How to sort a list in C# | List.Sort() Method Set -1

List<T>.Sort() Method is used to sort the elements or a portion of the elements in the List<T> using either the specified or default IComparer<T> implementation or a provided Comparison<T> delegate to compare list elements. There are total 4 methods in the overload list of this method as follows:

1. Sort(IComparer<T>)
2. Sort(Int32, Int32, IComparer)
3. Sort()
4. Sort(Comparison<T>)

Here, we will discuss the first two methods.

Sort(IComparer<T>) Method

This method is used to sort the elements in the entire List<T> using the specified comparer.

Syntax:

public void Sort (System.Collections.Generic.IComparer<T> comparer);

Here, the comparer is the IComparer<T> implementation to use when comparing elements, or null to use the default comparer Default.

Exceptions:

• InvalidOperationException: If the comparer is null, and the default comparer Default cannot find the implementation of the IComparable<T> generic interface or the IComparable interface for type T.
• ArgumentException: If the implementation of comparer caused an error during the sort. For example, comparer might not return 0 when comparing an item with itself.

Example 1:

// C# program to demonstrate the concept of
// List<T>.Sort(IComparer <T>) method
using System;
using System.Collections.Generic;

class GFG : IComparer<int>
{
    public int Compare(int x, int y)
    {
        if (x == 0 || y == 0)
        {
            return 0;
        }

        // CompareTo() method
        return x.CompareTo(y);

    }
}

public class geek
{

    // Main Method
    public static void Main()
    {

        // List initialize
        List<int> list1 = new List<int>
        {

            // list elements
            1,5,6,2,4,3

        };

        Console.WriteLine("Original List");

        foreach(int g in list1)
        {

            // Display Original List
            Console.WriteLine(g);

        }

        // "gg" is the object oif class GFG
        GFG gg = new GFG();

        Console.WriteLine("\nSort with a comparer:");

        // use of List<T>.Sort(IComparer<T>)
        // method. The comparer is "gg"
        list1.Sort(gg);

        foreach( int g in list1 )
        {

            // Display sorted list
            Console.WriteLine(g);

        }
    }
}

Output:

Original List
1
5
6
2
4
3

Sort with a comparer:
1
2
3
4
5
6

Example 2:

// C# program to demonstrate the concept of
// List<T>.Sort(IComparer <T>) method
using System;
using System.Collections.Generic;

class GFG : IComparer<string>
{
    public int Compare(string x, string y)
    {

        if (x == null || y == null)
        {
            return 0;
        }

        // "CompareTo()" method
        return x.CompareTo(y);

    }
}

public class geek
{

    // Main Method
    public static void Main()
    {
        List<string> list1 = new List<string>();

        // list elements
        list1.Add("A");
        list1.Add("I");
        list1.Add("G");
        list1.Add("B");
        list1.Add("E");
        list1.Add("H");
        list1.Add("F");
        list1.Add("C");
        list1.Add("J");

        Console.WriteLine("Original List");

        // Display Original List
        Display(list1);

        // "gg" is the object
        GFG gg = new GFG();

        Console.WriteLine("\nSort with a comparer:");

        // sort the list with a
        // specified comparer "gg"
        list1.Sort(gg);

        // Display sorted List
        Display(list1);

        Console.WriteLine("\nBinarySearch and Insert D");

        // Binary Search for "D"
        // using List.BinarySearch(T) method
        int index = list1.BinarySearch("D");

        if (index < 0)
        {

            // range++;
            list1.Insert(~index, "D");

        }

        // Display the List after
        // inserting "D"
        Display(list1);

    }

    // Display function
    public static void Display(List<string> list)
    {
        foreach( string g in list )
        {
            Console.WriteLine(g);
        }
    }
}

Output:

Original List
A
I
G
B
E
H
F
C
J

Sort with a comparer:
A
B
C
E
F
G
H
I
J

BinarySearch and Insert D
A
B
C
D
E
F
G
H
I
J

List<T>.Sort(Int32, Int32, IComparer<T>) Method

This method is used to sort the elements in a range of elements in List<T> using the specified comparer.

Syntax:

public void Sort(int index, int len, IComparer<T> comparer)

Parameters:

index : It is the zero-based starting index of the range in which sort will happen.

len : It is the length of the range.

comparer : When comparing elements then use the IComparer implementation or null to use the default comparer Default.

Exceptions:

• ArgumentOutOfRangeException : If indexor len is less than 0.
• ArgumentException : If index and count do not specify a valid range in the List.
• InvalidOperationException : If comparer is null.

Example:

// C# program to demonstrate the use of
// List<T>.Sort(Int32, Int32, IComparer<T>)
// Method
using System;
using System.Collections.Generic;

class GFG : IComparer<string>
{
    public int Compare(string x, string y)
    {
        if (x == null || y == null)
        {
            return 0;
        }

        // "CompareTo()" method
        return x.CompareTo(y);

    }
}

public class geek
{
    public static void Main()
    {
        List<string> list1 = new List<string>();

        // list elements
        list1.Add("C++");
        list1.Add("Java");
        list1.Add("C");
        list1.Add("Python");
        list1.Add("HTML");
        list1.Add("CSS");
        list1.Add("Scala");
        list1.Add("Ruby");
        list1.Add("Perl");

        int range = 4;
        Console.WriteLine("Original List");

        // Display Original List
        Display(list1);

        // "gg" is the object
        GFG gg = new GFG();

        Console.WriteLine("\nSort a range with comparer:");

        // sort the list within a
        // range of index 1 to 4
        // where range = 4
        list1.Sort(1, range, gg);

        // Display sorted List
        Display(list1);

        Console.WriteLine("\nBinarySearch and Insert Dart");

        // Binary Search and storing
        // index value to "index"
        int index = list1.BinarySearch(0, range,
                                    "Dart", gg);

        if (index < 0)
        {
            list1.Insert(~index, "Dart");
            range++;
        }

        // Display the List
        // after inserting "Dart"
        Display(list1);

    }

    // Display function
    public static void Display(List<string> list)
    {
        foreach(string g in list)
        {
            Console.WriteLine(g);
        }
    }
}

Output:

Original List
C++
Java
C
Python
HTML
CSS
Scala
Ruby
Perl

Sort a range with comparer:
C++
C
HTML
Java
Python
CSS
Scala
Ruby
Perl

BinarySearch and Insert Dart
C++
C
Dart
HTML
Java
Python
CSS
Scala
Ruby
Perl

Reference:

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