Remove, List

For a C# List, the Remove() method eliminates elements. We remove by index, value, or by condition (a lambda). Each approach has its proper use.

Method notes

The Remove method is effective for removing by value. But we may also use RemoveAt to remove an element at an index.

Remove example

We can remove based on the element value (with Remove), or based on the index (with RemoveAt). This is not equivalent to assigning an element to null.

using System;
using System.Collections.Generic;

// Step 1: create List.
List<string> values = new List<string>();
values.Add("AA");
values.Add("BB");
values.Add("CC");

// Step 2: remove by value.
values.Remove("BB");
Console.WriteLine(string.Join(";", values));

// Step 3: remove by index.
values.RemoveAt(1);
Console.WriteLine(string.Join(";", values));
AA;CC
AA

RemoveRange

This can remove elements in a certain series of indexes. One useful way to use this method is to remove the first or last several elements at once.

using System;
using System.Collections.Generic;

List<int> list = new List<int>();
list.Add(1);
list.Add(2);
list.Add(3);
list.Add(4);
list.Add(5);

// Remove all except last 2
int remove = Math.Max(0, list.Count - 2);
list.RemoveRange(0, remove);

foreach (int i in list)
{
    Console.Write(i);
}
45

RemoveRange, example 2

Here we use RemoveRange to remove the first N elements in a List. We also use Math.Min here to avoid arguments that are too large and would raise an exception.

using System;
using System.Collections.Generic;

List<int> list = new List<int>();
list.Add(1);
list.Add(2);
list.Add(3);
list.Add(4);
list.Add(5);

// Remove first 2 elements
int remove = Math.Min(list.Count, 2);
list.RemoveRange(0, remove);

foreach (int i in list)
{
    Console.Write(i);
}
345

RemoveAt

The RemoveAt() method removes one element. How is this method on the List type implemented in .NET? It requires an index argument.

using System;
using System.Collections.Generic;

var list = new List<string>();
list.Add("dot");
list.Add("net");
list.Add("perls");

list.RemoveAt(1);

foreach (string element in list)
{
    Console.WriteLine(element);
}
dot
perls

Notes, performance

Methods on List that remove elements require linear time. It could be faster to assign null to elements you want to erase, rather than removing them.

Discussion

When you call RemoveAt, all the element following the index you remove will be copied and shifted forward. This is not efficient.

A review

We invoked the Remove, RemoveAt, RemoveAll and RemoveRange methods. We found out how to remove the first and last elements, and reviewed algorithmic complexity.