Multiple interfaces

C# course · Intermediate · Lesson 49/116

Learn that a class can implement multiple interfaces.

Multiple interfaces

A class can implement more than one interface. They are separated by commas: class Button : IDrawable, IClickable. The class must implement all methods from both interfaces.

Multiple interfaces let a class take on different roles. A Button can be both drawable and clickable. Any of the interfaces can be used as the variable type: IDrawable d = new Button(); or IClickable c = new Button();. C# does not allow multiple inheritance for classes but it does for interfaces.

When to use multiple interfaces?

If a class has multiple roles, defining a separate interface for each role is a good fit. Example: a Button is both drawn and clicked; two interfaces keep the roles clear.

Prefer small, focused interfaces over a single large one. This is called the Interface Segregation Principle. A class implements only the interfaces it needs; it is not burdened with methods it doesn't use.

Exercise Button class

Define the Button class: it implements IDrawable and IClickable. The Draw method does Console.WriteLine("Drawn");, the Click method does Console.WriteLine("Clicked");. In Main, create a Button and call Draw and Click.

Show example solution

IDrawable, IClickable { public void Draw() { Console.WriteLine("Drawn"); } public void Click() { Console.WriteLine("Clicked"); } }`. In Main write `Button b = new Button(); b.Draw(); b.Click();`.
Example solution:
using System;

interface IDrawable
{
    void Draw();
}

interface IClickable
{
    void Click();
}

class Button : IDrawable, IClickable
{
    public void Draw()
    {
        Console.WriteLine("Drawn");
    }

    public void Click()
    {
        Console.WriteLine("Clicked");
    }
}

class Program
{
    static void Main()
    {
        Button b = new Button();
        b.Draw();
        b.Click();
    }
}

Test yourself

Fill in the blank: multiple interfaces

class Button ____
{
    public void Draw() { }
    public void Click() { }
}

Show answer

class Button : IDrawable, IClickable
{
    public void Draw() { }
    public void Click() { }
}

Match: multiple interfaces

comma

interface separator
two interfaces

two roles
IDrawable d = new Button()

drawing role
IClickable c = new Button()

clicking role
Which statement about multiple interfaces is correct?
- A class can implement multiple comma-separated interfaces.
- A class can implement at most two interfaces.
- When implementing multiple interfaces, `:` is written once.
- Multiple interfaces apply only to structs.
Show answer

A class can implement multiple comma-separated interfaces. — Correct. In C# classes can implement multiple comma-separated interfaces.

Multiple interfaces