Prevent empty tooltips at a wpf datagrid

Question

I am working on a calendar program, which consists mainly of a WPF DataGrid. As there is not always enough space to display all the entries of a day (which is a DataGridCell), a tooltip with all the entries of the day shell appear at mouse over. This works so far with the code snippet shown below. And now the (little) problem: If there are no entries for a day, no tooltip shell pop up. With the code below an empty tooltip pops up.

<DataGridTemplateColumn x:Name="Entry" 
                        IsReadOnly="True">
  <DataGridTemplateColumn.CellTemplate>
    <DataTemplate>
      <Grid>
        <TextBlock Text="{Binding EntryText}"
                   Foreground="{Binding EntryForeground}"
                   FontWeight="{Binding EntryFontWeight}">
        </TextBlock>
        <TextBlock Text="{Binding RightAlignedText}"
                   Foreground="Gray"    
                   Background="Transparent">
          <TextBlock.ToolTip>
            <TextBlock Text="{Binding AllEntriesText}"/>
          </TextBlock.ToolTip>
        </TextBlock>
      </Grid>
    </DataTemplate>
  </DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>

The Databinding is made via

myCalDataGrid.Itemssource = _listOfDays; 

in code behind, where a 'Day' is the view model for a single calendar row.

Answer 1

As H.B. suggested bind directly to the ToolTip property instead of using TextBlock and in case AllEntriesText is empty string you can apply a trigger on your TextBlock to disable your tooltip by setting the property ToolTipService.IsEnabled like this -

<TextBlock Text="{Binding RightAlignedText}"
           Foreground="Gray"    
           Background="Transparent"
           ToolTip="{Binding AllEntriesText}">
   <TextBlock.Style>
      <Style TargetType="TextBlock">
         <Style.Triggers>
            <Trigger Property="ToolTip"
                     Value="{x:Static system:String.Empty}">
               <Setter Property="ToolTipService.IsEnabled" Value="False" />
            </Trigger>
         </Style.Triggers>
       </Style>
    </TextBlock.Style>
</TextBlock>

Make sure to add namespace system in your xaml -

xmlns:system="clr-namespace:System;assembly=mscorlib"