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C# Extension Methods (How It Works For Developers)

ByChipego Kalinda

August 15, 2024

Updated June 22, 2025

Extension methods are a powerful feature in C# that allow you to add new functionality to existing types without modifying their source code. They can be extremely useful in making your code more readable and maintainable. In this guide, we'll explore the basics of extension methods and how to implement them.

What are Extension Methods?

Extension methods are special static methods that can be called as if they were instance methods of an existing type. They are a convenient way to add new methods to an existing class without changing the original source code or inheriting from the class.

To create an extension method, you need to define a static method inside a static class. The first parameter of the method should be the type you want to extend, prefixed with the this keyword. This special keyword tells the C# compiler that this is an extension method.

Implementing Extension Methods in C#

Now that we know what extension methods are, let's implement one. Imagine you have a string that you want to reverse. Instead of writing a separate function to do this, you can create an extension method for the string class.

First, let's create a new static class called StringExtensions. The class name is not important, but it's a common convention to use the name of the type being extended followed by "Extensions". Inside this class, we'll define a static method called Reverse:

public static class StringExtensions
{
    // This extension method reverses a given string.
    public static string Reverse(this string input)
    {
        // Convert the string to a character array.
        char[] chars = input.ToCharArray();
        // Reverse the array in place.
        Array.Reverse(chars);
        // Create a new string from the reversed character array and return it.
        return new string(chars);
    }
}

public static class StringExtensions
{
    // This extension method reverses a given string.
    public static string Reverse(this string input)
    {
        // Convert the string to a character array.
        char[] chars = input.ToCharArray();
        // Reverse the array in place.
        Array.Reverse(chars);
        // Create a new string from the reversed character array and return it.
        return new string(chars);
    }
}

Public Module StringExtensions
	' This extension method reverses a given string.
	<System.Runtime.CompilerServices.Extension> _
	Public Function Reverse(ByVal input As String) As String
		' Convert the string to a character array.
		Dim chars() As Char = input.ToCharArray()
		' Reverse the array in place.
		Array.Reverse(chars)
		' Create a new string from the reversed character array and return it.
		Return New String(chars)
	End Function
End Module

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In this example, we created a public static string method called Reverse with a single parameter. The this keyword before the string type indicates that this is an extension method for the string class.

Now, let's see how to use this new extension method in our Program class:

class Program
{
    static void Main(string[] args)
    {
        string example = "Hello, World!";
        // Call the extension method as if it were an instance method.
        string reversed = example.Reverse();
        Console.WriteLine(reversed); // Output: !dlroW ,olleH
    }
}

class Program
{
    static void Main(string[] args)
    {
        string example = "Hello, World!";
        // Call the extension method as if it were an instance method.
        string reversed = example.Reverse();
        Console.WriteLine(reversed); // Output: !dlroW ,olleH
    }
}

Friend Class Program
	Shared Sub Main(ByVal args() As String)
		Dim example As String = "Hello, World!"
		' Call the extension method as if it were an instance method.
		Dim reversed As String = example.Reverse()
		Console.WriteLine(reversed) ' Output: !dlroW ,olleH
	End Sub
End Class

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Notice that we didn't have to create an instance of the StringExtensions class. Instead, we used the Reverse method directly on the string instance as if it were an instance method.

Extension Method Syntax

Extension methods look and behave like instance methods, but there are a few important differences to keep in mind:

Extension methods cannot access private members of the extended type.
They also do not participate in inheritance or polymorphism.
You cannot override an existing method with an extension method.

If the extended type has a method with the same signature as an extension method, the instance method will always take precedence. Extension methods are only called when there is no matching instance method.

Real-Life Examples of Extension Methods

Now that we understand the basics of extension methods in C#, let's look at some real-life examples.

String Extension Method Word Count

Imagine you want to count the number of words in a string. You can create a WordCount extension method for the string class:

public static class StringExtensions
{
    // This extension method counts the number of words in a string.
    public static int WordCount(this string input)
    {
        // Split the string by whitespace characters and return the length of the resulting array.
        return input.Split(new[] { ' ', '\t', '\r', '\n' }, StringSplitOptions.RemoveEmptyEntries).Length;
    }
}

public static class StringExtensions
{
    // This extension method counts the number of words in a string.
    public static int WordCount(this string input)
    {
        // Split the string by whitespace characters and return the length of the resulting array.
        return input.Split(new[] { ' ', '\t', '\r', '\n' }, StringSplitOptions.RemoveEmptyEntries).Length;
    }
}

Imports Microsoft.VisualBasic

Public Module StringExtensions
	' This extension method counts the number of words in a string.
	<System.Runtime.CompilerServices.Extension> _
	Public Function WordCount(ByVal input As String) As Integer
		' Split the string by whitespace characters and return the length of the resulting array.
		Return input.Split( { " "c, ControlChars.Tab, ControlChars.Cr, ControlChars.Lf }, StringSplitOptions.RemoveEmptyEntries).Length
	End Function
End Module

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Now, you can easily count the number of words in a string like this:

string text = "Extension methods are awesome!";
int wordCount = text.WordCount();
Console.WriteLine($"The text has {wordCount} words."); // Output: The text has 4 words.

string text = "Extension methods are awesome!";
int wordCount = text.WordCount();
Console.WriteLine($"The text has {wordCount} words."); // Output: The text has 4 words.

Dim text As String = "Extension methods are awesome!"
Dim wordCount As Integer = text.WordCount()
Console.WriteLine($"The text has {wordCount} words.") ' Output: The text has 4 words.

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IEnumerable Extension Method Median

Suppose you have a collection of numbers, and you want to calculate the median value. You can create an extension method for IEnumerable<int>:

using System;
using System.Collections.Generic;
using System.Linq;

public static class EnumerableExtensions
{
    // This extension method calculates the median of a collection of integers.
    public static double Median(this IEnumerable<int> source)
    {
        // Sort the collection and convert it to an array.
        int[] sorted = source.OrderBy(x => x).ToArray();
        int count = sorted.Length;

        if (count == 0)
        {
            throw new InvalidOperationException("The collection is empty.");
        }

        // If the count is even, return the average of the two middle elements.
        if (count % 2 == 0)
        {
            return (sorted[count / 2 - 1] + sorted[count / 2]) / 2.0;
        }
        else
        {
            // Otherwise, return the middle element.
            return sorted[count / 2];
        }
    }
}

using System;
using System.Collections.Generic;
using System.Linq;

public static class EnumerableExtensions
{
    // This extension method calculates the median of a collection of integers.
    public static double Median(this IEnumerable<int> source)
    {
        // Sort the collection and convert it to an array.
        int[] sorted = source.OrderBy(x => x).ToArray();
        int count = sorted.Length;

        if (count == 0)
        {
            throw new InvalidOperationException("The collection is empty.");
        }

        // If the count is even, return the average of the two middle elements.
        if (count % 2 == 0)
        {
            return (sorted[count / 2 - 1] + sorted[count / 2]) / 2.0;
        }
        else
        {
            // Otherwise, return the middle element.
            return sorted[count / 2];
        }
    }
}

Imports System
Imports System.Collections.Generic
Imports System.Linq

Public Module EnumerableExtensions
	' This extension method calculates the median of a collection of integers.
	<System.Runtime.CompilerServices.Extension> _
	Public Function Median(ByVal source As IEnumerable(Of Integer)) As Double
		' Sort the collection and convert it to an array.
		Dim sorted() As Integer = source.OrderBy(Function(x) x).ToArray()
		Dim count As Integer = sorted.Length

		If count = 0 Then
			Throw New InvalidOperationException("The collection is empty.")
		End If

		' If the count is even, return the average of the two middle elements.
		If count Mod 2 = 0 Then
			Return (sorted(count \ 2 - 1) + sorted(count \ 2)) / 2.0
		Else
			' Otherwise, return the middle element.
			Return sorted(count \ 2)
		End If
	End Function
End Module

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With this extension method, you can easily find the median value of a collection:

int[] numbers = { 5, 3, 9, 1, 4 };
double median = numbers.Median();
Console.WriteLine($"The median value is {median}."); // Output: The median value is 4.

int[] numbers = { 5, 3, 9, 1, 4 };
double median = numbers.Median();
Console.WriteLine($"The median value is {median}."); // Output: The median value is 4.

Dim numbers() As Integer = { 5, 3, 9, 1, 4 }
Dim median As Double = numbers.Median()
Console.WriteLine($"The median value is {median}.") ' Output: The median value is 4.

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DateTime Extension Method StartOfWeek

Let's say you want to find the start of the week for a given date. You can create an extension method for the DateTime struct:

public static class DateTimeExtensions
{
    // This extension method calculates the start of the week for a given date.
    public static DateTime StartOfWeek(this DateTime dt, DayOfWeek startOfWeek = DayOfWeek.Monday)
    {
        // Calculate the difference in days between the current day and the start of the week.
        int diff = (7 + (dt.DayOfWeek - startOfWeek)) % 7;
        // Subtract the difference to get the start of the week.
        return dt.AddDays(-1 * diff).Date;
    }
}

public static class DateTimeExtensions
{
    // This extension method calculates the start of the week for a given date.
    public static DateTime StartOfWeek(this DateTime dt, DayOfWeek startOfWeek = DayOfWeek.Monday)
    {
        // Calculate the difference in days between the current day and the start of the week.
        int diff = (7 + (dt.DayOfWeek - startOfWeek)) % 7;
        // Subtract the difference to get the start of the week.
        return dt.AddDays(-1 * diff).Date;
    }
}

Public Module DateTimeExtensions
	' This extension method calculates the start of the week for a given date.
'INSTANT VB NOTE: The parameter startOfWeek was renamed since Visual Basic will not allow parameters with the same name as their enclosing function or property:
	<System.Runtime.CompilerServices.Extension> _
	Public Function StartOfWeek(ByVal dt As DateTime, Optional ByVal startOfWeek_Conflict As DayOfWeek = DayOfWeek.Monday) As DateTime
		' Calculate the difference in days between the current day and the start of the week.
		Dim diff As Integer = (7 + (dt.DayOfWeek - startOfWeek_Conflict)) Mod 7
		' Subtract the difference to get the start of the week.
		Return dt.AddDays(-1 * diff).Date
	End Function
End Module

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Now, you can easily find the start of the week for any date:

DateTime today = DateTime.Today;
DateTime startOfWeek = today.StartOfWeek();
Console.WriteLine($"The start of the week is {startOfWeek.ToShortDateString()}.");
// Output will depend on the current date, e.g. The start of the week is 17/06/2024.

DateTime today = DateTime.Today;
DateTime startOfWeek = today.StartOfWeek();
Console.WriteLine($"The start of the week is {startOfWeek.ToShortDateString()}.");
// Output will depend on the current date, e.g. The start of the week is 17/06/2024.

Dim today As DateTime = DateTime.Today
Dim startOfWeek As DateTime = today.StartOfWeek()
Console.WriteLine($"The start of the week is {startOfWeek.ToShortDateString()}.")
' Output will depend on the current date, e.g. The start of the week is 17/06/2024.

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Generating PDFs with IronPDF and Extension Methods

In this section, we'll introduce IronPDF, our industry-leading library for generating and working with PDF files in C#. We'll also see how we can leverage extension methods to create a more seamless and intuitive experience when working with this library.

IronPDF converts HTML to PDF in a way that preserves the layout and style of content as it would appear in a web browser. The library can work with raw HTML from files, URLs, and strings. Here’s a quick overview:

using IronPdf;

class Program
{
    static void Main(string[] args)
    {
        var renderer = new ChromePdfRenderer();

        // 1. Convert HTML String to PDF
        var htmlContent = "<h1>Hello, IronPDF!</h1><p>This is a PDF from an HTML string.</p>";
        var pdfFromHtmlString = renderer.RenderHtmlAsPdf(htmlContent);
        pdfFromHtmlString.SaveAs("HTMLStringToPDF.pdf");

        // 2. Convert HTML File to PDF
        var htmlFilePath = "path_to_your_html_file.html"; // Specify the path to your HTML file
        var pdfFromHtmlFile = renderer.RenderHtmlFileAsPdf(htmlFilePath);
        pdfFromHtmlFile.SaveAs("HTMLFileToPDF.pdf");

        // 3. Convert URL to PDF
        var url = "http://ironpdf.com"; // Specify the URL
        var pdfFromUrl = renderer.RenderUrlAsPdf(url);
        pdfFromUrl.SaveAs("URLToPDF.pdf");
    }
}

using IronPdf;

class Program
{
    static void Main(string[] args)
    {
        var renderer = new ChromePdfRenderer();

        // 1. Convert HTML String to PDF
        var htmlContent = "<h1>Hello, IronPDF!</h1><p>This is a PDF from an HTML string.</p>";
        var pdfFromHtmlString = renderer.RenderHtmlAsPdf(htmlContent);
        pdfFromHtmlString.SaveAs("HTMLStringToPDF.pdf");

        // 2. Convert HTML File to PDF
        var htmlFilePath = "path_to_your_html_file.html"; // Specify the path to your HTML file
        var pdfFromHtmlFile = renderer.RenderHtmlFileAsPdf(htmlFilePath);
        pdfFromHtmlFile.SaveAs("HTMLFileToPDF.pdf");

        // 3. Convert URL to PDF
        var url = "http://ironpdf.com"; // Specify the URL
        var pdfFromUrl = renderer.RenderUrlAsPdf(url);
        pdfFromUrl.SaveAs("URLToPDF.pdf");
    }
}

Imports IronPdf

Friend Class Program
	Shared Sub Main(ByVal args() As String)
		Dim renderer = New ChromePdfRenderer()

		' 1. Convert HTML String to PDF
		Dim htmlContent = "<h1>Hello, IronPDF!</h1><p>This is a PDF from an HTML string.</p>"
		Dim pdfFromHtmlString = renderer.RenderHtmlAsPdf(htmlContent)
		pdfFromHtmlString.SaveAs("HTMLStringToPDF.pdf")

		' 2. Convert HTML File to PDF
		Dim htmlFilePath = "path_to_your_html_file.html" ' Specify the path to your HTML file
		Dim pdfFromHtmlFile = renderer.RenderHtmlFileAsPdf(htmlFilePath)
		pdfFromHtmlFile.SaveAs("HTMLFileToPDF.pdf")

		' 3. Convert URL to PDF
		Dim url = "http://ironpdf.com" ' Specify the URL
		Dim pdfFromUrl = renderer.RenderUrlAsPdf(url)
		pdfFromUrl.SaveAs("URLToPDF.pdf")
	End Sub
End Class

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Creating a Simple PDF

Before we dive into extension methods, let's see how to create a simple PDF from HTML using IronPDF:

using IronPdf;

class Program
{
    static void Main(string[] args)
    {
        var renderer = new ChromePdfRenderer();
        var PDF = renderer.RenderHtmlAsPdf("Hello, World!");
        PDF.SaveAs("HelloWorld.PDF");
    }
}

using IronPdf;

class Program
{
    static void Main(string[] args)
    {
        var renderer = new ChromePdfRenderer();
        var PDF = renderer.RenderHtmlAsPdf("Hello, World!");
        PDF.SaveAs("HelloWorld.PDF");
    }
}

Imports IronPdf

Friend Class Program
	Shared Sub Main(ByVal args() As String)
		Dim renderer = New ChromePdfRenderer()
		Dim PDF = renderer.RenderHtmlAsPdf("Hello, World!")
		PDF.SaveAs("HelloWorld.PDF")
	End Sub
End Class

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This code snippet creates a PDF with the text "Hello, World!" and saves it to a file named "HelloWorld.PDF".

Extension Methods for IronPDF

Now, let's explore how we can use extension methods to enhance the functionality of IronPDF and make it easier to work with. For instance, we can create an extension method that takes an instance of the string class and generates a PDF directly from it.

using IronPdf;

public static class StringExtensions
{
    // This extension method converts a string containing HTML to a PDF and saves it.
    public static void SaveAsPdf(this string htmlContent, string filePath)
    {
        var renderer = new ChromePdfRenderer();
        var PDF = renderer.RenderHtmlAsPdf(htmlContent);
        PDF.SaveAs(filePath);
    }
}

using IronPdf;

public static class StringExtensions
{
    // This extension method converts a string containing HTML to a PDF and saves it.
    public static void SaveAsPdf(this string htmlContent, string filePath)
    {
        var renderer = new ChromePdfRenderer();
        var PDF = renderer.RenderHtmlAsPdf(htmlContent);
        PDF.SaveAs(filePath);
    }
}

Imports IronPdf

Public Module StringExtensions
	' This extension method converts a string containing HTML to a PDF and saves it.
	<System.Runtime.CompilerServices.Extension> _
	Public Sub SaveAsPdf(ByVal htmlContent As String, ByVal filePath As String)
		Dim renderer = New ChromePdfRenderer()
		Dim PDF = renderer.RenderHtmlAsPdf(htmlContent)
		PDF.SaveAs(filePath)
	End Sub
End Module

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With this extension method, we can now generate a PDF directly from a string:

string html = "<h1>Extension Methods and IronPDF</h1><p>Generating PDFs has never been easier!</p>";
html.SaveAsPdf("ExtensionMethodsAndIronPDF.PDF");

string html = "<h1>Extension Methods and IronPDF</h1><p>Generating PDFs has never been easier!</p>";
html.SaveAsPdf("ExtensionMethodsAndIronPDF.PDF");

Dim html As String = "<h1>Extension Methods and IronPDF</h1><p>Generating PDFs has never been easier!</p>"
html.SaveAsPdf("ExtensionMethodsAndIronPDF.PDF")

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Generating PDFs from URLs

Another useful extension method we can create is one that generates a PDF from a URL. We can extend the Uri class to achieve this:

using IronPdf;

public static class UriExtensions
{
    // This extension method converts a web URL to a PDF and saves it.
    public static void SaveAsPdf(this Uri url, string filePath)
    {
        var renderer = new ChromePdfRenderer();
        var PDF = renderer.RenderUrlAsPdf(url.AbsoluteUri);
        PDF.SaveAs(filePath);
    }
}

using IronPdf;

public static class UriExtensions
{
    // This extension method converts a web URL to a PDF and saves it.
    public static void SaveAsPdf(this Uri url, string filePath)
    {
        var renderer = new ChromePdfRenderer();
        var PDF = renderer.RenderUrlAsPdf(url.AbsoluteUri);
        PDF.SaveAs(filePath);
    }
}

Imports IronPdf

Public Module UriExtensions
	' This extension method converts a web URL to a PDF and saves it.
	<System.Runtime.CompilerServices.Extension> _
	Public Sub SaveAsPdf(ByVal url As Uri, ByVal filePath As String)
		Dim renderer = New ChromePdfRenderer()
		Dim PDF = renderer.RenderUrlAsPdf(url.AbsoluteUri)
		PDF.SaveAs(filePath)
	End Sub
End Module

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Now, we can easily generate a PDF from a URL like this:

Uri url = new Uri("https://www.ironpdf.com/");
url.SaveAsPdf("UrlToPdf.PDF");

Uri url = new Uri("https://www.ironpdf.com/");
url.SaveAsPdf("UrlToPdf.PDF");

Dim url As New Uri("https://www.ironpdf.com/")
url.SaveAsPdf("UrlToPdf.PDF")

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Conclusion

And voila - we explored the concept of extension methods in C#, learned how to implement them using static methods and static classes, and used real-life examples for various types. Furthermore, we introduced IronPDF, a library for generating and working with PDF files in C#. As you start using extension methods and IronPDF together, you'll see how much cleaner, more readable, and more efficient your code can become.

Ready to get your hands on IronPDF? You can start with our 30-day free trial of IronPDF. It’s also completely free to use for development purposes so you can really get to see what it’s made of. And if you like what you see, IronPDF starts as low as liteLicense for IronPDF licensing details. For even bigger savings, check out the purchase options for the Iron Software Suite where you can get all nine Iron Software tools for the price of two. Happy coding!

Csharp Extension Methods 1 related to Conclusion

Frequently Asked Questions

What are extension methods in C#?

Extension methods are special static methods that allow you to add new functionality to existing types in C# without modifying their source code. They are defined in static classes and can be called as if they were instance methods.

How do you implement an extension method in C#?

To implement an extension method, define a static method inside a static class. The first parameter of the method should be the type you want to extend, prefixed with the 'this' keyword.

Can extension methods override existing methods?

No, extension methods cannot override existing methods. If the extended type has a method with the same signature, the instance method will always take precedence.

What are some real-life examples of extension methods?

Examples include a 'Reverse' method for strings, a 'WordCount' method for strings, a 'Median' method for collections of integers, and a 'StartOfWeek' method for DateTime structs.

What are the limitations of extension methods?

Extension methods cannot access private members of the extended type and do not participate in inheritance or polymorphism.

How can extension methods be used with a PDF library?

Extension methods can be used to simplify generating PDFs with a PDF library. For example, you can create an extension method for strings to convert HTML content directly into a PDF using the library.

What is a PDF library used for in C#?

A PDF library is used for generating and working with PDF files in C#. It converts HTML to PDF while preserving layout and style, and it can work with HTML from files, URLs, and strings.

How can you generate a PDF from a URL using a PDF library?

You can extend the Uri class with an extension method that uses a PDF library to convert a web URL to a PDF and saves it to a specified file path.

Chipego Kalinda

Chat with engineering team now

Software Engineer

Chipego has a natural skill for listening that helps him to comprehend customer issues, and offer intelligent solutions. He joined the Iron Software team in 2023, after studying a Bachelor of Science in Information Technology. IronPDF and IronOCR are the two products Chipego has been focusing on, but his knowledge of all products is growing daily, as he finds new ways to support customers. He enjoys how collaborative life is at Iron Software, with team members from across the company bringing their varied experience to contribute to effective, innovative solutions. When Chipego is away from his desk, he can often be found enjoying a good book or playing football.

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