C# extension method
last modified July 5, 2023
In this article we show how to define and use extension methods in C#.
C# extension method
Extension methods are methods that can be inserted to existing types without creating a new derived type, recompiling, or otherwise modifying the original type.
Extension methods are static methods, but in C# they are called as if they were
normal C# methods. They use this modifier in their parameters.
Extension methods are only in scope when we explicitly import the namespace into your source code with a using directive.
In the standard library, we can find plenty of extension methods, most notably in LINQ.
C# LINQ extension methods
LINQ is a mini language built into C# that brings more advanced query abilities.
string[] words = { "falcon", "oak", "sky", "cloud", "tree", "tea", "water" };
Console.WriteLine(words.First(word => word.Length == 3));
Console.WriteLine(words.Last(word => word.Length == 3));
The example prints the first and the last word that has three letters.
Console.WriteLine(words.First(word => word.Length == 3)); Console.WriteLine(words.Last(word => word.Length == 3));
Both First and Last are C# extension methods that are
defined in the System.Linq namespace.
$ dotnet run oak tea
C# extension method example
In the following example, we add a new method to the string type.
var msg = "We walked for hours. We saw four hawks in the sky.";
int n = msg.WordCount();
Console.WriteLine(n);
public static class ExtensionMethods
{
public static int WordCount(this string str)
{
return str.Split(new char[] { ' ', '.', '?' },
StringSplitOptions.RemoveEmptyEntries).Length;
}
}
The WordCount method is a static methods defined in a static class.
Its parameter contains the this keyword.
int n = msg.WordCount();
The method is called directly on the string.
$ dotnet run 11
C# extension method example II
In the next example, we add an extension method to the int type.
int x = 20;
int y = -5;
Console.WriteLine(x.Abs());
Console.WriteLine(y.Abs());
public static class ExtensionMethods
{
public static int Abs(this int val)
{
if (val < 0)
{
return -val;
}
return val;
}
}
The example adds the Abs method, which returns an absolute value
of the given integer.
$ dotnet run 20 5
C# generic extension method
In the next example, we define a FindAll extension method for the
list type.
public static class ExtensionMethods
{
public static List<T> FindAll<T>(this List<T> vals, List<Predicate<T>> preds)
{
List<T> data = new List<T>();
foreach (T e in vals)
{
bool pass = true;
foreach (Predicate<T> p in preds)
{
if (!(p(e)))
{
pass = false;
break;
}
}
if (pass) data.Add(e);
}
return data;
}
}
The FindAll method returns list elements that fill all the
specified predicates.
public static List<T> FindAll<T>(this List<T> vals, List<Predicate<T>> preds)
The FindAll method takes a list of generic predicate functions as
a parameter. It returns a filtered generic list.
var preds = new List<Predicate<int>>();
preds.Add(e => e > 0);
preds.Add(e => e % 2 == 0);
var vals = new List<int> {-3, -2, -1, 0, 1, 2, 3, 4};
var filtered = vals.FindAll(preds);
foreach (var e in filtered) {
Console.WriteLine(e);
}
Console.WriteLine("---------------------");
var words = new List<string> {"sky", "wrath", "wet", "sun", "pick", "who",
"cloud", "war", "water", "jump", "ocean"};
var preds2 = new List<Predicate<string>>();
preds2.Add(e => e.StartsWith("w"));
preds2.Add(e => e.Length == 3);
var filtered2 = words.FindAll(preds2);
foreach (var e in filtered2) {
Console.WriteLine(e);
}
We define two lists: an integer list and a string list. From the integer list, we filter out all positive even values. From the string list, we get all words that start with 'w' and have three letters.
$ dotnet run 2 4 --------------------- wet who war
C# Base64 extension methods
In the following example, we create extension methods for encoding and decoding Base64 data. Base64 is a group of similar binary-to-text encoding schemes that represent binary data in a text string format.
using System.Text;
namespace Base64Ex;
class Program
{
static void Main()
{
string msg = "one 🐘 and three 🐋";
string base64 = msg.EncodeBase64();
string msg2 = base64.DecodeBase64();
Console.WriteLine(msg);
Console.WriteLine(base64);
Console.WriteLine(msg2);
}
}
static class ExtensionMethods
{
public static string EncodeBase64(this string value)
{
byte[] data = Encoding.UTF8.GetBytes(value);
return Convert.ToBase64String(data);
}
public static string DecodeBase64(this string value)
{
byte[] data = System.Convert.FromBase64String(value);
return Encoding.UTF8.GetString(data);
}
}
The program creates the EncodeBase64 and DecodeBase64
extension methods.
string msg = "one 🐘 and three 🐋"; string base64 = msg.EncodeBase64(); string msg2 = base64.DecodeBase64();
The extension methods can be directly called on the string.
$ dotnet run one 🐘 and three 🐋 b25lIPCfkJggYW5kIHRocmVlIPCfkIs= one 🐘 and three 🐋
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