Help with a ElementHost Null Exception

[JOSheaIV]

Here's a good one for everyone. I have been working on a Screensaver program for some time now (some of you may even remember questions related to this). Anyway, one of the biggest challanenges with developing it was finding a good way to transition images, smoothly, and not killing all my resources. I ended up using a WPF for this piece that is used to fade in and out images.

Once I had that little component developed, I created a new class, an object that inherited a Windows Form Panel, and then proceeded to add on a series of functions. This item includes housing the WPF I have build. The code looks like this (these are snippets)

//Fields setup for the Panel
private ElementHost WPFHost;
private ScreenSaverImageDisplay ImageDisplayer;

//Inside the custom Panel's initializer
this.ImageDisplayer = new ScreenSaverImageDisplay(new ImageFadeComplete(ImageFadeCompleteCheckIn));
this.ImageDisplayer.Size = new System.Windows.Size(Width, Height);

this.WPFHost = new ElementHost();
this.WPFHost.Location = new Point(0, 0);
this.WPFHost.Size = new Size(Width, Height);
this.WPFHost.Child = ImageDisplayer;
this.Controls.Add(WPFHost);

That's how I setup and initialize the thing, pretty simple.

However, while the code runs fine all the time when I debug through Visual Studio, there is this issue I have when I run it as an actual ScreenSaver (you know go to your properties choose it, and having it setup to start after so much time idle). The problem I am facing is that sometimes (I can't seem to find a pattern), is a null exception will be thrown on the start of the ScreenSaver. Again it doesn't happy everytime, and I can't even be sure if it only happens the first time it tries to run (that being like if I shut down the PC and start it back up, that would count as a first time when it runs the first time).

I get the usual exception window, and this

************** Exception Text **************
System.NullReferenceException: Object reference not set to an instance of an object.
at System.Windows.Forms.Integration.ElementHost.Select(Boolean directed, Boolean forward)
at System.Windows.Forms.Control.SelectNextControl(Control ctl, Boolean forward, Boolean tabStopOnly, Boolean nested, Boolean wrap)
at System.Windows.Forms.Form.set_Active(Boolean value)
at System.Windows.Forms.Form.WmActivate(Message& m)
at System.Windows.Forms.Form.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

I can not figure out what is causing this problem. This is all internal code, as I do not have any functions or anything like this. And like I said, I can never get this to occur when I am debugging so it makes it even more difficult to pin down.

I am hoping maybe someone has seen this before and knows what's causing it, because right now I am scratching my head. Null exceptions can be tricky, but this one is a stumper, even more because I have no clue where in my code this is occuring.

Thanks everyone for the help

[tobyITguy]

In your constructor for your class, do you instantiate all needed objects from the WPF?

[JOSheaIV]

There is one piece I have found that could blow the error.

If you notice above I assign a delegate called "ImageFadeComplete", which calls back to a function containing a timer in it. That timer was initialized after the code above. Is there any possibility that code was calling back to the delegate function? I thought delegates resides in the class the code resides, and not an instance related to the object though

Not leaning heavily to that though, cause I feel the error would be else where.

By the way when you say constructor, do you mean the Panel or WPF?

[JOSheaIV]

Well bad news, the timer wasn't the fix.

What's so weird is when debugging I can never create this error (of course if it did I probably wouldn't be in the current stumper). When you mentioned the constructor I assume you were talking abou the WPF's correct?

[JOSheaIV]

Alright I think I am going to mark this solved, as I finally believe I have found the fix.

I was doing some other work on the project and started to realize some of my images were loading in with issues. Turns out I was trying to load images in on a thread, and then Invoking a control on the form to assign an image to.

The problem I saw was that the component's Handler wasn't actually created yet, and causing null errors to below (or like that from what I can tell, I don't recall the exact error). I updated the code to pretty much no proceed with any tasks until the Handler is created for the component. So far all tests have proven successful. (now if I just knew exactly what the Handler did)