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

Question

Both of these generate an error saying they must be a compile-time constant:

void Foo(TimeSpan span = TimeSpan.FromSeconds(2.0))
void Foo(TimeSpan span = new         


        

Answer 1

void Foo(TimeSpan span = default(TimeSpan))
{
    if (span == default(TimeSpan)) 
        span = TimeSpan.FromSeconds(2); 
}

provided default(TimeSpan) is not a valid value for the function.

Or

//this works only for value types which TimeSpan is
void Foo(TimeSpan span = new TimeSpan())
{
    if (span == new TimeSpan()) 
        span = TimeSpan.FromSeconds(2); 
}

provided new TimeSpan() is not a valid value.

Or

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

This should be better considering chances of null value being a valid value for the function are rare.