Force ComboBox opens in Silverlight?

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

I have a ComboBox related to a dynamic dataset where items are added or removed. Here is the XAML for 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 fine, and the ComboBox opens up with a long list of items in ItemsContained. However, if I delete one of the items in ItemsContained, the ComboBox will suddenly switch from opening β€œup” to opening β€œdown” when you click on it, despite the fact that there are a lot of items in this collection, and there are only 20 or so pixels to open it. I can’t figure it out. I even tried to set MinHeight for ItemsPanelTemplate, but that does not help. Does anyone know how to make ComboBox always open up?

Also, even if I set MinHeight to something funny, like 10,000, for example, it still does.

EDIT: As an update, I got this to work, creating a whole new ComboBox every time ItemsContained changes. 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 like this is a bit ad hoc, so if anyone has a better idea let me know. Changing the height of the ScrollViewer inside the ComboBox also does not work.

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I think this is a Silverlight bug. When you set the ItemsSource property, the behavior of the combobox popup window changes from the very beginning.

I Clone my combobox in the case of DropDownClosed from each combobox, and this 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 
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I'm not sure that this solution will be less ad hoc than your current fix, but I decided that I would propose it.

I edited a copy of the ComboBox template, and where the original only had a ScrollViewer in ItemsPresenter, I added a Rectangle with Margin 0.5000,0,0, so that the internal Popup code would always place it on top. I also moved the template bindings to the ScrollViewer so that it does not stretch to the top, even if there were not enough elements.

The style is a little long, but here it is, just drop it into your xaml resources (and set the ComboBox style for ComboBoxStyle1). Let me know how it works for you!

 <ControlTemplate x:Key="ValidationToolTipTemplate"> <Grid x:Name="Root" Margin="5,0" Opacity="0" RenderTransformOrigin="0,0"> <Grid.RenderTransform> <TranslateTransform x:Name="xform" X="-25"/> </Grid.RenderTransform> <VisualStateManager.VisualStateGroups> <VisualStateGroup x:Name="OpenStates"> <VisualStateGroup.Transitions> <VisualTransition GeneratedDuration="0"/> <VisualTransition GeneratedDuration="0:0:0.2" To="Open"> <Storyboard> <DoubleAnimation Duration="0:0:0.2" To="0" Storyboard.TargetProperty="X" Storyboard.TargetName="xform"> <DoubleAnimation.EasingFunction> <BackEase Amplitude=".3" EasingMode="EaseOut"/> </DoubleAnimation.EasingFunction> </DoubleAnimation> <DoubleAnimation Duration="0:0:0.2" To="1" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="Root"/> </Storyboard> </VisualTransition> </VisualStateGroup.Transitions> <VisualState x:Name="Closed"> <Storyboard> <DoubleAnimation Duration="0" To="0" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="Root"/> </Storyboard> </VisualState> <VisualState x:Name="Open"> <Storyboard> <DoubleAnimation Duration="0" To="0" Storyboard.TargetProperty="X" Storyboard.TargetName="xform"/> <DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="Root"/> </Storyboard> </VisualState> </VisualStateGroup> </VisualStateManager.VisualStateGroups> <Border Background="#052A2E31" CornerRadius="5" Margin="4,4,-4,-4"/> <Border Background="#152A2E31" CornerRadius="4" Margin="3,3,-3,-3"/> <Border Background="#252A2E31" CornerRadius="3" Margin="2,2,-2,-2"/> <Border Background="#352A2E31" CornerRadius="2" Margin="1,1,-1,-1"/> <Border Background="#FFDC000C" CornerRadius="2"/> <Border CornerRadius="2"> <TextBlock Foreground="White" MaxWidth="250" Margin="8,4,8,4" TextWrapping="Wrap" Text="{Binding (Validation.Errors)[0].ErrorContent}" UseLayoutRounding="false"/> </Border> </Grid> </ControlTemplate> <Style x:Key="ComboBoxStyle1" TargetType="ComboBox"> <Setter Property="Padding" Value="6,2,25,2"/> <Setter Property="Background" Value="#FF1F3B53"/> <Setter Property="HorizontalContentAlignment" Value="Left"/> <Setter Property="BorderThickness" Value="1"/> <Setter Property="TabNavigation" Value="Once"/> <Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto"/> <Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto"/> <Setter Property="BorderBrush"> <Setter.Value> <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0"> <GradientStop Color="#FFA3AEB9" Offset="0"/> <GradientStop Color="#FF8399A9" Offset="0.375"/> <GradientStop Color="#FF718597" Offset="0.375"/> <GradientStop Color="#FF617584" Offset="1"/> </LinearGradientBrush> </Setter.Value> </Setter> <Setter Property="Template"> <Setter.Value> <ControlTemplate TargetType="ComboBox"> <Grid> <Grid.Resources> <Style x:Name="comboToggleStyle" TargetType="ToggleButton"> <Setter Property="Foreground" Value="#FF333333"/> <Setter Property="Background" Value="#FF1F3B53"/> <Setter Property="BorderBrush"> <Setter.Value> <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0"> <GradientStop Color="#FFA3AEB9" Offset="0"/> <GradientStop Color="#FF8399A9" Offset="0.375"/> <GradientStop Color="#FF718597" Offset="0.375"/> <GradientStop Color="#FF617584" Offset="1"/> </LinearGradientBrush> </Setter.Value> </Setter> <Setter Property="BorderThickness" Value="1"/> <Setter Property="Padding" Value="3"/> <Setter Property="Template"> <Setter.Value> <ControlTemplate TargetType="ToggleButton"> <Grid> <VisualStateManager.VisualStateGroups> <VisualStateGroup x:Name="CommonStates"> <VisualState x:Name="Normal"/> <VisualState x:Name="MouseOver"> <Storyboard> <DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="BackgroundOverlay"/> <ColorAnimation Duration="0" To="#7FFFFFFF" Storyboard.TargetProperty="(Shape.Fill).(GradientBrush.GradientStops)[3].(GradientStop.Color)" Storyboard.TargetName="BackgroundGradient"/> <ColorAnimation Duration="0" To="#CCFFFFFF" Storyboard.TargetProperty="(Shape.Fill).(GradientBrush.GradientStops)[2].(GradientStop.Color)" Storyboard.TargetName="BackgroundGradient"/> <ColorAnimation Duration="0" To="#F2FFFFFF" Storyboard.TargetProperty="(Shape.Fill).(GradientBrush.GradientStops)[1].(GradientStop.Color)" Storyboard.TargetName="BackgroundGradient"/> </Storyboard> </VisualState> <VisualState x:Name="Pressed"> <Storyboard> <DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="BackgroundOverlay2"/> <DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="(UIElement.Opacity)" Storyboard.TargetName="Highlight"/> <ColorAnimation Duration="0" To="#E5FFFFFF" Storyboard.TargetProperty="(Shape.Fill).(GradientBrush.GradientStops)[1].(GradientStop.Color)" Storyboard.TargetName="BackgroundGradient"/> <ColorAnimation Duration="0" To="#BCFFFFFF" Storyboard.TargetProperty="(Shape.Fill).(GradientBrush.GradientStops)[2].(GradientStop.Color)" Storyboard.TargetName="BackgroundGradient"/> <ColorAnimation Duration="0" To="#6BFFFFFF" Storyboard.TargetProperty="(Shape.Fill).(GradientBrush.GradientStops)[3].(GradientStop.Color)" Storyboard.TargetName="BackgroundGradient"/> <ColorAnimation Duration="0" To="#F2FFFFFF" Storyboard.TargetProperty="(Shape.Fill).(GradientBrush.GradientStops)[0].(GradientStop.Color)" Storyboard.TargetName="BackgroundGradient"/> </Storyboard> </VisualState> <VisualState x:Name="Disabled"/> </VisualStateGroup> <VisualStateGroup x:Name="CheckStates"> <VisualState x:Name="Checked"> <Storyboard> <DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="(UIElement.Opacity)" Storyboard.TargetName="BackgroundOverlay3"/> <DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="(UIElement.Opacity)" Storyboard.TargetName="Highlight"/> <DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="(UIElement.Opacity)" Storyboard.TargetName="BackgroundGradient2"/> <ColorAnimation Duration="0" To="#E5FFFFFF" Storyboard.TargetProperty="(Shape.Fill).(GradientBrush.GradientStops)[1].(GradientStop.Color)" Storyboard.TargetName="BackgroundGradient2"/> <ColorAnimation Duration="0" To="#BCFFFFFF" Storyboard.TargetProperty="(Shape.Fill).(GradientBrush.GradientStops)[2].(GradientStop.Color)" Storyboard.TargetName="BackgroundGradient2"/> <ColorAnimation Duration="0" To="#6BFFFFFF" Storyboard.TargetProperty="(Shape.Fill).(GradientBrush.GradientStops)[3].(GradientStop.Color)" Storyboard.TargetName="BackgroundGradient2"/> <ColorAnimation Duration="0" To="#F2FFFFFF" Storyboard.TargetProperty="(Shape.Fill).(GradientBrush.GradientStops)[0].(GradientStop.Color)" Storyboard.TargetName="BackgroundGradient2"/> </Storyboard> </VisualState> <VisualState x:Name="Unchecked"/> </VisualStateGroup> <VisualStateGroup x:Name="FocusStates"> <VisualState x:Name="Focused"> <Storyboard> <ObjectAnimationUsingKeyFrames Duration="0" Storyboard.TargetProperty="Visibility" Storyboard.TargetName="FocusVisualElement"> <DiscreteObjectKeyFrame KeyTime="0"> <DiscreteObjectKeyFrame.Value> <Visibility>Visible</Visibility> </DiscreteObjectKeyFrame.Value> </DiscreteObjectKeyFrame> </ObjectAnimationUsingKeyFrames> </Storyboard> </VisualState> <VisualState x:Name="Unfocused"/> </VisualStateGroup> </VisualStateManager.VisualStateGroups> <Rectangle x:Name="Background" Fill="{TemplateBinding Background}" RadiusY="3" RadiusX="3" Stroke="{TemplateBinding BorderBrush}" StrokeThickness="{TemplateBinding BorderThickness}"/> <Rectangle x:Name="BackgroundOverlay" Fill="#FF448DCA" Opacity="0" RadiusY="3" RadiusX="3" Stroke="#00000000" StrokeThickness="{TemplateBinding BorderThickness}"/> <Rectangle x:Name="BackgroundOverlay2" Fill="#FF448DCA" Opacity="0" RadiusY="3" RadiusX="3" Stroke="#00000000" StrokeThickness="{TemplateBinding BorderThickness}"/> <Rectangle x:Name="BackgroundGradient" Margin="{TemplateBinding BorderThickness}" RadiusY="2" RadiusX="2" Stroke="#FFFFFFFF" StrokeThickness="1"> <Rectangle.Fill> <LinearGradientBrush EndPoint=".7,1" StartPoint=".7,0"> <GradientStop Color="#FFFFFFFF" Offset="0"/> <GradientStop Color="#F9FFFFFF" Offset="0.375"/> <GradientStop Color="#E5FFFFFF" Offset="0.625"/> <GradientStop Color="#C6FFFFFF" Offset="1"/> </LinearGradientBrush> </Rectangle.Fill> </Rectangle> <Rectangle x:Name="BackgroundOverlay3" Fill="#FF448DCA" Opacity="0" RadiusY="3" RadiusX="3" Stroke="#00000000" StrokeThickness="{TemplateBinding BorderThickness}"/> <Rectangle x:Name="BackgroundGradient2" Margin="{TemplateBinding BorderThickness}" Opacity="0" RadiusY="2" RadiusX="2" Stroke="#FFFFFFFF" StrokeThickness="1"> <Rectangle.Fill> <LinearGradientBrush EndPoint=".7,1" StartPoint=".7,0"> <GradientStop Color="#FFFFFFFF" Offset="0"/> <GradientStop Color="#F9FFFFFF" Offset="0.375"/> <GradientStop Color="#E5FFFFFF" Offset="0.625"/> <GradientStop Color="#C6FFFFFF" Offset="1"/> </LinearGradientBrush> </Rectangle.Fill> </Rectangle> <Rectangle x:Name="Highlight" IsHitTestVisible="false" Margin="{TemplateBinding BorderThickness}" Opacity="0" RadiusY="2" RadiusX="2" Stroke="#FF6DBDD1" StrokeThickness="1"/> <ContentPresenter x:Name="contentPresenter" ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/> <Rectangle x:Name="FocusVisualElement" IsHitTestVisible="false" Margin="1" RadiusY="3.5" RadiusX="3.5" Stroke="#FF6DBDD1" StrokeThickness="1" Visibility="Collapsed"/> </Grid> </ControlTemplate> </Setter.Value> </Setter> </Style> </Grid.Resources> <VisualStateManager.VisualStateGroups> <VisualStateGroup x:Name="CommonStates"> <VisualState x:Name="Normal"/> <VisualState x:Name="MouseOver"/> <VisualState x:Name="Disabled"> <Storyboard> <DoubleAnimation Duration="00:00:00" To=".55" Storyboard.TargetProperty="(UIElement.Opacity)" Storyboard.TargetName="DisabledVisualElement"/> </Storyboard> </VisualState> </VisualStateGroup> <VisualStateGroup x:Name="FocusStates"> <VisualState x:Name="Focused"> <Storyboard> <DoubleAnimation Duration="00:00:00" To="1" Storyboard.TargetProperty="(UIElement.Opacity)" Storyboard.TargetName="FocusVisualElement"/> </Storyboard> </VisualState> <VisualState x:Name="Unfocused"/> <VisualState x:Name="FocusedDropDown"> <Storyboard> <ObjectAnimationUsingKeyFrames Duration="00:00:00" Storyboard.TargetProperty="(UIElement.Visibility)" Storyboard.TargetName="PopupBorder"> <DiscreteObjectKeyFrame KeyTime="00:00:00"> <DiscreteObjectKeyFrame.Value> <Visibility>Visible</Visibility> </DiscreteObjectKeyFrame.Value> </DiscreteObjectKeyFrame> </ObjectAnimationUsingKeyFrames> </Storyboard> </VisualState> </VisualStateGroup> <VisualStateGroup x:Name="ValidationStates"> <VisualState x:Name="Valid"/> <VisualState x:Name="InvalidUnfocused"> <Storyboard> <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="ValidationErrorElement"> <DiscreteObjectKeyFrame KeyTime="0"> <DiscreteObjectKeyFrame.Value> <Visibility>Visible</Visibility> </DiscreteObjectKeyFrame.Value> </DiscreteObjectKeyFrame> </ObjectAnimationUsingKeyFrames> </Storyboard> </VisualState> <VisualState x:Name="InvalidFocused"> <Storyboard> <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="ValidationErrorElement"> <DiscreteObjectKeyFrame KeyTime="0"> <DiscreteObjectKeyFrame.Value> <Visibility>Visible</Visibility> </DiscreteObjectKeyFrame.Value> </DiscreteObjectKeyFrame> </ObjectAnimationUsingKeyFrames> <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="IsOpen" Storyboard.TargetName="validationTooltip"> <DiscreteObjectKeyFrame KeyTime="0"> <DiscreteObjectKeyFrame.Value> <System:Boolean>True</System:Boolean> </DiscreteObjectKeyFrame.Value> </DiscreteObjectKeyFrame> </ObjectAnimationUsingKeyFrames> </Storyboard> </VisualState> </VisualStateGroup> </VisualStateManager.VisualStateGroups> <Border x:Name="ContentPresenterBorder"> <Grid> <ToggleButton x:Name="DropDownToggle" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" HorizontalAlignment="Stretch" HorizontalContentAlignment="Right" Margin="0" Style="{StaticResource comboToggleStyle}" VerticalAlignment="Stretch"> <Path x:Name="BtnArrow" Data="F1 M 301.14,-189.041L 311.57,-189.041L 306.355,-182.942L 301.14,-189.041 Z " HorizontalAlignment="Right" Height="4" Margin="0,0,6,0" Stretch="Uniform" Width="8"> <Path.Fill> <SolidColorBrush x:Name="BtnArrowColor" Color="#FF333333"/> </Path.Fill> </Path> </ToggleButton> <ContentPresenter x:Name="ContentPresenter" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"> <TextBlock Text=" "/> </ContentPresenter> </Grid> </Border> <Rectangle x:Name="DisabledVisualElement" Fill="White" IsHitTestVisible="false" Opacity="0" RadiusY="3" RadiusX="3"/> <Rectangle x:Name="FocusVisualElement" IsHitTestVisible="false" Margin="1" Opacity="0" RadiusY="2" RadiusX="2" Stroke="#FF6DBDD1" StrokeThickness="1"/> <Border x:Name="ValidationErrorElement" BorderBrush="#FFDB000C" BorderThickness="1" CornerRadius="1" Visibility="Collapsed"> <ToolTipService.ToolTip> <ToolTip x:Name="validationTooltip" DataContext="{Binding RelativeSource={RelativeSource TemplatedParent}}" Placement="Right" PlacementTarget="{Binding RelativeSource={RelativeSource TemplatedParent}}" Template="{StaticResource ValidationToolTipTemplate}"> <ToolTip.Triggers> <EventTrigger RoutedEvent="Canvas.Loaded"> <BeginStoryboard> <Storyboard> <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="IsHitTestVisible" Storyboard.TargetName="validationTooltip"> <DiscreteObjectKeyFrame KeyTime="0"> <DiscreteObjectKeyFrame.Value> <System:Boolean>true</System:Boolean> </DiscreteObjectKeyFrame.Value> </DiscreteObjectKeyFrame> </ObjectAnimationUsingKeyFrames> </Storyboard> </BeginStoryboard> </EventTrigger> </ToolTip.Triggers> </ToolTip> </ToolTipService.ToolTip> <Grid Background="Transparent" HorizontalAlignment="Right" Height="12" Margin="1,-4,-4,0" VerticalAlignment="Top" Width="12"> <Path Data="M 1,0 L6,0 A 2,2 90 0 1 8,2 L8,7 z" Fill="#FFDC000C" Margin="1,3,0,0"/> <Path Data="M 0,0 L2,0 L 8,6 L8,8" Fill="#ffffff" Margin="1,3,0,0"/> </Grid> </Border> <Popup x:Name="Popup" > <Border x:Name="PopupBorder" HorizontalAlignment="Stretch"> <Grid> <ScrollViewer BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" VerticalAlignment="Bottom" x:Name="ScrollViewer" Padding="1"> <ScrollViewer.Background> <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0"> <GradientStop Color="White" Offset="0"/> <GradientStop Color="#FFFEFEFE" Offset="1"/> </LinearGradientBrush> </ScrollViewer.Background> <ItemsPresenter/> </ScrollViewer> <Rectangle Margin="0,5000,0,0"/> </Grid> </Border> </Popup> </Grid> </ControlTemplate> </Setter.Value> </Setter> </Style> 
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