C# extension method

last modified July 7, 2024

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.

Program.cs

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.

Program.cs

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.

Program.cs

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.

ExtensionMethods.cs

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.

Program.cs

List<Predicate<int>> preds = [e => e > 0, e => e % 2 == 0];

List<int> vals = [-3, -2, -1, 0, 1, 2, 3, 4]; var filtered = vals.FindAll(preds);

foreach (var e in filtered) {

Console.WriteLine(e);

}

Console.WriteLine("———————");

List<string> words = [“sky”, “wrath”, “wet”, “sun”, “pick”, “who”, “cloud”, “war”, “water”, “jump”, “ocean”];

List<Predicate<string>> preds2 = [e => e.StartsWith(“w”), 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.

Program.cs

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 🐋

Source

Extention methods

In this article we have worked with extension methods in C#.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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