Defining interfaces

Learn to define a contract with interface.

Defining an interface

An interface is a contract that the implementing class must follow. It contains method signatures but no bodies; it cannot hold fields. Declaration: interface IGreeter { string Greet(string name); }.

When a class implements an interface, it must write the body of every method. This lets different classes follow the same contract. Interfaces cannot be instantiated with new; they are used through instances of implementing classes.

Interface declaration

An interface declaration starts with the interface keyword. The body is in curly braces and each method signature ends with a semicolon: string Greet(string name);.

Methods are not marked public; interface members are public by default. Interface names usually start with an I prefix: IGreeter, IShape, IComparable. This avoids confusion with classes and follows .NET convention.

Exercise Greeter interface

Define the IGreeter interface with the string Greet(string name); method signature. EnglishGreeter and Main are already in place; you only add the interface.

Show example solution

Add `interface IGreeter { string Greet(string name); }`.
Example solution:
using System;

interface IGreeter
{
    string Greet(string name);
}

class EnglishGreeter : IGreeter
{
    public string Greet(string name)
    {
        return "Hello " + name;
    }
}

class Program
{
    static void Main()
    {
        IGreeter g = new EnglishGreeter();
        Console.WriteLine(g.Greet("Alex"));
    }
}

Test yourself

Which statement about interfaces is correct?
- An interface can hold fields.
- An interface is a contract and the implementing class provides the methods.
- An interface can be instantiated with new.
- An interface can be implemented by only one class.
Show answer

An interface is a contract and the implementing class provides the methods. — Correct. An interface is a contract; the implementing class provides the methods.

Fill in the blank: interface method

interface IGreeter
{
    ____;
}

Show answer

interface IGreeter
{
    string Greet(string name);
}

Match: interface concepts

interface

contract
bodyless signature

ends with semicolon
I prefix

IShape, IGreeter
new IGreeter()

invalid

Next lesson: Implementing interfaces

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