title: Why does this extension method throw a NullReferenceException in VB.NET?
body: <p>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:</p>
  
  <pre><code>// code in static class
  static bool IsNull(this object obj) {
      return obj == null;
  }
  
  // code elsewhere
  object x = null;
  bool exists = !x.IsNull();
  </code></pre>
  
  <p>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 <em>thought</em> was the VB.NET equivalent of the above code, only to discover that it actually throws a <code>NullReferenceException</code>. The code I wrote was this:</p>
  
  <pre><code>' code in module '
  &lt;Extension()&gt; _
  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()
  </code></pre>
  
  <p>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 <strong>not</strong> 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.)</p>
  
  <p>Can anybody explain this one to me?</p>