C# Extension Methods

C# allows us to create and add new methods to existing class without creating a new child class. The existing class does not require recompiling the code. C# extension methods are the special type of the static methods that can be called as instance methods.

We can add extension methods in both C# predefined classes and user created custom classes. We need to consider the following points to define an extension method.

An extension method should be a static method.
It must have this keyword associate with class name.
The class name should be the first parameter in the parameter list.

In the following example, we are adding an extension method GetUpperCase() in C# String class.

C# Extension Methods Example 1

using System;
namespace CSharpFeatures
{
    public static class StringHelper
    {
        public static string GetUpperCase(this string name)
        {
            return name.ToUpper();
        }
    }
    public class ExtensionMethodExample
    {
        static string name = "javatpoint";
        static void Main(string[] args)
        {
            string str = name.GetUpperCase();  // calling extension method using string instance 
            Console.WriteLine(str);
        }
    }
}

Output

JAVATPOINT

C# Extension Methods Example 2

In the following example, we are adding extension method to a Student class.

using System;
namespace CSharpFeatures
{
    public static class StudentHelper
    {
        public static string GetUpperName(this Student student) // Using this before Student class
        {
            return student.name.ToUpper();
        }
    }
    public class Student
    {
        public string name = "javatpoint";
        public string GetName()
        {
            return this.name;
        }
    }
    public class ExtensionMethodExample
    {
        static void Main(string[] args)
        {
            Student student = new Student();
            Console.WriteLine(student.GetName());
            Console.WriteLine(student.GetUpperName());
        }
    }
}

Output