.net - Why does this extension method throw a NullReferenceException in VB.NET?

Question

From previous experience I had been under the impression that it's perfectly legal (though perhaps not advisable) to call extension methods on a null instance. So in C#, this code compiles and runs:

// code in static class
static bool IsNull(this object obj) {
    return obj == null;
}

// code elsewhere
object x = null;
bool exists = !x.IsNull();

However, I was just putting together a little suite of example code for the other members of my development team (we just upgraded to .NET 3.5 and I've been assigned the task of getting the team up to speed on some of the new features available to us), and I wrote what I thought was the VB.NET equivalent of the above code, only to discover that it actually throws a NullReferenceException. The code I wrote was this:

' code in module '
<Extension()> _
Function IsNull(ByVal obj As Object) As Boolean
    Return obj Is Nothing
End Function

' code elsewhere '
Dim exampleObject As Object = Nothing
Dim exists As Boolean = Not exampleObject.IsNull()

The debugger stops right there, as if I'd called an instance method. Am I doing something wrong (e.g., is there some subtle difference in the way I defined the extension method between C# and VB.NET)? Is it actually not legal to call an extension method on a null instance in VB.NET, though it's legal in C#? (I would have thought this was a .NET thing as opposed to a language-specific thing, but perhaps I was wrong.)

Can anybody explain this one to me?

Answer 1

You cannot extend the object type in VB.NET.

Mainly, we do not allow extension methods to be called off of any expression that is statically typed as "Object". This was necessary to prevent any existing late bound code you may have written from being broken by extension methods.

Reference:

• http://blogs.msdn.com/vbteam/archive/2007/01/24/extension-methods-and-late-binding-extension-methods-part-4.aspx

• http://johnwest.spaces.live.com/Blog/cns!EBA860160D5F5D75!463.entry