How to bind tooltip's template Width to TextBox Width? - wpf

Question

My problem is I can't set the width of a TextBox's Tooltip. The tooltip should have the same width as its containing TextBox.
please consider this code :
<TextBox Text="{Binding PatientSubStay.Observations, Mode=OneWay}" ToolTip="{Binding}" x:Name="ObservationsTextBox" MaxLength="100">
<TextBox.Resources>
<Style TargetType="ToolTip">
<Setter Property="Template" Value="{StaticResource ObservationsToolTipControlTemplate}"/>
<Setter Property="Width" Value="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type TextBox}}, Path=Width}"/>
</Style>
</TextBox.Resources>
</TextBox>
The Tooltip is set by a template, and my attempt to modify its Width with a setter is a failure:
thank you.
EDIT:
here is the template (it's a preversion):
<ControlTemplate x:Key="ObservationsToolTipControlTemplate">
<Border Background="White" BorderBrush="Black" BorderThickness="1" Padding="2" CornerRadius="2" MinWidth="150">
<Border.Effect>
<DropShadowEffect BlurRadius="10" ShadowDepth="0"/>
</Border.Effect>
<Grid>
<Grid.ColumnDefinitions>
<!-- Width="{Binding Source={StaticResource TooltipHeaderColumnWidth}}" MaxWidth="{Binding Source={StaticResource TooltipHeaderColumnWidth}}" -->
<ColumnDefinition/>
<ColumnDefinition/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition/>
<RowDefinition Height="20"/>
<RowDefinition Height="20"/>
</Grid.RowDefinitions>
</Grid>
</Border>
</ControlTemplate>
EDIT:
With the setter like the one of Peregrine's answer, I get this rendering:
I wonder why the widths are not equal.
for information, here is the code corresponding to the picture above:
<TextBox Text="{Binding PatientSubStay.Observations, Mode=OneWay}" ToolTip="{Binding}" x:Name="ObservationsTextBox" MaxLength="100">
<TextBox.Style>
<Style TargetType="TextBox">
<Style.Triggers>
<Trigger Property="IsFocused" Value="True">
<Setter Property="Background" Value="{StaticResource FocusedItemSolidColorBrush}"/>
</Trigger>
</Style.Triggers>
</Style>
</TextBox.Style>
<TextBox.Resources>
<Style TargetType="ToolTip">
<Setter Property="Template" Value="{StaticResource ObservationsToolTipControlTemplate}"/>
<Setter Property="Width" Value="{Binding PlacementTarget.ActualWidth, RelativeSource={RelativeSource Self}}"/>
<Setter Property="Placement" Value="Bottom"/>
</Style>
</TextBox.Resources>
</TextBox>

Answer 1

The issue is that Tooltips exist outside of the standard visual tree, and so can't reference another control by name or as a typed ancestor. The only thing that a tooltip knows about is its PlacementTarget - the control that it is tied to.
The following code demonstates how to bind to a property of the PlacementTarget
<TextBox Text="Some text for the text box" ToolTip="tool tip">
<TextBox.Resources>
<Style TargetType="{x:Type ToolTip}">
<Setter Property="Width" Value="{Binding PlacementTarget.ActualWidth, RelativeSource={RelativeSource Self}}"/>
</Style>
</TextBox.Resources>
</TextBox>
Note that you should use ActualWidth rather than Width to get the size of another control as Width may not actually be set to a value.

Answer 2

Try setting the ActualWidth of TextBox to the Tooltip. When you don't set the Width (
or Width=Auto) the value is double.NaN. The below code worked for me.
<Setter Property="Width" Value="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type TextBox}}, Path=ActualWidth}"/>