XAML error is shown when the content of a DataTemplate is not a View

[marchev-prgs]

Description

There is an error shown in VS when in XAML the Content of a DataTemplate is not a View:

    <DataTemplate x:Key="template1">
        <local:MyTabViewItem>
            <local:MyTabViewItem.Header>
                <Label Text="{Binding MyHeaderText}" />
            </local:MyTabViewItem.Header>
            <local:MyTabViewItem.Content>
                <Grid>
                    <Label Text="{Binding MyContentText}" />
                </Grid>
            </local:MyTabViewItem.Content>
        </local:MyTabViewItem>
    </DataTemplate>

The error says "Property DataTemplateContent does not support value of type MyTabViewItem".

Note that MyTabViewItem is not a View, it is a BindableObject:
public class MyTabViewItem : BindableObject { }

Rationale:
In our case, we have a custom TabView. Each TabView-item contains two views - one header view, and one content view. This is one of the reasons we chose for the MyTabViewItem to not be a view, and it has two properties that contain the two views - Header and Content properties.
While we understand that most items-controls will use the DataTemplate with a View in them, this is not necessarily a restriction, as the DataTemplate can create content of type object (for reference see the signature of the CreateContent() method of the DataTemplate -

maui/src/Controls/src/Core/ElementTemplate.cs

Line 73 in f2cc45e

public object CreateContent()

).

This is why we would like for that error to be removed as users of our custom TabView may think there is an error with their code.

Steps to Reproduce

1. Download the repo from the link (https://github.com/telerik/ms-samples) and go to Maui/XamlErrorForDataTemplate
2. Open the project
3. Go to MainPage.xaml

Expected Behavior: No error should be displayed.

Actual Behavior: The whole DataTemplate is underlined and an error is displayed - "Property DataTemplateContent does not support value of type MyTabViewItem".

Link to public reproduction project repository

https://github.com/telerik/ms-samples/tree/main/Maui/XamlErrorForDataTemplate

Version with bug

8.0.7 SR2

Is this a regression from previous behavior?

No, this is something new

Last version that worked well

8.0.7 SR2

Affected platforms

iOS, Android, Windows, macOS

Affected platform versions

No response

Did you find any workaround?

I did not find a workaround.

Relevant log output

No response

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[ninachen03]

Verified this issue with Visual Studio 17.10 Preview 7 (8.0.40 & 8.0.3). Can repro it.

[PureWeen]

@mgoertz-msft @StephaneDelcroix thoughts?

[mgoertz-msft]

What is the property type of DataTemplate.DataTemplateContent? Is MyTabViewItem assignable to it?

[marchev-prgs]

What is the property type of DataTemplate.DataTemplateContent? Is MyTabViewItem assignable to it?

Yes, it is assignable, the sample runs correctly. It's just the shown error that casts doubt.

The signature of the method in the DataTemplate is object CreateContent() (link). And in my case a tab-item is a BindableObject.