C# Extension Methods

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Inheritence is a powerfull way to add new features to an existing type. But it is not useful in some scenarios. Suppose if we have a type in our code that is used in many places. Now If we want to add some new methods to this type then one way we can do this is by using inheritance we can create a new type and provide the new methods we want in this type. This solution however has few disadvantages.

  1. If we are using the class variables in many place in our code then we will have to replace all the existing class variables with the the new class variables. This needs refactoring of the entire application.
  1. We can not use inheritance with structures.

The solution is using extension methods.

Using extension methods we can add new methods to an existing Type. If we have lets say an int type and we want to add a new method to this type then by using extension methods we can do this. To achieve this we need to define a static class and a static method in that static class. And we need to use the this keyword along with the type we are adding this extension method to as the first method parameter type.

Here is a short example of an extension method that multiplies the integer entered by the user by 5 using extension method.

public static class MyInt
    public static int MultiplyByFive(this int num)
        num *= 5;
        return num;

class ExtensionMethods

    public static void Main(string[] args)
        int x;
        Console.WriteLine("Enter an integer"); 
        int y=x.MultiplyByFive();
        Console.WriteLine("Integer {0} multiplied by 5 is {1}",x,y);


Remember that extension methods must be defined inside a top level static class and not inside a nested class and the method itself must be declared as static. System. Linq namespace has a lot of extension methods that makes easier working with Linq.

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