Disable Save button in WPF if validation fails

Question

I've adopted what appears to be the standard way of validating textboxes in WPF using the IDataErrorInfo interface and styles as shown below. However, how can I disable the Save button when the page becomes invalid? Is this done somehow through triggers?

Default Public ReadOnly Property Item(ByVal propertyName As String) As String Implements IDataErrorInfo.Item
    Get
        Dim valid As Boolean = True
        If propertyName = "IncidentCategory" Then
            valid = True
            If Len(IncidentCategory) = 0 Then
                valid = False
            End If
            If Not valid Then
                Return "Incident category is required"
            End If
        End If

        Return Nothing

    End Get
End Property

<Style TargetType="{x:Type TextBox}">
    <Setter Property="Margin" Value="3" />
    <Setter Property="Height" Value="23" />
    <Setter Property="HorizontalAlignment" Value="Left" />
    <Setter Property="Validation.ErrorTemplate">
        <Setter.Value>
            <ControlTemplate>
                <DockPanel LastChildFill="True">
                    <Border BorderBrush="Red" BorderThickness="1">
                        <AdornedElementPlaceholder Name="MyAdorner" />
                    </Border>
                </DockPanel>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
    <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>

Answer 1

A couple of things:

First, I would recommend using the RoutedCommand ApplicationCommands.Save for implementing the handling of the save button.

If you haven't checked out the WPF Command model, you can get the scoop here.

<Button Content="Save" Command="Save">

Now, to implement the functionality, you can add a command binding to the Window/UserControl or to the Button itself:

    <Button.CommandBindings>
        <CommandBinding Command="Save" 
                        Executed="Save_Executed" CanExecute="Save_CanExecute"/>
    </Button.CommandBindings>
</Button>

Implement these in code behind:

private void Save_Executed(object sender, ExecutedRoutedEventArgs e)
{
}

private void Save_CanExecute(object sender, CanExecuteRoutedEventArgs e)
{
}

In Save_CanExecute, set e.CanExecute based on the validity of the binding on the text box.

If you want to implement using the MVVM (Model-View-ViewModel) design pattern, check out Josh Smith's post on CommandSinkBinding.

One final note: If you want the enable/disable to be updated as soon as the value in the TextBox is changed, set UpdateSourceTrigger="PropertyChanged" on the binding for the TextBox.

EDIT: If you want to validate/invalidate based on all of the bindings in the control, here are a few suggestions.

1) You are already implementing IDataErrorInfo. Try implementing the IDataErrorInfo.Error property such that it returns the string that is invalid for all of the properties that you are binding to. This will only work if your whole control is binding to a single data object. Set e.CanExecute = string.IsNullOrEmpty(data.Error);

2) Use reflection to get all of the public static DependencyProperties on the relevant controls. Then call BindingOperations.GetBindingExpression(relevantControl, DependencyProperty) in a loop on each property so you can test the validation.

3) In the constructor, manually create a collection of all bound properties on nested controls. In CanExecute, iterate through this collection and validate each DependencyObject/DepencyProperty combination by using BindingOperation.GetBindingExpression() to get expressions and then examining BindingExpression.HasError.