Partial declarations must not specify different base classes?

c# wpf xaml inheritance

12,700

I had the same issue when I was working with Windows Phone. I can't remember the exact exception, but you can see the XAML here on GitHub, the page code here, and the base page code here (mine was a base page, not base control). I needed to add a new XAML namespace and change the <UserControl/> declaration:

Code Assumption

namespace RCO_Manager
{
    // Inherits **Base**UserControl, not UserControl
    public partial class EnterNewRequest : BaseUserControl
    {
        // Magic goes here
        ...
    }
}

XAML

<local:BaseUserControl
    xmlns:local="clr-namespace:RCO_Manager"
    x:Class="RCO_Manager.EnterNewRequest"

Side Note

According to Baboon, you don't need to specify it in your code-behind once you specify the base class in the XAML, so you can then change the code-behind to show the following. I can't verify it right now, but you can give this a try after you get it working.

public partial class EnterNewRequest // Don't specify BaseUserControl here
{
    ...

12,700

Author by

cost

Updated on June 19, 2022

Comments

cost about 2 years
I see a number of other people asking about this error message in other questions, but I don't seem to understand enough about what's going on to fix this for myself. I created this error by having a WPF UserControl
```
public partial class EnterNewRequest : UserControl
```
But then later on I wanted to add a method to UserControl, so I used inheritance to stick it in there (can't use an extension because I need to override this method). But now my usercontrol is upset, and I'm not sure what in the xaml I need to change. The UserControl change block is in the namespace RCO_Manager. This is my xaml:
```
<UserControl x:Class="RCO_Manager.EnterNewRequest"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
```
Louis Kottmann about 12 years

By the way, declaring base class in both XAML and code-behind is redundant.
Chris Benard about 12 years

At least with Windows Phone, I got the same WindowIsUpsetException when I didn't only put the BaseUserControl in the code and not the XAML. Maybe it'll fix his issue too? I'll update my answer to say I had the same problem.
Louis Kottmann about 12 years

But yes, making a class that inherits UserControl then making your control inherit from it should be the way to go.
cost about 12 years

This still isn't working, I've tried this with both regular UserControl and with a newly inherited one. My xaml is <local:UserControl x:Class="RCO_Manager.EnterNewRequest" xmlns:local="clr-namespace.RCO_Manager" But all it say is "The type local:usercontrol was not found." What am I missing here?
Chris Benard about 12 years

You typed it wrong. It's clr-namespace:RCO_Manager, not clr-namespace.RCO_Manager. Colon, not period. Also, it's not <local:UserControl/>, it's <local:BaseUserControl/>. Please read my answer again.
cost about 12 years

When I typed clr-namespace:RCO_Manager, Visual Studio balked at me about it being improperly formatted (so I assumed it was a typo, which it thought clr-namespace.RCO_manager was fine) though now it likes the : so I don't know what has changed. Regardless, I use local:UserControl because that's what my class is named, not BaseUserControl. Apparently xaml doesn't like me using the class names in that way though
Chris Benard about 12 years

@cost, Your class isn't called UserControl. It's called EnterNewRequest. What does it inherit from? You said you were inheriting from something that is not UserControl.
cost about 12 years

Perhaps I didn't word it in my question very well. EnterNewRequest inherits from UserControl, but then later on I tried to modify UserControl to add a method to it (instead of creating a new class and having it inherit UserControl and then having EnterNewRequest inherit that). But when I added the method to UserControl, EnterNewRequest failed to work
Chris Benard about 12 years

let us continue this discussion in chat
cost about 12 years

Hey, thanks for all of your help with this, but this created a new issue which I didn't notice last night. Now the design view window doesn't work. I get an InstanceBuilderException. "Cannot create an instance of BaseUserControl." The project still compiles and works fine, but now I can't see anything on the design view screen. Googling this looks like it's a bug characteristic of older VS versions, not 2010. Any thoughts on a solution?
cost about 12 years

This goes back to it being an abstract class, doesn't it. I think I may know how to fix this..
cost about 12 years

And one more comment to my friend Chris Bernard, who was nice enough to explain the usefulness of abstract to me in chat. It turns out I can't use abstract with a usercontrol like this, because the design view needs to be able to instantiate the object to show it. I didn't want to use an interface because I needed to store some variables as well.
Chris Benard about 12 years

You need to add a default constructor to your base class. Then it'll work fine in the designer.