Extension methods are used to extend a class’s functionality.
when are extension methods useful ?

• extend a class’s functionality that we can not modify and have no access to its source code.
• keep code clean by writing simple methods that can give us more features .

Steps

Create a static class that will house our static extension method, This method returns if the supplied int is greater than the int we are working on.

public static class Extension
{
    public static bool IsGreaterThan(this int input, int secondValue)
    {
        return input > secondValue;
    }
}

Call the extension Method.

int intToTestAgainst = 23;

int secondInt = 25;

var isGreater = intToTestAgainst.IsGreaterThan(25);

Console.WriteLine($"Is {intToTestAgainst} > {secondInt} ? \n {isGreater}"); 

Console.ReadLine();

Extension methods can work on any type of class where they can solve more complex problems but this is a simple example to let you understand how it works.