Вы можете привязать к RowDefinitions.Count, но потребуется обновить привязку при добавлении строк вручную.
Редактировать: Фактически только частично вручнуюXaml:
<StackPanel Orientation="Vertical">
<Grid Name="GridThing">
<Grid.ColumnDefinitions>
<ColumnDefinition/>
<ColumnDefinition/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition />
</Grid.RowDefinitions>
<Grid.Children>
<Button Content="TopRight" Grid.Row="0" Grid.Column="1"/>
<Button Content="LowerRight" Grid.Row="1" Grid.Column="1"/>
<Button Content="Span Rows" Name="BSpan" Grid.RowSpan="{Binding RelativeSource={RelativeSource AncestorType=Grid}, Path=RowDefinitions.Count, Mode=OneWay}"/>
</Grid.Children>
</Grid>
<Button Click="Button_Click" Content="Add Row" />
</StackPanel>
Код:
private void Button_Click(object sender, RoutedEventArgs e)
{
GridThing.RowDefinitions.Add(new RowDefinition() { Height = new GridLength(20) });
foreach (FrameworkElement child in GridThing.Children)
{
BindingExpression exp = child.GetBindingExpression(Grid.RowSpanProperty);
if (exp != null)
{
exp.UpdateTarget();
}
}
}