Why does WPF Style to show validation errors in ToolTip work for a TextBox but fails for a ComboBox?
- by Mike B
I am using a typical Style to display validation errors as a tooltip from IErrorDataInfo for a textbox as shown below and it works fine.
<Style TargetType="{x:Type TextBox}">
<Style.Triggers>
<Trigger Property="Validation.HasError" Value="true">
<Setter Property="ToolTip"
Value="{Binding RelativeSource={RelativeSource Self},
Path=(Validation.Errors)[0].ErrorContent}"/>
</Trigger>
</Style.Triggers>
</Style>
But when i try to do the same thing for a ComboBox like this it fails
<Style TargetType="{x:Type ComboBox}">
<Style.Triggers>
<Trigger Property="Validation.HasError" Value="true">
<Setter Property="ToolTip"
Value="{Binding RelativeSource={RelativeSource Self},
Path=(Validation.Errors)[0].ErrorContent}"/>
</Trigger>
</Style.Triggers>
</Style>
The error I get in the output window is:
System.Windows.Data Error: 17 : Cannot get 'Item[]' value (type 'ValidationError') from '(Validation.Errors)' (type 'ReadOnlyObservableCollection`1'). BindingExpression:Path=(0)[0].ErrorContent; DataItem='ComboBox' (Name='ownerComboBox'); target element is 'ComboBox' (Name='ownerComboBox'); target property is 'ToolTip' (type 'Object') ArgumentOutOfRangeException:'System.ArgumentOutOfRangeException: Specified argument was out of the range of valid values.Parameter name: index'
Oddly it also attempts to make invalid Database changes when I close the window if I change any ComboBox values (This is also when the binding error occurs)!!!
Cannot insert the value NULL into column 'EmpFirstName', table 'OITaskManager.dbo.Employees'; column does not allow nulls. INSERT fails.
The statement has been terminated.
Simply by commenting the style out everyting works perfectly. How do I fix this?
Just in case anyone needs it one of the comboBox' xaml follows:
<ComboBox ItemsSource="{Binding Path=Employees}"
SelectedValuePath="EmpID"
SelectedValue="{Binding Path=SelectedIssue.Employee2.EmpID,
Mode=OneWay, ValidatesOnDataErrors=True}"
ItemTemplate="{StaticResource LastNameFirstComboBoxTemplate}"
Height="28" Name="ownerComboBox" Width="120" Margin="2"
SelectionChanged="ownerComboBox_SelectionChanged" />
<DataTemplate x:Key="LastNameFirstComboBoxTemplate">
<TextBlock>
<TextBlock.Text>
<MultiBinding StringFormat="{}{1}, {0}" >
<Binding Path="EmpFirstName" />
<Binding Path="EmpLastName" />
</MultiBinding>
</TextBlock.Text>
</TextBlock>
</DataTemplate>
SelectionChanged: (I do plan to implement commanding before long but, as this is my first WPF project I have not gone full MVVM yet. I am trying to take things in small-medium sized bites)
// This is done this way to maintain the DataContext Integrity
// and avoid an error due to an Object being "Not New" in Linq-to-SQL
private void ownerComboBox_SelectionChanged(object sender,
SelectionChangedEventArgs e)
{
Employee currentEmpl = ownerComboBox.SelectedItem as Employee;
if (currentEmpl != null &&
currentEmpl != statusBoardViewModel.SelectedIssue.Employee2)
{
statusBoardViewModel.SelectedIssue.Employee2 = currentEmpl;
}
}
