Why are C# 4 optional parameters defined on interface not enforced on implementing class?

UPDATE: This question was the subject of my blog on May 12th 2011. Thanks for the great question!

Suppose you have an interface as you describe, and a hundred classes that implement it. Then you decide to make one of the parameters of one of the interface's methods optional. Are you suggesting that the right thing to do is for the compiler to force the developer to find every implementation of that interface method, and make the parameter optional as well?

Suppose we did that. Now suppose the developer did not have the source code for the implementation:

// in metadata:
public class B 
{ 
    public void TestMethod(bool b) {}
}

// in source code
interface MyInterface 
{ 
    void TestMethod(bool b = false); 
}
class D : B, MyInterface {}
// Legal because D's base class has a public method 
// that implements the interface method

How is the author of D supposed to make this work? Are they required in your world to call up the author of B on the phone and ask them to please ship them a new version of B that makes the method have an optional parameter?

That's not going to fly. What if two people call up the author of B, and one of them wants the default to be true and one of them wants it to be false? What if the author of B simply refuses to play along?

Perhaps in that case they would be required to say:

class D : B, MyInterface 
{
    public new void TestMethod(bool b = false)
    {
        base.TestMethod(b);
    }
}

The proposed feature seems to add a lot of inconvenience for the programmer with no corresponding increase in representative power. What's the compelling benefit of this feature which justifies the increased cost to the user?

UPDATE: In the comments below, supercat suggests a language feature that would genuinely add power to the language and enable some scenarios similar to the one described in this question. FYI, that feature -- default implementations of methods in interfaces -- will be added to C# 8.

An optional parameter is just tagged with an attribute. This attribute tells the compiler to insert the default value for that parameter at the call-site.

The call obj2.TestMethod(); is replaced by obj2.TestMethod(false); when the C# code gets compiled to IL, and not at JIT-time.

So in a way it's always the caller providing the default value with optional parameters. This also has consequences on binary versioning: If you change the default value but don't recompile the calling code it will continue to use the old default value.

On the other hand, this disconnect means you can't always use the concrete class and the interface interchangeably.

You already can't do that if the interface method was implemented explicitly.

Because default parameters are resolved at compile time, not runtime. So the default values does not belong to the object being called, but to the reference type that it is being called through.

Optional parameters are kind of like a macro substitution from what I understand. They are not really optional from the method's point of view. An artifact of that is the behavior you see where you get different results if you cast to an interface.

Just want to add my take here, as the other answers do provide reasonable explanations, but not ones that fully satisfy me.

Optional parameters are syntactic sugar for compile-time injection of the default value at the call site. This doesn't have anything to do with interfaces/implementations, and it can be seen as purely a side-effect of methods with optional parameters. So, when you call the method,

public void TestMethod(bool value = false) { /*...*/ }

like SomeClass.TestMethod(), it is actually SomeClass.TestMethod(false). If you call this method on an interface, from static type-checking, the method signature has the optional parameter. If you call this method on a deriving class's instance that doesn't have the optional parameter, from static type-checking, the method signature does not have the optional parameter, and must be called with full arguments.

Due to how optional parameters are implemented, this is the natural design result.

Thanks for your explanation @eric-lippert

Here is some code example:

[Fact]
public void TestOptionalMethodArgument()
{
    var implementation = new TestHello();
    IHello @interface = implementation;

    Assert.Equal(23, @interface.Action());
    Assert.Equal(40, implementation.Action());
}

public class TestHello : IHello
{
    public int Action(int number = 40)
        => number;
}

public interface IHello
{
    int Action(int number = 23);
}

var obj = new MyClass();        
obj.TestMethod(); // compiler error

var obj2 = new MyClass() as MyInterface;
obj2.TestMethod(); // prints false

Use

MyInterface obj = new MyClass();        
obj.TestMethod(); // compiler error

var obj2 = new MyClass() as MyInterface;
obj2.TestMethod(); // prints false

and both result in false