Overriding default parameters in C#

Question

Really simple to replicate, the output is bizarre;

Expected output is "bbb bbb" Actual output is "aaa bbb"

Has anyone got any MSDN explanation of this behaviour? I couldn't find any.

((a)new b()).test();
new b().test();


public class a
{
    public virtual void test(string bob = "aaa ")
    {
        throw new NotImplementedException();
    }
}

public class b : a
{
    public override void test(string bob = "bbb ")
    {
        HttpContext.Current.Response.Write(bob);
    }
}

I believe that it is invalid to change the default value of an optional parameter while overriding a method. Doesn't that change the method signature ? — Frederik Gheysels, Dec 17 '13 at 12:26
Only if you dont understand the `virtual` and `override` keywords! — Jamiec, Dec 17 '13 at 12:27
@Frederik: Shouldn't this throw a compiler warning if it is invalid? — user1567896, Dec 17 '13 at 12:28
Indeed, the compiler should complain when it is invalid. It doesn't, but Resharper does ... — Frederik Gheysels, Dec 17 '13 at 12:35
@Frederik It's not *invalid*, it's just potentially *unwise*. — Rawling, Dec 17 '13 at 12:40

score 12 · Accepted Answer · answered Dec 17 '13 at 12:26

Why do you expect "bbb bbb"?

Since you are casting the instance to a, the only information to the compiler on the first call is the version with "aaa", so that value is what is used.

In the second version without the cast, the compiler can see the "bbb", so that value is what is used.

Polymorphism impacts which method is invoked - but it doesn't impact the parameters passed. Essentially, the default values are supplied by the compiler (at the call-site), so your code is actually equivalent to:

((a)new b()).test("aaa");
new b().test("bbb");

where the "aaa" and "bbb" is supplied at compile time, by inspection of the resolved method.

So, to re-emphasize, default values are implemented at compile time, not runtime? — Boluc Papuccuoglu, Dec 17 '13 at 12:28
@BolucPapuccuoglu yes; I already added in a bit more to highlight that — Marc Gravell, Dec 17 '13 at 12:28

Overriding default parameters in C#

1 Answers1