64. Default interface methods#

In traditional object oriented programming, interfaces have been contracts that classes can implement, dictating a set of methods that the implementing classes must provide. This is also the picture we painted in the chapter on interfaces.

However, a newer feature known as “default interface methods” change this narrative by allowing you to define a method within an interface and provide a default implementation for it. This interesting feature brings a new dimension to interface design that in some sense can be thought of as multiple inheritance.

Key points

  • Default interface methods allow us to add methods with implementation to an interface.

  • Default methods are only available on objects whose compile-time type is the same as the one in which the method is defined.

Let’s start with a basic example. Consider an interface ISequence which captures the idea of sequences of integers.

interface ISequence
    int Next();

    int[] Take(int n)
        int[] result = new int[n];
        for (int i = 0; i < n; i++)
            result[i] = Next();
        return result;

Notice how the interface above not only demands that implementors of the interface supplies an implementation for the method called Next but also itself defines a method called Take.

Let’s now write a class that implements this interface.

class NaturalNumbers : ISequence
    int n = -1;
    public int Next() => n++;

The class NaturalNumbers implements ISequence but is not required to provide an implementation for Take. Should we want a specialized implementation of Take in the class we could however of course define it.

A key aspect of default interface methods is their relationship with compile-time and run-time types. The default method is available on an instance of a class only if the compile-time type of the reference is the interface with the default method. This distinction is crucial and can affect how your code behaves at runtime.

NaturalNumbers seq = new NaturalNumbers();
seq.Take(10); // ❌ Does not compile!
(2,5): error CS1061: 'NaturalNumbers' does not contain a definition for 'Take' and no accessible extension method 'Take' accepting a first argument of type 'NaturalNumbers' could be found (are you missing a using directive or an assembly reference?)

ISequence seq = new NaturalNumbers();
seq.Take(10); // ✅ Compiles!

Why did the language designers decide that we cannot call the method Take unless the compile-time type is the same type as the one where the method is defined? Classes can implement multiple interfaces and since these interfaces could contain methods with the same signature, we need some way of determine which implementation should be executed. Consider the following code:

interface IPositionable
    int X { get; set; }
    int Y { get; set; }

    void Print() => Console.WriteLine($"({X}, {Y})");
interface ISizable
    int Width { get; set; }
    int Height { get; set; }

    void Print() => Console.WriteLine($"{Width} x {Height}");
class Rectangle : IPositionable, ISizable
    public int X { get; set; }
    public int Y { get; set; }
    public int Width { get; set; }
    public int Height { get; set; }

If we now instantiate a Rectangle and store it in a variable whose compile-time type is Rectangle, which implementation should be executed if we call Print()?

Rectangle rect = new Rectangle();
(2,6): error CS1061: 'Rectangle' does not contain a definition for 'Print' and no accessible extension method 'Print' accepting a first argument of type 'Rectangle' could be found (are you missing a using directive or an assembly reference?)

Since there is no obvious answer, the language designers chose to simply make this code not compile. If however we store the object in a variable whose type is one of the interfaces then the call is no longer ambiguous and the compiler can determine which implementation should be executed.

ISizable rect = new Rectangle();
0 x 0
IPositionable rect = new Rectangle();
(0, 0)

In conclusion, the introduction of default interface methods in C# has significantly altered the traditional understanding of interfaces. By allowing interfaces to provide default implementations, C# extends their utility and flexibility, enabling a form of multiple inheritance.

This change is a notable shift in the language, opening up new possibilities in object oriented design. However, it also introduces complexities, especially regarding compile-time and run-time types. Understanding these nuances is crucial to effectively leverage this feature and avoid potential pitfalls.