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I want to show below the cell that contains the error.
First of all, the community toolkit DataGrid is different from the DataGrid in WPF. The Input Validation in the community toolkit DataGrid will only change the background row. You could also check this behavior in the official document: Input Validation.
For your scenario, I'd suggest you might need to custom the RowDetailsTemplate which could be used to display the error message. You could follow the document here: Display and Configure Row Details in the DataGrid Control. What you need to do is to create a DataTemplate that defines the appearance of the row details section first. Then place the DataTemplate inside the <DataGrid.RowDetailsTemplate> tags.
The code looks like this:
<controls:DataGrid> <controls:DataGrid.RowDetailsTemplate> <DataTemplate> <StackPanel Margin="20,10" Padding="5" Spacing="3"> <TextBlock Margin="20" Text="Error messages"/> </StackPanel> </DataTemplate> </controls:DataGrid.RowDetailsTemplate> </controls:DataGrid>
You could also try to submit a feature request in the Community Toolkit GitHub Issue about this.
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