Lambda List List Remove Predicate string.Join

`RemoveAll`

This C# method filters and removes elements. It can be used with a lambda expression. This reduces the size of your code and improves its clarity.

Some notes

The List RemoveAll method accepts a Predicate expression for this purpose. We can read it like a sentence: remove all elements matching this condition.

First example

Here we demonstrate the RemoveAll method on the C# List. We use a function argument (a lambda) to specify which elements to remove.

Step 1 We create a List of ints. We then call Add() with 5 separate integers to populate the list.

Step 2 We delete all elements with values of 2. The lambda expression, with the arrow syntax, matches elements with value of 2.

Step 3 We use a foreach loop on the list that we modified with RemoveAll. And we print the elements.

Tip You can use the delegate keyword with an optional parameter list and a method body instead of a lambda expression.

using System;
using System.Collections.Generic;

// Step 1: create List.
List<int> list = new List<int>();
list.Add(1);
list.Add(2);
list.Add(2);
list.Add(4);
list.Add(5);

// Step 2: remove all list items with value of 2.
// ... The lambda expression is the Predicate.
list.RemoveAll(item => item == 2);

// Step 3: display results.
foreach (int i in list)
{
    Console.WriteLine(i);
}1
4
5

Return value

When called, RemoveAll will invoke the Predicate method passed to it. After this is done, it will return an int equal to the number of elements it removed from the List.

Return The return int is the count of items removed—so if 2 items are removed, the return value is 2.

Note If no elements were removed, RemoveAll will return 0. We can check for 0 to see if the list remained unchanged.

Here This example displays the List contents by calling the string.Join method, which supports a List argument.

using System;
using System.Collections.Generic;

class Program
{
    static bool IsRemovable(string item)
    {
        return item.StartsWith("b");
    }

    static bool IsRemovableNothingMatches(string item)
    {
        return item == "x";
    }

    static void Main()
    {
        var items = new List<string> { "bird", "frog", "bat" };
        Console.WriteLine("ITEMS: {0}", string.Join(",", items));

        // Remove 2 items, the result value is 2.
        var result = items.RemoveAll(IsRemovable);

        Console.WriteLine("COUNT OF ITEM REMOVED: {0}", result);
        Console.WriteLine("ITEMS: {0}", string.Join(",", items));

        // Nothing is removed with this call.
        var result2 = items.RemoveAll(IsRemovableNothingMatches);

        // The result is 0.
        Console.WriteLine("COUNT, NOTHING REMOVED: {0}", result2);
    }
}ITEMS: bird,frog,bat
COUNT OF ITEM REMOVED: 2
ITEMS: frog
COUNT, NOTHING REMOVED: 0

Notes, lambda

When reading the lambda expression, it helps to think of it as "goes to." So you can say that the RemoveAll invocation above "removes all elements where the value goes to 2."

A discussion

There are other removal methods on List like the Remove and RemoveAt methods. If you want to remove an isolated element, use those methods instead.

Tip The simplest method—the one that is the easiest to read—is usually the best choice.

A summary

You can use the RemoveAll method on the List. We noted how to use a lambda expression as the Predicate object to this method, and how you can read lambda expressions.

Using the RemoveAll method instead of a complex loop with multiple tests and copies can make code more readable. But be careful to keep your lambda expressions as short as possible.