Using WPF Validation rules and the disabling of a 'Save' button

Question

I have a page where a few textboxes cannot be empty before clicking a Save button.

Answer 1

Here is a helper method which tracks validation errors on the dependency objects (and all its children) and calls delegate to notify about the change. It also tracks removal of the children with validation errors.

 public static void AddErrorHandler(DependencyObject element, Action<bool> setHasValidationErrors)
        {
            var errors = new List<Tuple<object, ValidationError>>();

            RoutedEventHandler sourceUnloaded = null;

            sourceUnloaded = (sender, args) =>
                {
                    if (sender is FrameworkElement)
                        ((FrameworkElement) sender).Unloaded -= sourceUnloaded;
                    else
                        ((FrameworkContentElement) sender).Unloaded -= sourceUnloaded;

                    foreach (var error in errors.Where(err => err.Item1 == sender).ToArray())
                        errors.Remove(error);

                    setHasValidationErrors(errors.Any());
                };

            EventHandler<ValidationErrorEventArgs> errorHandler = (_, args) =>
                {
                    if (args.Action == ValidationErrorEventAction.Added)
                    {
                        errors.Add(new Tuple<object, ValidationError>(args.OriginalSource, args.Error));

                        if (args.OriginalSource is FrameworkElement)
                            ((FrameworkElement)args.OriginalSource).Unloaded += sourceUnloaded;
                        else if (args.OriginalSource is FrameworkContentElement)
                            ((FrameworkContentElement)args.OriginalSource).Unloaded += sourceUnloaded;
                    }
                    else
                    {
                        var error = errors
                            .FirstOrDefault(err => err.Item1 == args.OriginalSource && err.Item2 == args.Error);

                        if (error != null) 
                            errors.Remove(error);
                    }

                    setHasValidationErrors(errors.Any());
                };


            System.Windows.Controls.Validation.AddErrorHandler(element, errorHandler);
        } 

Answer 2

Because it's still missing, here is an adaption of Developer's answer in case the link ever goes away:

XAML:

<TextBox.Text Validation.Error="handleValidationError">
    <Binding Path ="LastName" 
             UpdateSourceTrigger="PropertyChanged"
             NotifyOnValidationError="True">
        <Binding.ValidationRules>
            <local:StringRequiredValidationRule />
        </Binding.ValidationRules>                              
    </Binding>
</TextBox.Text>
<Button IsEnabled="{Binding HasNoValidationErrors}"/>

CodeBehind/C#:

private int _numberOfValidationErrors;
public bool HasNoValidationErrors => _numberOfValidationErrors = 0;

private void handleValidationError(object sender, ValidationErrorEventArgs e)
{
    if (e.Action == ValidationErrorEventAction.Added)
        _numberOfValidationErrors++;
    else
        _numberOfValidationErrors--;
}

Answer 3

This website has the code you're looking for: https://www.wpfsharp.com/2012/02/03/how-to-disable-a-button-on-textbox-validationerrors-in-wpf/

For posterity the button code should look like this if you are using a ValidationRule override on the input fields:

<Button Content="<NameThisButton>" Click="<MethodToCallOnClick>" >
                <Button.Style>
                    <Style TargetType="{x:Type Button}">
                        <Setter Property="IsEnabled" Value="false" />
                        <Style.Triggers>
                            <MultiDataTrigger>
                                <MultiDataTrigger.Conditions>                                    
                                    <Condition Binding="{Binding ElementName=<TextBoxName>, Path=(Validation.HasError)}" Value="false" />
                                    <Condition Binding="{Binding ElementName=<TextBoxName>, Path=(Validation.HasError)}" Value="false" />
                                </MultiDataTrigger.Conditions>
                                <Setter Property="IsEnabled" Value="true" />
                            </MultiDataTrigger>
                        </Style.Triggers>
                    </Style>
                </Button.Style>
            </Button>

Answer 4

Here is the complete sample what you need.

http://codeblitz.wordpress.com/2009/05/08/wpf-validation-made-easy-with-idataerrorinfo/

https://skydrive.live.com/?cid=2c6600f1c1d5e3be&id=2C6600F1C1D5E3BE%21203