Why do we have interfaces?

From a theoretical point of view, both interface implementation and class inheritance solve the same problem: They allow you to define a subtype relationship between types.

So why do we have both in C#? Why do we need interfaces at all? Can’t we just define an interface as an abstract class, just as we do, for example, in C++?

The reason for this is the diamond problem: _{(Image source)}

If both B and C implement A.DoSomething() differently, which implementation should D inherit? That’s a hard problem, and the Java as well as the C# designers decided to avoid it by allowing multiple inheritance only for special base types which do not include any implementation. They decided to call these special base types interfaces.

So, there is no “principle of interface”. Interfaces are just a “tool” to solve a particular problem.

So why do we need default implementations?

Backwards compatibility. You wrote a vastly successful library used by thousands of developers worldwide. Your library contains some interface I, and now you decide that you need an extra method M on it. The problem is:

• You can’t add another method M to I, because that would break existing classes implementing I (because they don’t implement M), and
• you can’t change I to an abstract base class, because that, as well, would break existing classes implementing I, and you will lose the ability to do multiple inheritance.

So how do default implementations avoid the diamond problem?

By not inheriting those default methods (example inspired by the one in this article, see the full article for some interesting corner cases):

interface I1
{
    void M() { Console.WriteLine("I1.M"); } // default method
}

interface I2
{
    void M() { Console.WriteLine("I2.M"); } // default method
}

class C : I1, I2 { }

class Program
{
    static void Main(string[] args)
    {
        // c, i1 and i2 reference the same object
        C c = new C();
        I1 i1 = c;
        I2 i2 = c;

        i1.M(); // prints "I1.M"
        i2.M(); // prints "I2.M"
        c.M();  // compile error: class 'C' does not contain a member 'M'
    }
}