Question

Got an interesting oddity – thought someone might be able to help.

This came out of some fun with nullable types from this question:

How to check if an object is nullable?

Option Strict On

Module Test
  ' Call this overload 1
  <Extension()>
  Function IsNullable(obj As ValueType) As Boolean
    Return False
  End Function

  ' Call this overload 2
  <Extension()>
  Function IsNullable(Of T As {Structure})(obj As Nullable(Of T)) As Boolean
    Return True
  End Function

  Sub Test() 
    ' a is an integer!
    Dim a As Integer = 123

    ' calling IsNullable as an extension method calls overload 1 and returns false
    Dim result1 As Boolean = a.IsNullable()

    ' calling IsNullable as method calls overload 2 and returns true
    Dim result2 As Boolean = IsNullable(a)

    ' why? surely the compiler should treat both those calls as equivalent
  End Sub
End Module

I would expect that both calls to IsNullable would be treated the same by the compiler, but that is not the case. The extension method call uses a different overload to the normal method call even though the argument “a” is unchanged.

My question is why? What makes the compiler change its mind between the two calls?

FTR: We are using Visual Studio 2010, .NET Framework 4.

Answer 1

I think this is a bug, or at least a VB.NET “feature”. (I’m just not sure which of VB.NET or C# is wrong.)

I have tried in LINQPad 4 (because that’s what I’ve got on the machine I’m using) and for C# I got False for both results, for every value type and enum except for Nullable types, of course.

Whereas for VB.NET I get the False and True for all value types and enums, except for Nullable types, and ValueType and [Enum] which return False, False because you can’t have a ValueType? or [Enum]?. With Option Strict Off, Object causes late binding, and fails at runtime to locate either overload, but the second result is False also because you can’t have Object?.

For completeness, Nullable types return True, True for both languages as expected.

The fact that C# is doing something different (assuming my test is correct) confirms the reference to the C# “Better Conversion” check is wrong (or being misread at least – in that C# is not doing what is being interpreted as why VB.NET is doing what it is doing).

However, I do agree that the issue is probably related to the implicit conversion to Nullable(Of T) existing and somehow being a higher priority to the implicit conversion to ValueType.

Here’s my LINQPad 4 “Query” (C# Program):

void Main()
{
    Test.test();
}

// Define other methods and classes here
static class Test
{
    static bool IsNullable(this ValueType obj)
    {
        return false;
    }

    static bool IsNullable<T>(this T? obj) where T:struct
    {
        return true;
    }

    public static void test()
    {
        int x = 42;

        bool result1 = x.IsNullable();
        bool result2 = IsNullable(x);

        result1.Dump("result1");
        result2.Dump("result2");
    }
}