WPF validation launched on focus gained - wpf

Question

I'm developing a IDataErrorInfo to validate the textboxes I have inside my application. I have the following code:
The .cs class to validate:
public class UserInformation : IDataErrorInfo
{
public string _name;
public string _surname;
public string Name
{
get { return _name; }
set { _name = value; }
}
public string Surname
{
get { return _surname; }
set { _surname = value; }
}
public override string ToString()
{
return Name + " " + Surname;
}
public string this[string columnName]
{
get
{
if (columnName == null) return string.Empty;
string result = string.Empty;
if (columnName.Equals("Name"))
{
if (string.IsNullOrEmpty(_name))
result = "Name cannot be empty.";
}
return result;
}
}
public string Error { get; private set; }
}
The .xaml:
<TextBox Grid.Column="3" Grid.Row="0" Name="TextBoxName"
Style="{DynamicResource InnerTextBox}"
Validation.ErrorTemplate="{StaticResource ValidationErrorTemplate}">
<TextBox.Text>
<Binding Path="Name" Source="{StaticResource UserInformation}"
ValidatesOnDataErrors="True" UpdateSourceTrigger="PropertyChanged">
<Binding.ValidationRules>
<ExceptionValidationRule />
</Binding.ValidationRules>
</Binding>
</TextBox.Text>
</TextBox>
And the ErrorTemplate:
<ControlTemplate x:Key="ValidationErrorTemplate">
<DockPanel >
<StackPanel Orientation="Horizontal" DockPanel.Dock="Top">
<Grid Width="20" Height="20">
<Ellipse Width="20" Height="20" Fill="Tomato" HorizontalAlignment="Center" VerticalAlignment="Center" />
<TextBlock Foreground="White" FontWeight="Heavy" FontSize="8" HorizontalAlignment="Center" VerticalAlignment="Center" TextAlignment="Center"
ToolTip="{Binding ElementName=ErrorAdorner, Path=AdornedElement.(Validation.Errors)[0].ErrorContent}">X</TextBlock>
</Grid>
<TextBlock Foreground="Tomato" FontWeight="12" Margin="2,0,0,0" FontSize="20"
Text="{Binding ElementName=ErrorAdorner, Path=AdornedElement.(Validation.Errors)[0].ErrorContent}" />
</StackPanel>
<AdornedElementPlaceholder x:Name="ErrorAdorner" />
</DockPanel>
</ControlTemplate>
The code works fine when I'm typing. But when the TextBox is loaded, the validation occurs too. And I don't want it to happen when it gains focus, only when it looses it or I change the text (like the one published here).
How can I avoid the validation error to be considered on first TextBox load?
NOTE: Even if I set the UpdateSourceTrigger to LostFocus, it is still making the validations.

Answer 1

To acheive you goal you need to:
First, remove ValidatesOnDataErrors="True" on your Binding. As said in docs:
Setting this property provides an alternative to using the
DataErrorValidationRule element explicitly
And we're gonna use it explicitly. Then use DataErrorValidationRule instead of ExceptionValidationRule for correctly working with IDataErrorInfo and data errors.
And last, we need to use some properties that this rule gives us:
<Binding.ValidationRules>
<DataErrorValidationRule ValidatesOnTargetUpdated="False" />
</Binding.ValidationRules>
ValidatesOnTargetUpdated on false will not trigger validation when target itself changes (i.e. on load). You can also play with ValidationStep property for additional control.
Edit:
Ok, I see that you need to skip validation on load and you need to validate on lost focus even if the value was not changed. Well, validation rules does not support that, because if the value was not updated, then no changed events will be called and no validation will occur, regardless of UpdateSourceTrigger setting.
The easy way out is to emulate this functionality by adding LostFocus handler to TextBox itself:
private void ValidatedTextBox_LostFocus(object sender, RoutedEventArgs e)
{
var txt = (TextBox)sender;
txt.GetBindingExpression(TextBox.TextProperty).UpdateSource();
}
If you need this for several TextBoxes, you can move the code to some static class.
The same results can be achieved using Explicit update source trigger, wich can be a little bit more cleaner.

Answer 2

I dont have any example atm, because I moved to what you have. But You will need to create a class which will Inherit from ValidationRule which exist in system.windows.controls, and then override Validate method.
Then your xaml textbox would look something like this instead
<TextBox.Text>
<Binding Path="your binding here" UpdateSourceTrigger="LostFocus" >
<Binding.ValidationRules>
<validationClass:yourRule/> define this at the top of xaml page
</Binding.ValidationRules>
</Binding>
You should be able to find examples on msdn, and here about validation rules