C# 4.0: Can I use a TimeSpan as an optional parameter with a default value?

You can work around this very easily by changing your signature.

void Foo(TimeSpan? span = null) {

   if (span == null) { span = TimeSpan.FromSeconds(2); }

   ...

}

I should elaborate - the reason those expressions in your example are not compile-time constants is because at compile time, the compiler can't simply execute TimeSpan.FromSeconds(2.0) and stick the bytes of the result into your compiled code.

As an example, consider if you tried to use DateTime.Now instead. The value of DateTime.Now changes every time it's executed. Or suppose that TimeSpan.FromSeconds took into account gravity. It's an absurd example but the rules of compile-time constants don't make special cases just because we happen to know that TimeSpan.FromSeconds is deterministic.

My VB6 heritage makes me uneasy with the idea of considering "null value" and "missing value" to be equivalent. In most cases, it's probably fine, but you might have an unintended side effect, or you might swallow an exceptional condition (for example, if the source of span is a property or variable that should not be null, but is).

I would therefore overload the method:

void Foo()
{
    Foo(TimeSpan.FromSeconds(2.0));
}
void Foo(TimeSpan span)
{
    //...
}

This works fine:

void Foo(TimeSpan span = default(TimeSpan))

Note: default(TimeSpan) == TimeSpan.Zero