Unable to set the parent datacontext for datatemplate tooltip

Question

Unable to set the parent datacontext for datatemplate tooltip.

Below is the xaml code. Just once combo box and in the combobox datatemplate added textbox.

Xaml

<UserControl x:Class="WpfApplication1.UserControl1"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             Name="UC"
             d:DesignHeight="50" d:DesignWidth="200">
    <Grid>
        <ComboBox Width="200" Height="50" ItemsSource="{Binding Coll}">
            <ComboBox.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding Length}">
                        <TextBlock.ToolTip>
                            <ToolTip Content="{Binding Path=DataContext.ToolTipValue, 
                                                       RelativeSource={RelativeSource FindAncestor, 
                                                       AncestorType={x:Type UserControl}}}"/>
                        </TextBlock.ToolTip>
                    </TextBlock>
                </DataTemplate>
            </ComboBox.ItemTemplate>
        </ComboBox>
    </Grid>
</UserControl>

ViewModel

    private List<string> _coll;

    public List<string> Coll
    {
        get { return _coll; }
        set { _coll = value; OnPropertyChanged(); }
    }

    private string _ToolTipValue;

    public string ToolTipValue
    {
        get { return _ToolTipValue; }
        set { _ToolTipValue = value; OnPropertyChanged(); }
    }

    public ViewModel()
    {
        _coll = new List<string>(){ "1", "2", "3"};
        _ToolTipValue = "Demo";
    }

Can you please help me why DataContext is not set.

Is it binding issue?

Answer 1

if you don't need any special things try not to nest the tooltip:

<TextBlock ToolTip="{Binding DataContext.ToolTipValue, 
                     RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=UserControl}}"
           Text="{Binding Length}"/>