Question

I have a ComboBox in Silverlight that has very inconsistent behavior.

I have the ComboBox bound to a dynamic collection of data where elements are added or removed. Here is the XAML for the ComboBox:

<ComboBox Margin="0,-1,0,0" Width="20" Grid.Row="0" Grid.Column="0" ItemsSource="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=ItemsContained}" x:Name="TabComboBox" >
    <ComboBox.ItemsPanel>
        <ItemsPanelTemplate>
            <StackPanel Background="White" MinWidth="250" />
        </ItemsPanelTemplate>
    </ComboBox.ItemsPanel>
</ComboBox>

So this works great and the ComboBox opens “up” with the long list of items in ItemsContained. However, if I delete one of the items in ItemsContained, the ComboBox all of a sudden will switch from opening “up” to opening “down” when you click on it, despite the fact there are a lot of items in this collection and there is only 20 or so pixels of room for it to open down. I cannot figure this out. I have even tried setting the ItemsPanelTemplate to have a MinHeight, but that does not help. Does anyone know how to make the ComboBox always open “up”?

Also, even if I set the MinHeight to something ridiculous, such as 10,000, it still does this.

EDIT: As an update, I have gotten this to work by creating a whole new ComboBox every time ItemsContained is changed. This is the code:

scrollingGrid.Children.Remove(tabComboBox);
tabComboBox.ItemsSource = null;
ComboBox boxy = new ComboBox()
{
    ItemsSource = ItemsContained
};
scrollingGrid.Children.Add(boxy);
tabComboBox = boxy;

I feel this is a little ad hoc, so if anyone has a better idea, let me know. Changing the height of the ScrollViewer inside the ComboBox does not work either.

Answer 1

I think this is a Silverlight Bug. When you set the ItemsSource property the behavior of the combobox popup changes from up to down.

I Clone my combobox in the event DropDownClosed of each combobox and that works for me.

 Private Sub cbRow1_DropDownClosed(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cbRow1.DropDownClosed, cbRow2.DropDownClosed, cbRow3.DropDownClosed, cbRow4.DropDownClosed, cbRow5.DropDownClosed
    Dim co As ComboBox = CType(sender, ComboBox)
    CloneComboBox(co)
End Sub

Private Sub CloneComboBox(ByVal co As ComboBox)
    Dim cbClon = New ComboBox()
    cbClon.Margin = co.Margin
    cbClon.Width = co.Width
    cbClon.Height = co.Height
    cbClon.Name = co.Name
    cbClon.Tag = co.Tag

    cbClon.ItemsSource = co.ItemsSource
    cbClon.SelectedIndex = co.SelectedIndex

    LayoutRoot.Children.Remove(co)
    LayoutRoot.Children.Add(cbClon)

    //this is because I have my comboboxes inside a grid
    System.Windows.Controls.Grid.SetRow(cbClon, cbClon.Tag)
    System.Windows.Controls.Grid.SetColumn(cbClon, 0)


    AddHandler cbClon.DropDownClosed, AddressOf cbRow1_DropDownClosed

End Sub