Hi @Takeshi ,Welcome to Microsoft Q&A, you could try the following code to get what you wanted.
<Grid Width="500">
<Grid.Resources>
<Style TargetType="Border">
<Setter Property="BorderThickness" Value="1,1,1,1" />
<Setter Property="BorderBrush" Value="Black" />
</Style>
</Grid.Resources>
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition />
<RowDefinition />
<RowDefinition />
</Grid.RowDefinitions>
<Border Grid.Row="0" />
<Border Grid.Row="1" />
<Border Grid.Row="2" />
<Border Grid.Row="3" />
</Grid>
You can check this documentation: XAML object elements, you need Completing the tag with a forward slash and closing angle bracket in succession (/>).
Best Regards,
Jiale
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