Unable to use Previewer with Dependency Injection

[laolarou726]

Describe the bug

Currently, AvaloniaUI's previewer can not work well when I use Microsoft.Extensions.DependencyInjection to inject all the types into the constructor. Previewer will show the following messages:

13:58:10.093 [Information] 45904 Connection initialized
13:58:12.131 [Error] 45904 UpdateXamlResult error
System.Xaml.XamlException: “Unable to find public constructor for type xxx:xxxx.xxxx.xxxViewModel() Line 26, position 10.”

To Reproduce

1. Create a simple view with constructor and parameters
2. Build the project

Expected behavior

The preview window should have the rendered controls instead of errors

Environment

• OS: Windows 11 23H2
• Avalonia-Version: 11.0.5

[timunie]

Try if(Design.IsDesignTime)

[laolarou726]

Try if(Design.IsDesignTime)

Is there a way to detect the previewer using the compile symbol? I don't want those codes to be present in the production publish.

[timunie]

Then you need to find another solution. Maybe #IF DEBUG can help here, but I never tested this with the designer.

[maxkatz6]

It doesn't look like an issue. The error message is pretty descriptive and specific.
Can you tell what exactly are you trying to do? I.e. specific XAML code.

[thevortexcloud]

I don't mind doing that when setting up the DI container if I need to. But as far as I can tell it's not possible to get that to work. But doing it in every view with a service dependency is not really worth the effort and not much different from using the XAML design time stuff. I suppose I could use use a locator call only on debug builds to resolve the view model with the test dependencies, but I am still not a fan of doing that.

[timunie]

well if anyone has a better idea, we are open to it.

[thevortexcloud]

Hmm. The simplest option I can think of, of the top of my head which I suspect may be somewhat contentious would be to allow for Avalonia to resolve view models from it's own locator, either on an opt in or opt out basis. Probably with some sort of property on the view to control it (which may have other problems if it needs to instantiate the view to see the property value). But this then requires people to interact with Splat in some way, either to directly register things or to make it use another DI framework. In which case it would probably be a good idea to provide some sort of abstraction over it in the event Avalonia ever decides to move to a different internal DI.

But as I said, I can't think of any particularly great generic ideas for doing this, which is why I have not really complained that much about it.

[jp2masa]

In my projects I usually create a dummy DI container and then do something like Design.DataContext={markup:DesignResolve {x:Type vm:MainViewModel}}, with a custom markup extension that resolves view models from the dummy container.

I believe it could be simpler, for example by creating a base class for views or overriding the default value for Design.DataContextProperty.

Additionally, if you want to use DI with views as well, I think overriding the default value for Design.PreviewWithProperty would work.

[thevortexcloud]

I just tried that and it works. I had not thought about using a markup extension. Very nice. Is there a chance we can get something like this added to Avalonia itself to avoid having to copy this code everywhere I need it?

On a related note: is there a way to get the x:DataType if I have a reference to the control? It would allow me to make this even simpler.

[timunie]

I don't think this is well suited for vanilla Avalonia. But it may make it into Avalonia.Labs or can be added to a community third party package which can be easily added via nuget.

[thevortexcloud]

Fair enough.

[leowest77]

@jp2masa Hi, I am in similar issue using DI and ViewLocator could you provide an example of how to use the options you provided? Apparently I cannot figure it out myself. But I really would love to get ride of this issue Cannot compile Build method. Parameters mismatch.Type needs to have a parameterless ctor or a ctor with a single IServiceProvider argument.Or x:Arguments directive with matching arguments needs to be set even if I see the a parameterless ctor it still does no like it.

[agalanrosa]

Can someone please share a complete implementation about this, I'm not able to get it working, I have the @thevortexcloud implementation and Added the Design.DataContext={markup:DesignResolve {x:Type vm:MainViewModel}} to my view .xaml file but it's not working.

cc @jp2masa

Now I'm getting the same error as @leowest77

LastUpdate: It's wokring now, I had to add constructor with the Type field, and add the markup namespace to the view

[thevortexcloud]

As far as I am aware, my code should work as is. It however has been a long time since I tried it.