range.

Syntax:

public void Reverse (int index, int count);

Parameters:

index: It is the zero-based starting index of the range which is to be reversed.

count: It is the number of elements in the range which is to be reversed.

Exceptions:

• ArgumentOutOfRangeException: If the index or count is less than zero.
• ArgumentException: If the index and count do not denote a valid range of elements in the List<T>.

Below programs illustrate the use of above discussed method:

Example 1:

// C# Program to reverse the order of
// sub range in the List<T>
using System;
using System.Collections;
using System.Collections.Generic;

class Geeks {

    // Main Method
    public static void Main()
    {

        // Creating an List<T> of Integers
        List<string> firstlist = new List<string>();

        // Adding elements to List
        firstlist.Add("1st");
        firstlist.Add("2nd");
        firstlist.Add("3rd");
        firstlist.Add("4th");
        firstlist.Add("5th");
        firstlist.Add("6th");
        firstlist.Add("7th");

        Console.WriteLine("Elements Present in List:");

        // Displaying the elements of List
        foreach(string k in firstlist)
        {
            Console.WriteLine(k);
        }

        Console.WriteLine(" ");

        Console.WriteLine("After Reversing: ");

        // Reversing the sub-range
        // that starts from index 2
        // i.e 3rd element, and
        // count is 4 elements
        firstlist.Reverse(2, 4);

        // Displaying the elements of List
        foreach(string k in firstlist)
        {
            Console.WriteLine(k);
        }
    }
}

Output:

Elements Present in List:
1st
2nd
3rd
4th
5th
6th
7th
 
After Reversing: 
1st
2nd
6th
5th
4th
3rd
7th

Example 2:

// C# Program to reverse the order of
// sub range in the List<T>
using System;
using System.Collections;
using System.Collections.Generic;

class Geeks {

    // Main Method
    public static void Main()
    {

        // Creating an List<T> of Integers
        List<string> firstlist = new List<string>();

        // Adding elements to List
        firstlist.Add("1st");
        firstlist.Add("2nd");
        firstlist.Add("3rd");
        firstlist.Add("4th");
        firstlist.Add("5th");
        firstlist.Add("6th");
        firstlist.Add("7th");

        Console.WriteLine("Elements Present in List:");

        // Displaying the elements of List
        foreach(string k in firstlist)
        {
            Console.WriteLine(k);
        }

        Console.WriteLine(" ");

        Console.WriteLine("After Reversing: ");

        // taking negative index will give error
        firstlist.Reverse(-1, 4);

        // Displaying the elements of List
        foreach(string k in firstlist)
        {
            Console.WriteLine(k);
        }
    }
}

RUntimeError:

Unhandled Exception:
System.ArgumentOutOfRangeException: Non-negative number required.
Parameter name: index

Reference:

• https://docs.microsoft.com/en-us/dotnet/api/system.collections.generic.list-1.reverse?view=netframework-4.7.2