WPF custom control XAML null error - wpf

Question

Yet another strange WPF error:
I have a custom control in a simple XAML page. The project builds and runs perfectly, with no errors.
Here's the XAML:
<Window x:Class="Grapher2.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:graph="clr-namespace:Grapher2"
Title="Grapher" Width="800" Height="600">
<StackPanel Name="container" Width="700" Height="500">
<graph:GraphCanvas x:Name="graphCanvas" Width="700" Height="500" Background="#ddd" />
</StackPanel>
But when I try to view the XAML page in the XAML designer window, I get the message:
"Problem Loading--The document contains errors that must be fixed..."
When I hit the "Reload the designer" link, an error pops up in the Error List which says:
"Object reference not set to an instance of an object"
and the line starting with "graph:GraphCanvas..." is underlined.
I was originally developing on our development server, and there, everything was the same, except the error was:
"Request for the permission of type 'System.Security.Permissions.SecurityPermission (...blah blah... failed.)"
Again, everything works perfectly, except that I can't view the page in the XAML designer window, which is keeping me from developing the rest of the app.
I've tried cleaning, building, rebuilding, and all combinations of orders of these commands, with no success.
EDIT:
Please bear in mind this is my first custom control, and I could be doing something horribly, horribly wrong. Like I said, the app compiles and works beautifully, so I'm kinda mystified.
EDIT #2:
My derived canvas is not a partial class. From my understanding of how InitializeComponent works (What does InitializeComponent() do, and how does it work in WPF?), there is no auto-generated partial class that contains InitializeComponent linked to my control. Again, the control works perfectly without it, except for breaking the XAML design view.

Answer 1

I had a similar issue recently. Basically my understanding is that in designer some things happens in not exactly the same order as during run-time and some things that you would think could never be null actually are null during design-time.
I solved the problem this way: commented large parts of code in my control to the point there was no error in Cider and then uncommented them until I got the error again. Then when source of the error was localized, I surrounded problematic parts with
if (something != null)
{
...
}
Even when I felt that there's no way that could be null. And after some time I got rid of the error.
Not very "scientific" approach but it solved my problem. :)

Answer 2

Unfortunately, there are problems with the Cider designer in Visual Studio 2008 where this type of error is all too common. The version in VS2010 is vastly improved, but that's no comfort when you hit this issue in VS2008.

Answer 3

I'm not sure what "Grapher2" is but you might check to make sure that there isn't code in its constructor that can't run correctly when the designer instantiates it. Something like a database call would be problematic.

Answer 4

I've seen similar issues.
This is only a partial solution as it won't render in the parent in design mode, but it'll get rid of the error. It's the best solution I've been able to find so far.
In the Constructor of your custom control.
if (!System.ComponentModel.DesignerProperties.GetIsInDesignMode(this))
InitializeComponent();
Hope that helps, or maybe it'll help you find a better solution. If you do please post it so I can use it ;)
Edit
I get a different error when not using the code above:
Could not create an instance of type ''
So may be a different issue then what I've seen, but sounds like it might be related.