Image first loaded, then it isn't? (XNA) - xna

Question

I am very confused at the Moment.
I have the following Class: (Just a part of the class):
public class GUIWindow
{
#region Static Fields
//The standard image for windows.
public static IngameImage StandardBackgroundImage;
#endregion
}
IngameImage is just one of my own classes, but actually it contains a Texture2D (and some other things).
In another class I load a list of GUIButtons by deserializing a XML file.
public static GUI Initializazion(string pXMLPath, ContentManager pConMan)
{
GUI myGUI = pConMan.Load<GUI>(pXMLPath);
GUIWindow.StandardBackgroundImage = new
IngameImage(pConMan.Load<Texture2D>(myGUI.WindowStandardBackgroundImagePath),
Vector2.Zero, 1024, 600, 1, 0, Color.White, 1.0f,
true, false, false);
System.Console.WriteLine("Image loaded? " +
(GUIWindow.StandardBackgroundImage.ImageStrip != null));
myGUI.Windows = pConMan.Load<List<GUIWindow>>(myGUI.GUIFormatXMLPath);
System.Console.WriteLine("Windows loaded");
return myGUI;
}
Here this line: System.Console.WriteLine("Image loaded? " +
(GUIWindow.StandardBackgroundImage.ImageStrip != null));
Prints "true".
To load the GUIWindows I need an "empty" constructor, which looks like that:
public GUIWindow()
{
Name = "";
Buttons = new List<Button>();
ImagePath = "";
System.Console.WriteLine("Image loaded? (In win) " +
(GUIWindow.StandardBackgroundImage.ImageStrip != null));
//Image = new IngameImage(StandardBackgroundImage);
//System.Console.WriteLine(
//Image.IsActive = false;
SelectedButton = null;
IsActive = false;
}
As you can see, I commented lines out in the constructor. Because: Otherwise this would crash.
Here the line System.Console.WriteLine("Image loaded? (In win) " +
(GUIWindow.StandardBackgroundImage.ImageStrip != null));
Doesn't print anything, it just crashes with the following errormessage:
Building content threw NullReferenceException: Object reference not set to an object instance.
Why does this happen?
Before the program wants to load the List, it prints "true". But in the constructor, so in the loading of the list it prints "false".
Can anybody please tell me why this happens and how to fix it?

Answer 1

My best guess at the NullReferenceException is that GUIWindow.StandardBackgroundImage is null, so it throws this exception when you try to access GUIWindow.StandardBackgroundImage.ImageStrip.
Are you familiar with the Visual Studio debugger? If not, you should be. I'd set some breakpoints and step through any code that reads or writes StandardBackgroundImage.
Really, though, your organization could be improved. Why is StandardBackgroundImage a static field of the GUIWindow class? It should be a field of the class which loads it - wherever the Initialization method is. Then pass it into the constructor of GUIWindow.
You are treating the StandardBackgroundImage field like a global, and thus are feeling the effects of that decision - some things are reading and modifying it, and you can't keep track of what order they are doing so.
Take this advice on globals.