WPF Nested binding in a controltemplate

Question

I have successfully created a textbox that displays / collapses an error message depending upon a validation rule set in its model / vm. The code goes like this for the emai

Answer 1

You won't need the Grid.Row and Grid.Column bindings in the Template StackPanel since the StackPanel won't be the direct child of a Grid anyway,

TemplateBinding is always a OneWay binding so the Text property for the Templated TextBox will never get updated. Change it to a regular Binding with RelativeSource and TwoWay

Change ElementName=validableText to RelativeSource={RelativeSource TemplatedParent} in the bindings for ContentPresenter since we want to perform the validation check on the Templated TextBox and not the TextBox inside the Template.

<ControlTemplate x:Key="FormTextBox" TargetType="{x:Type TextBox}">
    <StackPanel>
        <TextBox x:Name="validableText"
                 MaxLength="{TemplateBinding MaxLength}"
                 Style="{StaticResource SectionEditPropertyTextBox}"
                 Text="{Binding RelativeSource={RelativeSource TemplatedParent},
                                Path=Text,
                                Mode=TwoWay,
                                UpdateSourceTrigger=PropertyChanged}" />
        <ContentPresenter Visibility="{Binding RelativeSource={RelativeSource TemplatedParent},
                                               Path=(Validation.HasError),
                                               Converter={StaticResource BooleanToVisibilityConverter}
                                               ConverterParameter=True}"
                            Content="{Binding RelativeSource={RelativeSource TemplatedParent},
                                              Path=(Validation.Errors).CurrentItem}"
                            HorizontalAlignment="Left">
            <ContentPresenter.ContentTemplate>
                <DataTemplate>
                    <Label Style="{StaticResource SectionEditErrorLabel}" Content="{Binding Path=ErrorContent}"/>
                </DataTemplate>
            </ContentPresenter.ContentTemplate>
        </ContentPresenter>
    </StackPanel>
</ControlTemplate>

On a side note, another alternative that you have here is to create a UserControl with the original piece of Xaml that you had. You could introduce the Dependency Properties needed for your scenario (Text etc.). It would only require small changes.