Home/ Questions/Q 389695
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-12T15:53:31+00:00

Yet another strange WPF error: I have a custom control in a simple XAML

  • 0

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.

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    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. 🙂

    • 0