x:DataType doesn't work with nested types from other assemblies

Describe the bug

Using x:DataType doesn’t work with nested types from other assemblies.

Consider the following viewmodel code:

public class RootViewModel
{
    public IEnumerable<object> Sections { get; } = new[] { new NestedViewModel() }; 

    public class NestedViewModel
    {
        public string SampleText => "Hello world";
    }
}

NOTE: this code must be placed in a separate project (eg. a .NET Standard 2.0 lib) to repro the issue. It works fine if the classes are right in the UWP app project.

And the following XAML code:

<Page
    x:Class="BindToNestedTypeRepro.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:libs="using:BindToNestedTypeReproLibs">
    <Page.DataContext>
        <libs:RootViewModel x:Name="ViewModel"/>
    </Page.DataContext>

    <Grid>
        <ListView ItemsSource="{x:Bind ViewModel.Sections}">
            <ListView.ItemTemplate>
                <DataTemplate x:DataType="local:RootViewModel+NestedViewModel">
                    <TextBlock Text="{x:Bind SampleText}"/>
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
    </Grid>
</Page>

The app will fail to build and that data type for the template will not be recognized.

Steps to reproduce the bug

Steps to reproduce the behavior:

1. Create a .NET Standard 2.0 project and add the viewmodel code
2. Create a new blank UWP app and reference the other project
3. Paste that XAML code above in MainPage
4. Try to build and run

Expected behavior The app should run normally and XAML should recognize the data type and see the properties in the model.

Screenshots

Version Info

NuGet package version: n/a

Windows app type:

UWP	Win32
Yes

Windows 10 version	Saw the problem?
Insider Build (xxxxx)
May 2020 Update (19041)	Yes
November 2019 Update (18363)
May 2019 Update (18362)
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)

Device form factor	Saw the problem?
Desktop	Yes
Xbox
Surface Hub
IoT

[baskren]

Thanks Sergio for being a good sport. Sorry … I guess I just needed to vent a bit.

[Sergio0694]

Yes, using XAML is clearly a terrible idea because of this one bug. For the same reason, using C# is also a terrible idea because one time I found a bug in the C# compiler.

This is not the right place to have this conversation, and that comment is completely off topic. There is a separate proposal to create data templates in C#, I’d recommend subscribing there if you’re interested 🙂