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POWhy does this extension method throw a NullReferenceException in VB.NET?
 

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  1. POWhy does this extension method throw a NullReferenceException in VB.NET?
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    <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>
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    1. USDan Tao
    1. USDan Tao
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    1. COIs the IsNull method just an example or are do you actually want to use x.IsNull() instead of 'x Is Nothing' or 'x == null'?
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      1. USjrummell
    2. CO@jrummell: It's just an example. As I mentioned, I was writing some example code to illustrate how extension methods work for some of my team members. I intended for this method to include a comment along the lines of "You can actually do this with extension methods, although I wouldn't recommend it" -- just to show how under the hood an extension method is really just a static (Shared) method. But then I discovered that I *couldn't* do it in VB, which surprised me.
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      1. POWhy does this extension method throw a NullReferenceException in VB.NET?
      1. USDan Tao
    3. COAs has been found, this is due to support for late binding...like you (Dan) I didn't realise VB had such support for late binding, I'm guessing it's a compatibility thing with VB6. As a slight aside, if you're looking into setting out some standards/training for other developers you may wish to mandate Option Strict as I feel it helps pick up a lot potential problems. YMMV of course.
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      1. USGareth Wilson
 

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