Why doesn't calling a null event handler raise an exception?

Question

After reading this question, it seems like the following code should fail:

private void Form1_Load(object sender, EventArgs e)
{
   EventHandler myHandler = null;
   myHandler(this, null);
}

But when I run it, it works just fine (and does nothing). How does that code behave any differently than the following?

private void Form1_Load(object sender, EventArgs e)
{
   EventHandler myHandler = null;
   EventHandler myCopy = myHandler;
   if (myCopy != null)
   {
      myHandler(this, null);
   }
}

Edit: Catching the exception this way works, according to Lasse V. Karlsen's answer:

private void Form1_Load(object sender, EventArgs e)
{
   try
   {
      EventHandler myHandler = null;
      myHandler(this, null);
   }
   catch (Exception ex)
   {
      this.Text = "Exception!";
   }
}

`Why doesn't calling a null event handler raise an exception?` - You're not calling a `null` event handler. You're just passing `null` as a parameter to an existing (non-null) event handler. — Federico Berasategui, Mar 05 '13 at 20:52
It *certainly* looks like he's calling a `null` event handler (delegate) reference, doesn't it? What am I missing here? — Lasse V. Karlsen, Mar 05 '13 at 20:55
@LasseV.Karlsen Did the OP ninja edit that extra line in perhaps? — Servy, Mar 05 '13 at 20:55
That could certainly be, I can't see any edit history on the question, but he might've edited that in quickly enough for the edits to merge into the original post. — Lasse V. Karlsen, Mar 05 '13 at 20:56

score 6 · Accepted Answer · edited May 23 '17 at 12:13

The problem here is that the Load event is swallowing your exception.

There are other questions here on SO about this and other posts on the net about it:

Why the form load can't catch exception?
Paul Betts’s personal website / blog / what-have-you The case of the disappearing OnLoad exception – user-mode callback exceptions in x64

In short, in certain circumstances (the most often cited reason is a 32-bit .NET program running on 64-bit Windows) any exceptions in the Load event of a WinForms Form will be swallowed.

You can wrap the Form Load event in a try/catch block to catch it, and determine how to react to it.

In short 2: The code does indeed cause a null reference exception as you expected, you're just not seeing it

That's it, thank you. Catching myself worked as you suggested. — jtpereyda, Mar 05 '13 at 21:27

score 2 · Answer 2 · answered Mar 05 '13 at 20:53

2

How are you determining that this code runs just fine? It's very likely that this code is throwing an exception under the hood which is just then swallowed by the Windows Forms Runtime code. There are several reasons why this exception swallowing could be handled silently by the debugger / runtime

I would try debugging this or code or barring that put a Messagebox.Show line just under the delegate invoke and see if it executes.

answered Mar 05 '13 at 20:53

JaredPar

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Isn't his point that `myHandler` itself is null, so the "call" could be interpreted as `null(this, null)`? – Eric J. Mar 05 '13 at 20:54
I must be blind or whatnot, because it definitely looks like he's calling a delegate that is null. – Lasse V. Karlsen Mar 05 '13 at 20:54
@LasseV.Karlsen i haven't had enough coffee and read past that. I updated my answer – JaredPar Mar 05 '13 at 20:57

score 0 · Answer 3 · answered Mar 05 '13 at 20:57

0

Are you sure the code does nothing?

I do get a NullReferenceException when I try this:

class Program
{
    static void Main(string[] args)
    {
        EventHandler myhandler = null;
        myhandler(null, null);
    }
}

Maybe your code is never executed (e.g. your Form1_Load event handler is not called), or maybe your exception is swallowed (e.g. by a different thread)?

answered Mar 05 '13 at 20:57

Daniel A.A. Pelsmaeker

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See my answer for the culprit. – Lasse V. Karlsen Mar 05 '13 at 21:00

Why doesn't calling a null event handler raise an exception?

3 Answers3