No parameterless constructor in custom theme

[Flo0806]

Describe the bug

Hello,

I created a library with some custom controls and styles. I followed the guides and searched the code. I created an entry point like this:

C# (ControlThemes.axaml.cs)

using Avalonia.Controls;
using Avalonia.Markup.Xaml;
using Avalonia.Styling;
using System;

namespace SD.Avalonia.UI.Controls
{
    public class ControlThemes : Styles
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="ControlThemes"/> class.
        /// </summary>
        /// <param name="sp">The parent's service provider.</param>
        public ControlThemes(IServiceProvider? sp = null)
        {
            AvaloniaXamlLoader.Load(sp, this);
        }
    }
}

ControlThemes.axaml

<Styles x:Class="SD.Avalonia.UI.Controls.ControlThemes"
		xmlns="/~https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
	<FluentTheme />

	<Styles.Resources>
		<ResourceDictionary>
			<ResourceDictionary.MergedDictionaries>
				<ResourceInclude Source="/Controls/Themes/Styles.axaml" />
				<ResourceInclude Source="/Controls/Themes/Generic.axaml" />
			</ResourceDictionary.MergedDictionaries>
		</ResourceDictionary>
	</Styles.Resources>

</Styles>

In the end all works as well if I start the application. But the designer show an error:
Cannot dynamically create an instance of type 'SD.Avalonia.UI.Controls.ControlThemes'. Reason: No parameterless constructor defined.

I wanna understand what to do... I checked other libraries and I haven't found any differences.

Greetings, Flo

To Reproduce

You can use this package, create a second Project (Avalon MVVM as example), add the library project to it and after a build open the AvaloniaThemeTemplate.axaml to see the error.

Link to the Repo: Here

Expected behavior

No errors :-)

Avalonia version

11.1.0 - 11.2.4

OS

Windows

Additional context

No response

[maxkatz6]

It looks fine. How is this file referenced? Repro project doesn't show that.

"Cannot dynamically create an instance" doesn't seem to be an Avalonia exception.

[Flo0806]

It looks fine. How is this file referenced? Repro project doesn't show that.

It is referenced as a Avalonia XAML. In the App self its referenced like this:

<Application xmlns="/~https://github.com/avaloniaui"
             xmlns:control="using:SD.Avalonia.UI.Controls"
             ...
             />
    <Application.Styles>
        <FluentTheme/>
        <control:ControlThemes/> <!-- HERE -->
    </Application.Styles>

The Thing is that the designer throws this error. The app starts and runs without any errors after the build.

Greetings, Flo

[maxkatz6]

@Flo0806 do you have a complete repro? As you mentioned above, normally this code just works. I don't see anything unusual.

[Flo0806]

@maxkatz6 Yes, here: REPO.

Load the project, build it und then open the AvaloniaThemeTemplate.axaml in VS Designer. If you run the app you will see a pink button - The proof that the lib will work correctly and all Resources are found successfully.

By the way: Is it normal that MergeResourceInclude throws a InnerException like "Unable to resolve XAML resource xxxx"? In build/run mode all is fine, no errors. Only in the Designer, again.

[Flo0806]

@maxkatz6 Is the Repo ok? Can you reproduce the error?

[maxkatz6]

@Flo0806 designer works here.

[maxkatz6]

Or are you trying to preview AvaloniaThemeTemplate? What's the goal? Avalonia technically could create an empty IServiceProvider to preview files like this, but that's a rare use case, with easy workaround (second ctor without IServiceProvider), and more importantly - there is nothing to preview.

[maxkatz6]

By the way: Is it normal that MergeResourceInclude throws a InnerException like "Unable to resolve XAML resource xxxx"? In build/run mode all is fine, no errors. Only in the Designer, again.

Not really normal. You can use full avares path instead, should make designer happier.