Question

Having mocked up a custom control as a window and got all the behaviour right, I’m now trying to turn it into a proper custom control (called “When”, it’s a date-time widget).

I have prepared a When.XAML file in which sub-elements are named PART_xxx

<ResourceDictionary
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:glob="clr-namespace:System.Globalization;assembly=mscorlib"
  xmlns:local="clr-namespace:Widgets">
  <local:DatePartPositionValueConverter x:Key="DatePartPositionValueConverter" />
  <local:DatePartVisibilityValueConverter x:Key="DatePartVisibilityValueConverter" />
  <Style TargetType="{x:Type local:When}">
    <Setter Property="Template">
      <Setter.Value>
        <ControlTemplate TargetType="{x:Type local:When}">
          <Border Background="{TemplateBinding Background}"
                  BorderBrush="{TemplateBinding BorderBrush}"
                  BorderThickness="{TemplateBinding BorderThickness}">
            <Border BorderThickness="1" BorderBrush="{DynamicResource 
                {x:Static SystemColors.ControlDarkBrushKey}}">
              <Grid HorizontalAlignment="Left" Margin="4,0,0,0">
                <Grid.ColumnDefinitions>
                  <ColumnDefinition />
                  ...
                  <ColumnDefinition />
                </Grid.ColumnDefinitions>
                <TextBlock x:Name="PART_year" Grid.Column="{Binding 
                    Converter={StaticResource DatePartPositionValueConverter}, 
                    ConverterParameter=y}">
                  <TextBlock.Text>
                  ...

The custom control project Generic.XAML file references When.XAML

<ResourceDictionary
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <ResourceDictionary.MergedDictionaries>
    <ResourceDictionary Source="/Widgets;component/Themes/When.xaml" />
  </ResourceDictionary.MergedDictionaries>
</ResourceDictionary>

My code, however, does not seem to be able to resolve the PART_ names.

I’d like to be able to compare _focussedElement to (eg) PART_year to provide context for validation checks. It must be possible to refer directly to PART_xxx from the code of a custom control, otherwise it would be impossible to use code to bind event handlers to the elements of a template.

What have I failed to apprehend?

---

To paraphrase and expand on the excellent answer below, PART_year isn’t in scope because there’s no opportunity for the IDE’s code generation magic to bring it into scope. So you bring it into scope yourself, like this:

MenuItem PART_MenuItemToday, PART_MenuItemNow, 
  PART_MenuItemMonthEnd, PART_MenuItemMonthStart;
public override void OnApplyTemplate()
{
  base.OnApplyTemplate();
  PART_MenuItemMonthEnd = GetTemplateChild("PART_ContextMenuMonthEnd") as MenuItem;
  PART_MenuItemMonthEnd.Click += PART_ContextMenuMonthEnd_Click;
  ...
}

When you need to hook the same set of handlers to several widgets you can do this

private void BindGenericHandlers(TextBlock textBlock)
{
  textBlock.GotFocus += PART_GotFocus;
  textBlock.LostFocus += PART_LostFocus;
  textBlock.MouseDown += PART_MouseDown;
  textBlock.MouseEnter += PART_MouseEnter;
  textBlock.MouseLeave += PART_MouseLeave;
}

TextBlock _focussedElement, PART_year, PART_month, PART_day, PART_hour, PART_minute, PART_second, PART_designator;

public override void OnApplyTemplate()
{
  base.OnApplyTemplate();
  BindGenericHandlers(PART_day = GetTemplateChild("PART_day") as TextBlock);
  BindGenericHandlers(PART_designator = GetTemplateChild("PART_designator") as TextBlock);
  BindGenericHandlers(PART_hour = GetTemplateChild("PART_hour") as TextBlock);
  BindGenericHandlers(PART_minute = GetTemplateChild("PART_minute") as TextBlock);
  BindGenericHandlers(PART_month = GetTemplateChild("PART_month") as TextBlock);
  BindGenericHandlers(PART_second = GetTemplateChild("PART_second") as TextBlock);
  BindGenericHandlers(PART_year = GetTemplateChild("PART_year") as TextBlock);
  ...
}

Answer 1

You retreive references to your PARTs in the code by using the GetTemplateChild method inside the OnApplyTemplate override. So in the code for your control, you’ll have the following:

private const string PART_TEXTINPUT = "PART_TEXT";
private TextBox _textInput;

public override void OnApplyTemplate()
{
   base.OnApplyTemplate();
   _textInput = GetTemplateChild(PART_TEXTINPUT) as TextBox;
}

Since you are using PARTs, you seem to be making a lookless control, and therefore, you can’t have a direct reference to the element from the XAML (since a custom ControlTemplate could replace it with something unexpected). So you retrieve references to your PARTs with the GetTemplateChild method.

NOTE: Be sure to use the lowest possible type for the part (in your code) in case someone replaces your expected control with a different implementation.