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

The RemoveAll() method in C# removes all elements from a List<T> that match the conditions defined by a predicate. A predicate is a method that takes an element as input and returns a boolean value indicating whether the element should be removed.

Syntax

Following is the syntax for the RemoveAll() method −

public int RemoveAll(Predicate<T> match)

Parameters

• match − A predicate delegate that defines the conditions for removal. It takes an element of type T and returns true if the element should be removed.

Return Value

The method returns an int representing the number of elements that were removed from the list.

Using RemoveAll() with Named Method Predicate

In this example, we create a predicate method that removes all occurrences of the value 500 −

using System;
using System.Collections.Generic;

public class Demo {
    private static bool IsEqual500(int i) {
        return (i == 500);
    }

    public static void Main(String[] args) {
        List<int> list = new List<int>();
        list.Add(100);
        list.Add(500);
        list.Add(300);
        list.Add(400);
        list.Add(500);
        list.Add(600);
        list.Add(500);

        Console.WriteLine("Original list elements:");
        foreach (int i in list) {
            Console.WriteLine(i);
        }

        int removedCount = list.RemoveAll(IsEqual500);
        Console.WriteLine("\nRemoved " + removedCount + " elements");
        Console.WriteLine("List after removing elements 500:");
        foreach (int i in list) {
            Console.WriteLine(i);
        }
    }
}

The output of the above code is −

Original list elements:
100
500
300
400
500
600
500

Removed 3 elements
List after removing elements 500:
100
300
400
600

Using RemoveAll() with Mathematical Condition

This example removes all numbers that are divisible by 10 −

using System;
using System.Collections.Generic;

public class Demo {
    private static bool IsDivisibleBy10(int i) {
        return ((i % 10) == 0);
    }

    public static void Main(String[] args) {
        List<int> list = new List<int>();
        list.Add(100);
        list.Add(295);
        list.Add(355);
        list.Add(450);
        list.Add(550);

        Console.WriteLine("Original list elements:");
        foreach (int i in list) {
            Console.WriteLine(i);
        }

        int removedCount = list.RemoveAll(IsDivisibleBy10);
        Console.WriteLine("\nRemoved " + removedCount + " elements divisible by 10");
        Console.WriteLine("List after removing elements:");
        foreach (int i in list) {
            Console.WriteLine(i);
        }
    }
}

The output of the above code is −

Original list elements:
100
295
355
450
550

Removed 3 elements divisible by 10
List after removing elements:
295
355

Using RemoveAll() with Lambda Expression

You can also use lambda expressions for more concise code −

using System;
using System.Collections.Generic;

public class Demo {
    public static void Main(String[] args) {
        List<string> names = new List<string> {"Alice", "Bob", "Charlie", "Anna", "David"};

        Console.WriteLine("Original list:");
        foreach (string name in names) {
            Console.WriteLine(name);
        }

        int removedCount = names.RemoveAll(name => name.StartsWith("A"));
        Console.WriteLine("\nRemoved " + removedCount + " names starting with 'A'");
        Console.WriteLine("Updated list:");
        foreach (string name in names) {
            Console.WriteLine(name);
        }
    }
}

The output of the above code is −

Original list:
Alice
Bob
Charlie
Anna
David

Removed 2 names starting with 'A'
Updated list:
Bob
Charlie
David

Conclusion

The RemoveAll() method provides an efficient way to remove multiple elements from a List based on a condition. It accepts a predicate delegate, removes all matching elements in a single operation, and returns the count of removed elements.