WPF Designer “Could not create an instance of type

Question

In my UI XAML I'm essentially inheriting from a class "BaseView" that contains functionality common to several forms, however this is preventing the designer from displaying the form: "Could not create instance of type BaseView". The code will compile and run, but it is frustrating not to be able to see the form in the Designer. Is there a better way? Thanks.

XAML:

<vw:BaseView 
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:vw="clr-namespace:ReviewClient"   
    x:Class="ReviewClient.MainPage"

...

Answer 1

I have the problem that my MVVM class have acces to the database in the constructor and that was the problem, it throws an exception. I only have to check if application is runing in Design mode.

Answer 2

In WinForms, it's possible to use the designer with abstract controls if you use a custom TypeDescriptionProvider to inform the designer of a concrete implementation:

I'm using the solution in this answer to another question, which links this article. The article recommends using a custom TypeDescriptionProvider and concrete implementation of the abstract class. The designer will ask the custom provider which types to use, and your code can return the concrete class so that the designer is happy while you have complete control over how the abstract class appears as a concrete class.

Answer 3

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.

Answer 4

Yet another possible cause.

I have a user control which has child controls which generate events e.g. selection_changed on list control. The select_changed event handler makes changes to other child controls.

During initialisation the selected item property of the list box gets changed and triggers a selection_changed event. The handler tries to update the other child controls but cannot because they have not yet been instantiated. This leads to a null pointer exception and causes the problem.

Once the null pointer problem was handled the control was able to be instantiated and appeared in the parent control.

Answer 5

And another possible situation (this is actual for at least SL for WP):

If you create instanse of your class (ex. <local:MyDataSource />) then it should be public. If your class is internal, it will work at design-time but will fail with this exception at runtime.

Answer 6

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.