How to Create Extension Method in C# - Simple Guide

In C#, create an extension method by defining a static method inside a static class with the first parameter preceded by this keyword to specify the type it extends. This allows you to call the method as if it were part of the original type.

Syntax

An extension method must be a static method inside a static class. The first parameter uses the this keyword followed by the type you want to extend. This tells C# which type the method extends.

• static class: container for extension methods
• static method: the extension method itself
• this TypeName param: the type being extended

csharp

public static class ExtensionClass
{
    public static ReturnType MethodName(this TypeName param, OtherParameters)
    {
        // method body
    }
}

Example

This example adds an extension method IsEven to the int type. It returns true if the number is even, otherwise false. You can call it like a normal method on any integer.

csharp

using System;

public static class IntExtensions
{
    public static bool IsEven(this int number)
    {
        return number % 2 == 0;
    }
}

class Program
{
    static void Main()
    {
        int value = 10;
        bool result = value.IsEven();
        Console.WriteLine($"Is {value} even? {result}");
    }
}

Output

Is 10 even? True

Common Pitfalls

• Extension methods must be in a static class; forgetting this causes errors.
• The extension method itself must be static.
• The first parameter must use this keyword to specify the extended type.
• Extension methods cannot access private members of the extended type.
• Be careful not to name extension methods the same as existing methods to avoid confusion.

csharp

/* Wrong: Missing static class */
// public class Extensions
// {
//     public static bool IsPositive(this int number) => number > 0;
// }

/* Correct: Static class and method */
public static class Extensions
{
    public static bool IsPositive(this int number) => number > 0;
}

Quick Reference

Remember these key points when creating extension methods:

• Use static class to hold extension methods.
• Extension methods must be static.
• The first parameter uses this keyword to specify the type extended.
• Call extension methods like instance methods on the extended type.

Key Takeaways

Extension methods are static methods in static classes with the first parameter using 'this' to specify the extended type.

They let you add new methods to existing types without changing their source code.

Always place extension methods inside static classes and mark them static.

Extension methods cannot access private members of the extended type.

Use extension methods to write cleaner and more readable code by calling them like instance methods.