Add Methods to Built-in Classes in C#

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Star Gazers

2 min readSep 19, 2021

Consider, you are writing a program where you work with the arrays, lists. Let’s say you need to check if the list is empty or not. What would you do ?

Code Without Extension Method

Probably, if you do not know about the extension methods, you would write something like this:

List<int> list = new List<int>();if (list.Count == 0) {
   // Do somethings beautiful
}

Code After Writing Extension Method

Would not it be better if we would write code like this:

List<int> list = new List<int>();if (list.IsEmpty()) {
   // Do somethings beautiful
}

This definitely would be helpful. So, let’s see how we can do it:

Writing Extension Method

I am going to create a class for only extension methods. This is not required, but it would be much easier to reuse and maintain the code.

public static class ListExtension
{
    public static bool IsEmpty(this List<int> list)
    {
        return list?.Count == 0;
    }
}

First of all, this class should be static and non-generic.

Note: Non-Generic means that we cannot write a generic type like this:

public static class ListExtension<MyType>
{
    
}

How Does It Work ?

Although we make these extensions methods static, they actually will not be static. Instead, they will be used as instance methods. I will give an example in a minute.

When a List<int> instance calls IsEmpty() method, that instance will be sent as a parameter to this method. this keyword tells the compiler to recognize IsEmpty() method as the built-in List<int> class’ method.

Then, we have a return list?.Count == 0 which first checks if the list is null or not (with ? operator, you can read more here) and return a boolean that indicates if the list’s item count is 0 or not.

Example

In this example, we have created a list with three items in it. When we call IsEmpty() method, the output is false obviously.

public class Program
{
    public static void Main(string[] args)
    {
        List<int> list = new List<int>() { 1, 2, 3 };
        
        Console.WriteLine(list.IsEmpty()); // outputs false    }
}

If you are reading this sentence, then it means that you have read the whole article and have a good understanding of extension methods.

I hope it was helpful for you and see you in the next articles :).