WPF Designer "Could not create an instance of type"

Another cause. My control class had a static field that was initialized from resources, like this:

 static Color s_ImgColor = (Color)TheApp.Resources["PhoneForegroundColor"];

That would throw a null reference exception in XAML editor, since the resources are not available in design mode. Were it not a color resource (say, a brush), this won't be a problem, but a typecast to value type throws up on a null reference.

I found a very useful solution to this on : http://www.progware.org/Blog/post/WPF-Designer-Error-Could-not-create-an-instance-of-type.aspx.

This link explains how the WPF designer window runs the Constructor to display the UI in XAML and the remedy: adding the following snippet to any part of constructor code which might be giving error:

if(!System.ComponentModel.DesignerProperties.GetIsInDesignMode(this))
{
   //code producing exception         
}

the function name is self explanatory. :) This link also provides solutions on debugging issues with XAML.

Another possible cause, as we just found here, so I'm adding this answer for future users, is if the project is hosted on an untrusted source, such as a file server.

In that case, the designer wouldn't load the assembly and so gave the same "Could not create instance..." error. The solution would still build and debug OK.

The problem was that the base class was defined as abstract. This caused the designer to fail. This problem is described in more detail in the comments section of Laurent Bugnion's blog: http://geekswithblogs.net/lbugnion/archive/2007/03/02/107747.aspx