Force validation on bound controls in WPF

Question

Hello.

I have a WPF dialog with a couple of textboxes on it. Textboxes are bound to my business object and have WPF validation rules attached.

The problem is that user can perfectly click 'OK' button and close the dialog, without actually entering the data into textboxes. Validation rules never fire, since user didn't even attempt entering the information into textboxes.

Is it possible to force validation checks and determine if some validation rules are broken?

I would be able to do it when user tries to close the dialog and prohibit him from doing it if any validation rules are broken.

Thank you.

Answer 1

We have this issue in our application as well. The validation only fires when bindings update, so you have to update them by hand. We do this in the Window's Loaded event:

public void Window_Loaded(object sender, RoutedEventArgs e)
{
    // we manually fire the bindings so we get the validation initially
    txtName.GetBindingExpression(TextBox.TextProperty).UpdateSource();
    txtCode.GetBindingExpression(TextBox.TextProperty).UpdateSource();
}

This will make the error template (red outline) appear, and set the Validation.HasError property, which we have triggering the OK button to disable:

<Button x:Name="btnOK" Content="OK" IsDefault="True" Click="btnOK_Click">
    <Button.Style>
        <Style TargetType="{x:Type Button}">
            <Setter Property="IsEnabled" Value="false" />
            <Style.Triggers>
                <!-- Require the controls to be valid in order to press OK -->
                <MultiDataTrigger>
                    <MultiDataTrigger.Conditions>
                        <Condition Binding="{Binding ElementName=txtName, Path=(Validation.HasError)}" Value="false" />
                        <Condition Binding="{Binding ElementName=txtCode, Path=(Validation.HasError)}" Value="false" />
                    </MultiDataTrigger.Conditions>
                    <Setter Property="IsEnabled" Value="true" />
                </MultiDataTrigger>
            </Style.Triggers>
        </Style>
    </Button.Style>
</Button>

Answer 2

Hi

I have tried this but can't get it to work. The code in the Loaded event handler doesn't trigger the validation. If I instead place it in a button event handler and press the button. Then the validation is triggered.

What am I missing?

Per Grill

I Figured it out:
I had the following lines in the Loaded event handler:

person = new Person();
this.DataContext = person;

I moved these lines to the constructor instead right after InitializeComponent() and it worked

Answer 3

In 3.5SP1 / 3.0SP2, they also added a new property to the ValidationRule base, namely, ValidatesOnTargetUpdated="True". This will call the validation as soon as the source object is bound, rather than only when the target control is updated. That may not be exactly what you want, but it's not bad to see initially all the stuff you need to fix.

Works something like this:

<TextBox.Text>
 <Binding Path="Amount" StringFormat="C">
  <Binding.ValidationRules>
   <validation:RequiredValidationRule ErrorMessage="The pledge amount is required." ValidatesOnTargetUpdated="True"  />
   <validation:IsNumericValidationRule ErrorMessage="The pledge amount must be numeric." ValidationStep="ConvertedProposedValue" ValidatesOnTargetUpdated="True"  />
  </Binding.ValidationRules>
 </Binding>
</TextBox.Text>

Answer 4

using the INotifyPropertychanged on your data object

public class MyObject : INotifyPropertyChanged
{
    string _MyPropertyToBind = string.Empty;
    public string MyPropertyToBind
    {
        get
        {
            return _MyPropertyToBind;
        }
        set
        {
            _MyPropertyToBind = value;
            NotifyPropertyChanged("MyPropertyToBind");
        }
    }

    public void NotifyPropertyChanged(string property)
    {
        if (this.PropertyChanged != null)
        {
            this.PropertyChanged(this, new PropertyChangedEventArgs(property));
        }
    }
    #region INotifyPropertyChanged Members

    public event PropertyChangedEventHandler PropertyChanged;

    #endregion

}

you can add the following code to your control

<TextBox Text="{Binding MyPropertyToBind, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" >

The textbox susbscribe to the propertychanged event of the datacontext object ( MyObjet in our example) and assumes it is fired when the source data has been updated

it automatically forces the refresh to the control

No need to call yourself the UpdateTarget method