[Mono-list] Re: Mono-list Digest, Vol 4, Issue 5 sender for your account)

[mono-list.1.tracyanne at spamgourmet.com]

I realised what I was doing wrong. I didn't have the test for null "if
(InputError != null)" in my original code, and as I wasn't at the time
subscribing to the event, it was naturally going to error out.

By the way the code I sent in is pretty much what you'll find on page
309 (Chapter 6 Advanced C# Topics) of Wrox's Professional C#. That's
where i copied it from, for my project.

regards

Tracy Barlow



> Date: Thu, 04 Aug 2005 01:56:52 -0700
> From: Jackson Harper <jackson at ximian.com>
> Subject: Re: [Mono-list] Problem with Events
> To: mono-list at lists.ximian.com
> Message-ID: <1123145813.7867.18.camel at willy.site>
> Content-Type: text/plain
> 
> On Thu, 2005-08-04 at 17:30 +1000, mono-list.1.tracyanne at spamgourmet.com
> wrote:
> > Can anyone see anything wrong with this code. It looks to me to be
> > correct syntax for defining and raising an event. It compiles without
> > error, but when I run it I get the following error message
> 
> Nothing is ever subscribing to your event, so it is null when you are
> trying to invoke it. You need something like:
> 
> OnInputError += new InputError (ErrorHandlerFunction); somewhere.
> 
> A few general style comments:
> 
> 1. Normally event handler delegates are named <something>Handler. So
> your InputError delegate would be named InputErrorHandler.
> 
> 2. Normally events names don't start with On. So OnInputError would be
> InputError.
> 
> 3. It's common to wrap the instantiation of an event in a protected
> method named On<Event Name>. So you would have a method that looks like
> this:
> 
> protected void OnInputError (InputErrorEventArgs e)
> {
>         if (InputError != null)
>                 InputError (this, e);
> }
> 
> 
> Cheers,
> Jackson
> 
> 
> > 
> > Unhandled Exception: System.NullReferenceException: Object reference not
> > set to an instance of an object
> > in [0x00001] (at /home/tracy/MonoDevelop/TestEvents/Main.cs:22)
> > RaiseAnEvent:Run ()
> > in [0x00007] (at /home/tracy/MonoDevelop/TestEvents/Main.cs:11)
> > MainClass:Main (System.String[] args)
> > 
> > I'm using Mono_1.1.8-2.
> > 
> > // project created on 04/08/2005 at 17:17
> > using System;
> > 
> > class MainClass
> > {
> > 				
> > 	public static void Main(string[] args)
> > 	{
> > 		RaiseAnEvent RAE = new RaiseAnEvent();
> > 		RAE.Run();
> > 	}
> > }
> > 
> > class RaiseAnEvent
> > {
> > 	public delegate void InputError(object sender, InputErrorEventArgs e);
> > 	public event InputError OnInputError;
> > 	
> > 	public void Run()
> > 	{
> > 		OnInputError(this, new InputErrorEventArgs("DF", "OI"));
> > 	}
> > 
> > }
> > 
> > 	
> > //Class for passing Input Error messages to the application
> > class InputErrorEventArgs : EventArgs
> > {
> > 	private string errorMessage;
> > 	private string errorSource;
> > 	
> > 	public string ErrorMessage
> > 	{
> > 		get
> > 		{
> > 			return errorMessage;
> > 		}
> > 	}
> > 	
> > 	public string ErrorSource
> > 	{
> > 		get
> > 		{
> > 			return errorSource;
> > 		}
> > 	}
> > 	
> > 	public InputErrorEventArgs(string Message, string Source) : base()
> > 	{
> > 		errorMessage = Message;
> > 		errorSource = Source;
> > 	}
> > }
> > _______________________________________________
> > Mono-list maillist  -  Mono-list at lists.ximian.com
> > http://lists.ximian.com/mailman/listinfo/mono-list
> 
> 
> 
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> End of Mono-list Digest, Vol 4, Issue 5
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