CODEWITHSUNDEEP
wpfxamlinheritancedesigner
jchadhowell
jchadhowell

Reputation: 1131

WPF Designer "Could not create an instance of type"

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"

...

Upvotes: 18

Views: 25373

Answers (11)

jchadhowell
jchadhowell

Reputation: 1131

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: Link

Upvotes: 21

Sophie Chai
Sophie Chai

Reputation: 37

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Yeay! I solved it. The form ended up showing on the Display & error gone after I set the Control Display Options to Only Display Platform Control. Check out the image to locate it.

Upvotes: 3

A. Morel
A. Morel

Reputation: 10344

I don't know what the problem is, but I solved it like this:

You should not execute the code in the constructor of your BaseClass but from the constructor of the parent class.

You create a method in the BaseClass that you execute from the constructor of the parent class.

Upvotes: 0

TechnoTim
TechnoTim

Reputation: 3205

The reason why I was getting this error was simple but tough for me to track down. My converter class was not public. Simply changing the class's accessibility fixed it.

    public class StringToLowerConverter : IValueConverter

Upvotes: 0

Maxim Kamalov
Maxim Kamalov

Reputation: 765

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.

Upvotes: 0

angad.stjudes
angad.stjudes

Reputation: 186

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.

Upvotes: 9

Carl G
Carl G

Reputation: 18260

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.

Upvotes: 0

RoQ
RoQ

Reputation: 11

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.

Upvotes: 0

Seva Alekseyev
Seva Alekseyev

Reputation: 61378

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.

Upvotes: 2

paul
paul

Reputation: 13516

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.

Upvotes: 1

Andy Dent
Andy Dent

Reputation: 17969

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.

Upvotes: 4

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