# Implementing an Interface

*phrase*

say it: **“IM-pluh-ment-ing an IN-ter-fays”** **/ˈɪmplɪmɛntɪŋ ən ˈɪntərfeɪs/**

A class keeping an interface's promise by writing every member the interface lists.

## What does **implementing an interface** mean in C#?

**Implementing an Interface** — A class keeping the promise an interface makes: it names the interface after its colon and writes a public version of every member the interface lists. The compiler refuses the class until every member is there.

```csharp
interface IShippable { decimal ShippingCost(); }

class Book : IShippable
{
    public decimal ShippingCost() => 3.99m;
}
```

## In a sentence

- “By **implementing an interface**, IShippable, Book promises a ShippingCost method.”

Level: Journeyman · [More C# terms at this level](https://www.ask-codey.com/glossary?category=c-sharp&level=3)

Updated: October 28, 2026

Source: https://www.ask-codey.com/glossary/implementing-an-interface