Graeme Hill's Dev Blog

Forcing form validation in WPF before user input

Star date: 2009.043

By default, validation is not enforced on a binding until after the value has been changed once. Consider this situation: A form has a text box whose contents should not be empty, but its starting value is the empty string. The text box will not show an error when the form is loaded. Instead, it will only show its error style after the user types in some text and then deletes it. Some people would call this a feature, but I prefer to have the validation checked right away so that I immediately know what the required fields are, and which starting values are invalid.

A quick hack

One way to tackle this is to hook a loaded event to the control and then call UpdateSource() from code:

Private Sub myTextBox_Loaded(ByVal sender As System.Object, _
        ByVal e As System.Windows.RoutedEventArgs) Handles myTextBox.Loaded

    BindingOperations.GetBindingExpression(myTextBox, _
        TextBox.TextProperty).UpdateSource()

End Sub

Unfortunately, this really isn't a desirable solution because it requires some pretty ugly code-behind (not that there's any other kind) and needs to be done over again for every binding. What we really want is for this functionality to be a part of the binding markup extension.

Creating a wrapper around the binding markup extension

At first I thought it would be easy to just inherit from the Binding class and override ProvideValue, but of course, it is marked as NotOverridable (sealed in C#). Instead, I create a class called ValidationBinding that inherits directly from MarkupExtension and has the new class manage an instance of Binding. The code looks like this:

Imports System.Windows.Markup

Public Class ValidationBinding
    Inherits MarkupExtension

    Private _binding As New Binding
    Private _dependencyObject As DependencyObject
    Private _dependencyProperty As DependencyProperty

    Public Sub New()
        _binding.ValidatesOnDataErrors = True
        _binding.ValidatesOnExceptions = True
    End Sub

    Public Sub New(ByVal path As String)
        Me.New()
        _binding.Path = New PropertyPath(path)
    End Sub

    Public Overrides Function ProvideValue _
        (ByVal serviceProvider As System.IServiceProvider) As Object

        Dim valueTarget = _
            DirectCast(serviceProvider.GetService(GetType(IProvideValueTarget)),  _
            IProvideValueTarget)

        _dependencyObject = valueTarget.TargetObject
        _dependencyProperty = valueTarget.TargetProperty

        If TypeOf _dependencyObject Is FrameworkElement Then
            Dim element = DirectCast(_dependencyObject, FrameworkElement)
            If element.IsLoaded Then
                ForceValidation()
            Else
                AddHandler element.Loaded, AddressOf ElementLoaded
            End If
        Else
            ForceValidation()
        End If

        Return _binding.ProvideValue(serviceProvider)

    End Function

    Private Sub ForceValidation()
        BindingOperations.GetBindingExpression(_dependencyObject, _
                                               _dependencyProperty).UpdateSource()
    End Sub

    Private Sub ElementLoaded(ByVal sender As System.Object, _
                              ByVal e As System.Windows.RoutedEventArgs)
        ForceValidation()
    End Sub

    Public Property Path() As PropertyPath
        Get
            Return _binding.Path
        End Get
        Set(ByVal value As PropertyPath)
            _binding.Path = value
        End Set
    End Property

    ... the rest of the binding properties go here

End Class

The binding can then be used like this (where my is an imported namespace containing ValidationBinding):

<TextBox Margin="5" Text="{my:ValidationBinding Path=Text}" />

As an example, I have exposed the Binding's Path property, but you actually have to do this for all of the public properties in Binding. A reference of all the properties that should be implemented can be found here.

ProvideValue returns the result of the Binding's ProvideValue function, but first it checks whether the binding target has finished loading. If it has already finished loading then it forces validation by calling UpdateSource() on the target. In the much more likely scenario that the control has not yet loaded (this will be the case when you set your binding in XAML) it attaches a handler to the Loaded event so that the ForceValidation subroutine can be deferred until it is finished loading.

Also, notice that I set both ValidatesOnDataErrors and ValidatesOnExceptions to True in the constructor so that I didn't need to specify it in the XAML. Chances are that whenever you use this markup extension you will want those enabled anyways.

It may seem like a lot of work, but it is a very reusable solution that gives you significantly more power.

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