Interesting observation on C# 4’s optional parameter

The other day I had an interesting observation on the optional parameters in C# 4, whereby if you specify a parameter as optional on an interface you don’t actually have to make that parameter optional on any implementing class:

   1: public interface MyInterface

   2: {

   3:     void TestMethod(bool flag=false);

   4: }

   5:

   6: public class MyClass : MyInterface

   7: {

   8:     public void TestMethod(bool flag)

   9:     {

  10:         Console.WriteLine(flag);

  11:     }

  12: }

Which means you won’t be able to use the implementing class and the interface interchangeably:

   1: var obj = new MyClass();

   2: obj.TestMethod(); // compiler error

   3:

   4: var obj2 = new MyClass() as MyInterface;

   5: obj2.TestMethod(); // prints false

Naturally, this bags the question of why the compiler doesn’t enforce the implementation to match the default value specified by the contract?

Luckily, my subsequent question on SO was answered by Eric Lippert from the C# compiler team, not to waste time and effort repeating what’s already been said, check out his answer and it’s clear to see the rationale here and why it would be impractical and inconvenient should the compiler does it differently.

References:

My question on StackOverflow

Article on positives and pitfalls of using optional parameters