C# | Remove all elements of a List that match the conditions defined by the predicate

List<T>.RemoveAll(Predicate<T>) Method is used to remove all the elements that match the conditions defined by the specified predicate.

Properties of List:

• It is different from the arrays. A list can be resized dynamically but arrays cannot.
• List class can accept null as a valid value for reference types and it also allows duplicate elements.
• If the Count becomes equals to Capacity then the capacity of the List increases automatically by reallocating the internal array. The existing elements will be copied to the new array before the addition of the new element.

Syntax:

public int RemoveAll (Predicate<T> match);

Parameter:

match: It is the Predicate<T> delegate that defines the conditions of the elements which is to be removed.

Return Value: This method returns the number of elements that to be remove from the List<T>.

Exception: This method will give ArgumentNullException if the match is null.

Below programs illustrate the use of List<T>.RemoveAll(Predicate<T>) Method:

Example 1:

// C# Program to remove all elements of
// a List that match the conditions
// defined by the predicate
using System;
using System.Collections;
using System.Collections.Generic;

class Geeks {

    // function which checks whether an
    // element is even or not. Or you can
    // say it is the specified condition
    private static bool isEven(int i)
    {
        return ((i % 2) == 0);
    }

    // Main Method
    public static void Main(String[] args)
    {

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

        // Adding elements to List
        for (int i = 1; i <= 10; i++) {
            firstlist.Add(i);
        }

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

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

        Console.WriteLine(" ");

        Console.Write("Number of Elements Removed: ");

        // Removing the elements which is even
        // This will return 5 as it removed 5
        // even elements from the list
        Console.WriteLine(firstlist.RemoveAll(isEven));

        Console.WriteLine(" ");

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

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

Output:

Elements Present in List:

1
2
3
4
5
6
7
8
9
10
 
Number of Elements Removed: 5
 
Remaining Elements in List:
1
3
5
7
9

Example 2:

// C# Program to remove all elements of
// a List that match the conditions
// defined by the predicate
using System;
using System.Collections;
using System.Collections.Generic;

class Geeks {

    // function which checks whether an
    // element is even or not. Or you can
    // say it is the specified condition
    private static bool isEven(int i)
    {
        return ((i % 2) == 0);
    }

    // Main Method
    public static void Main(String[] args)
    {

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

        // Adding items to List
        firstlist.Add(13);
        firstlist.Add(17);
        firstlist.Add(19);
        firstlist.Add(11);

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

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

        Console.WriteLine(" ");

        Console.Write("Number of Elements Removed: ");

        // Removing the elements which is even
        // This will return 0 as it no elements
        // are even in List
        Console.WriteLine(firstlist.RemoveAll(isEven));

        Console.WriteLine(" ");

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

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

Output:

Elements Present in List:

13
17
19
11
 
Number of Elements Removed: 0
 
Remaining Elements in List:
13
17
19
11

Reference:

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