[DataGrid]: using RowDetailsTemplate sets wrong DataContext

[GundlackFlorian]

Describe the bug

Hello,

When using a RowDetailsTemplate of a DataGrid, the RowDetailsTemplate will have the parent DataContext set, which causes a crash.

To Reproduce

• Create a list of items
• Assign to a DataGrid
• Launch => the items are displayed.
• Add a RowDetailsTemplate that display some property
• Launch => crash

The issue also happens if there is no item in the list.

(example project included)

Expected behavior

The RowDetailsTemplate DataContext should be the Item and not the parent.
If the list is empty RowDetailsTemplate should not be called since there is no item.

Example Project

DataGridBug.zip

In the example project if you launch with the RowDetailsTemplate commented no problem.
But if you uncomment the RowDetailsTemplate you will get a crash because the wrong data context was set:
Unable to cast object of type 'DataGridBug.ViewModels.ItemListViewModel' to type 'DataGridBug.ViewModels.ItemViewModel'.

Even worse if you use the ContentControl instead of the TextBlock it will recursively show the wrong View causing a StackOverFlow.

Informations

• Avalonia Version 11.0.0

Edit: updated post

[timunie]

I'm unable to reproduce with the sample you provided. Can you double-check and tell me what to do to reproduce?

[GundlackFlorian]

Hello,
I double checked:

• In Rider (2023.1.3) i'm still getting the exception (in debug), but i can resume without the app crashing.

• In Visual Studio 2022 even in debug mode it doesn't throw the exception.

But i just added a new UserControl to print the DataContext

public partial class DataContextPrinter : UserControl
{
    public DataContextPrinter()
    {
        InitializeComponent();
    }

    private void InitializeComponent()
    {
        AvaloniaXamlLoader.Load( this );
    }

    protected override void OnPropertyChanged( AvaloniaPropertyChangedEventArgs change )
    {
        base.OnPropertyChanged( change );

        if ( change.Property == DataContextProperty )
        {
                Trace.WriteLine($"DataContext={change.NewValue}");
        }
    }
}

And in ItemListView.axaml i replaced <TextBlock Text="{Binding Id}" /> with <views:DataContextPrinter />

When launching with one item it prints:

DataContext=DataGridBug.ViewModels.ItemListViewModel
DataContext=DataGridBug.ViewModels.ItemViewModel

and when launching with an empty list

DataContext=DataGridBug.ViewModels.ItemListViewModel
DataContext=

So the parent DataContext is injected first even when there is no Item causing the invalid cast exception.

Maybe it's normal behavior and Rider is too "sensitive" and i was just assuming that when the debbuger stops it was a crash (which is most of the time tbh).

[GundlackFlorian]

In my answer i forgot also about the ContentControl:

If you set GenerateItems to 0 (or comment it) and use <ContentControl Content="{Binding .}" /> instead of <TextBlock Text="{Binding Id}" /> you will get a StackOverflow.
Probably because the content Control is trying to display ItemListView recursively.
And this even on Visual Studio.

Here is the updated project
DataGridBug.zip

[GundlackFlorian]

Hello,

I spent some time digging in the Avalonia Source code today and i think i found the issue and how to to fix it.

First i reproduced the issue in the ControlCatalog.

i Added a RowDetailsTemplate to the first Tab in the DataGridPage.

DataGridPage.xaml

//we are in the dataGrid1, after line 39
<DataGrid.RowDetailsTemplate>
    <DataTemplate>
        <ContentControl Content="{ReflectionBinding .}" />
    </DataTemplate>
</DataGrid.RowDetailsTemplate>

then i removed the ItemsSource assignation to set an empty list

DataGridPage.xaml.cs line 39

//dg1.ItemsSource = collectionView1;
dg1.ItemsSource = new List<Country>();

Now when launching and going to the DataGridPage we are getting the StackOverflow instead of getting an empty DataGrid.

After some debuging and digging, i found out that the problems comes from here

https://github.com/AvaloniaUI/Avalonia/blob/194044692eb3967b8c6bd0ed140a954f53b48e0e/src/Avalonia.Controls.DataGrid/DataGridRows.cs#L2973-L2976

In our case the dataItem is null, but the detailsContent.DataContext is set to ControlCatalog.Pages.DataGridPage

so when it enters the function bellow detailsContent.Measure i think it's measuring the wrong stuff and goes on a recursive loop.

If i comment the check the problem is solved.

//if (dataItem != null)
{
    detailsContent.DataContext = dataItem;
}

Now if i relaunch the ControlCatalog and go to the DataGridPage:

• When the list is empty, the grid is empty.
• When there an item in the list it will print in the RowDetailsTemplate ControlCatalog.Models.Country like it should.
• If i set the RowDetailsVisibilityMode to Visible, also no problem, it prints ControlCatalog.Models.Country for every item.

So:

• when the dataitem is not null detailsContent.DataContext is set to the right item.
• if the dataItem is null, the detailsContent.DataContext is set to null, and it's not measuring the wrong thing at some point in the underlying measure function.

I didn't investigate further in the underlying function, so maybe i'm wrong and/or there is another deeper issue.

I hope it helps.
Thanks.

[timunie]

Good finding. Can you file a PR?

[GundlackFlorian]

Done AvaloniaUI/Avalonia#12174