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C# List Remove ExamplesUse Remove, RemoveRange and RemoveAt on Lists. Delete elements by value and index.
dot net perls

Remove, List. In the winter storm, a branch is removed from the tree. For a List, the Remove() method eliminates elements. We remove by index, value, or by condition (a lambda).

List

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.

Step 1 We create a List and add several strings to it. This List is used to demonstrate how Remove methods work.

Step 2 We call Remove to remove an element by its value. So the element with the value "BB" is removed.

Step 3 The example shows RemoveAt. It removes the element at index 1, which is the second element, "BB."

C# program that uses Remove, RemoveAt on List
using System; using System.Collections.Generic; class Program { static void Main() { // 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.

Next We remove all elements except the last two. The code is robust because it uses Math.Max to avoid negative parameters.

Math.Max, Min

C# program that uses RemoveRange method
using System; using System.Collections.Generic; class Program { static void Main() { 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.

C# program that uses RemoveRange on first elements
using System; using System.Collections.Generic; class Program { static void Main() { 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); } } }
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RemoveAt. RemoveAt removes one element. How is this method on the List type implemented in the .NET Framework? It requires an index argument.

Example In this program we specify that a List be instantiated with three strings: "dot", "net" and "perls".

Next We remove the string at index 1, which is the second string. The List now contains only two strings: "dot" and "perls".

Var

String Literal

C# program that uses RemoveAt
using System; using System.Collections.Generic; class Program { static void Main() { 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.

Note RemoveAt is faster than Remove. It doesn't need to iterate through the number of elements equal to index.

RemoveAll You can use RemoveAll to remove all elements in the List that match a certain predicate expression.

Tip It is often easiest to use the lambda expression syntax with the RemoveAll method, which can reduce line count.

RemoveAll

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

However If you want to remove an element but eliminate copying, you could simply assign it to null or default(T).

Null

Default

Also The Insert method on List suffers from a similar performance drawback. It causes excessive element copying.

Insert

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.

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