Default interface methods

C# course · Intermediate · Lesson 50/116

Learn backward compatibility with default interface methods.

Default interface methods

From C# 8 onwards interface methods can have a body. These are called default interface methods. Declaration: void Write(string message) { Console.WriteLine(message); }.

When a class implements this interface, it does not have to override the default method; the body from the interface is used. This preserves backward compatibility when adding methods to existing interfaces. A default method can only be called through an interface-typed variable, not a class-typed one.

Calling default methods

If a class does not override the default method, the call uses the interface's body. The call must be made through the interface type: IWriter w = new ConsoleWriter(); w.Write("Hello");.

If the class writes its own body, that one is used. Default methods suit helper methods and backward compatibility. Providing a default body when adding a new method doesn't break existing implementing classes.

Exercise Default writer

Add a default body to IWriter's void Write(string message) method: Console.WriteLine(message);. In Main, create IWriter w = new ConsoleWriter(); and call w.Write("Hello").

Show example solution

Add `void Write(string message) { Console.WriteLine(message); }` to IWriter. In Main write `IWriter w = new ConsoleWriter(); w.Write("Hello");`.
Example solution:
using System;

interface IWriter
{
    void Write(string message)
    {
        Console.WriteLine(message);
    }
}

class ConsoleWriter : IWriter
{
}

class Program
{
    static void Main()
    {
        IWriter w = new ConsoleWriter();
        w.Write("Hello");
    }
}

Test yourself

Which statement about default interface methods is correct?
- Default methods must be static.
- Default methods must be overridden in the implementing class.
- Default methods exist only in C# 7.
- Default methods have a body and provide backward compatibility.
Show answer

Default methods have a body and provide backward compatibility. — Correct. Default methods have a body and make it safe to add methods to existing interfaces.
Match: default method parts

body

in curly braces
no override

default body is used
interface type

call route
backward compatibility

new method added safely

Fill in the blank: default method body

interface IWriter
{
    void Write(string message)
    ____
        Console.WriteLine(message);
    }
}

Show answer

interface IWriter
{
    void Write(string message)
    {
        Console.WriteLine(message);
    }
}

Default interface methods