Question

I have come across something confusing or potentially a bug in Visual Studio 2010 when defining an interface in my VB application: When defining an interface method with a default parameter of type Double, using the Double.NaN constant as the default value causes the code editor/intellisense/precompiler some issues.

The following code underlines “INaNTest” and “INaNTest.DoSomething” claiming that ‘DoSomething’ cannot implement ‘DoSomething’ because there is no matching sub on interface ‘INaNTest’:

Public Class NaNTest
    Implements INaNTest
    Public Sub DoSomething(ByVal x As Double,
                           Optional ByVal a As Double = Double.NaN)
                           Implements INaNTest.DoSomething
    End Sub
End Class

Public Interface INaNTest
    Sub DoSomething(ByVal x As Double,
                    Optional ByVal a As Double = Double.NaN)
End Interface

Removing the implementation and starting from:

Public Class NaNTest
    Implements INaNTest
End Class

Public Interface INaNTest
    Sub DoSomething(ByVal x As Double,
                    Optional ByVal a As Double = Double.NaN)
End Interface

where now “NaNTest” is underlined (Class ‘NaNTest’ must …), hitting the return key at the end of the line “Implements INaNTest” (i.e. automatically insert implementation) adds the implementation:

    Public Sub DoSomething(ByVal x As Double,
                           Optional ByVal a As Double = -1.#IND)
                           Implements INaNTest.DoSomething

    End Sub

in which the code editor then underlines ‘#’ (Identifier expected.). Thus the code automatically added code that is not right.

Alternatively now, starting with the original code above, using the Error Correction Options button on the underlined “INaNTest.DoSomething” and selecting ‘Generate method stub for ‘DoSomething’ in ‘INaNTest’, the added method stub is:

    Sub DoSomething(ByVal x As Double,
                    Optional ByVal a As Double = NaN)

where now “NaN” has been divorced from the “Double.” prefix and underlined (‘NaN’ is not declared. It may be inaccessible due to its protection level.) The code editor has inserted invalid code again.

Is there a correct solution to using Double.NaN as the default value for a method as defined on an interface, in VB.net, or is there a fundamental reason why this is impossible?

Many thanks,
JCollins

Answer 1

Ugh, that’s fugly. Hard to characterize this as anything other than a bug. The default formatting for NaN when you let the IDE generate the method signature shows what language the VB.NET team uses, that’s the way the C++ runtime library formats NaN. Attempts to convince it you know what you are doing are indeed futile, afaict.

You can report this at connect.microsoft.com. While you wait for the ‘fixed in the next version of Visual Studio’ to see the light of day, you might consider using nullable types as the workaround:

Public Class NaNTest
    Implements INaNTest
    Public Sub DoSomething(ByVal x As Double, Optional ByVal a As Double? = Nothing) Implements INaNTest.DoSomething
        If a.HasValue Then
            '' etc..
        End If
    End Sub
End Class

Public Interface INaNTest
    Sub DoSomething(ByVal x As Double,
                    Optional ByVal a As Double? = Nothing)
End Interface

Fwiw, it does work when you use Double.Epsilon as the default value. Kinda silly but not an entirely unreasonable workaround either. Just don’t let the IDE generate the implementation, then it gets silly.