Second Question – What would you expect this to do?

I expected the call to Bar() to call the correct bar (not the one with a default parameter) Not sure why you would want to provide two methods of the same name where one has a default. I think the idea of the parameter defaults is so you don’t have to provide extra overloads.

I tested it and the two following tests both threw NotImplementedExceptions.

public interface IFooJoey
{
int ParamValue { get; }
void Bar();
void Bar(int paramValue = 1);
}

public class FooJoey : IFooJoey
{
public int ParamValue { get; private set; }

public void Bar()
{
throw new NotImplementedException();
}

public void Bar(int paramValue)
{
ParamValue = paramValue;
}
}

[TestClass]
public class UnitTest1Joey
{
[TestMethod]
public void Should_get_the_concrete_class_default_value()
{
FooJoey f1 = new FooJoey();
f1.Bar();
f1.ParamValue.ShouldBeEqualTo(1);
}

[TestMethod]
public void Should_get_the_interface_default_value()
{
IFooJoey f = new FooJoey();
f.Bar();
f.ParamValue.ShouldBeEqualTo(1);
}
}

IFoo firstFoo = new Foo(); firstFoo.Bar() // 1
Foo secondFoo = new Foo(); secondFoo.Bar() // 1000

Second question:
How would method Overloading work in this situation?

public interface IFoo
{
int ParamValue { get; }
void Bar();
void Bar(int paramValue = 1);
}

((IFoo) new Foo()).Bar() – would use the value of ‘1’.

Seems like it violated the principle of least surprise. Maybe it is just me though.