Focusing a TextBox that’s inside a Custom Control isn’t so easy at startup of your application. Let me explain the problem that is also discussed on http://forums.silverlight.net/forums/t/151235.aspx. Imagine you’ve created a custom control that has a TextBox as Part-element. The Style that sets the Template would look like this:

<Style TargetType="local:SimpleControl">
  <Setter Property="Template">
    <Setter.Value>
      <ControlTemplate  TargetType="local:SimpleControl">
        <Border Background="{TemplateBinding Background}"
          BorderBrush="{TemplateBinding BorderBrush}"
          BorderThickness="{TemplateBinding BorderThickness}">
          <TextBox x:Name="PART_Text" Margin="10"></TextBox>
        </Border>
      </ControlTemplate>
    </Setter.Value>
  </Setter>
</Style>

You get the TextBox from the Template in the OnApplyTemplate-Method

public override void OnApplyTemplate()
{
  base.OnApplyTemplate();
  _partTextBox = this.GetTemplateChild("PART_Text") as TextBox;
}

Now imaginge you want to focus your SimpleControl in the constructor of your MainPage like below:

public MainPage()
{
  InitializeComponent();
  HtmlPage.Plugin.Focus();
  yourControl.Focus();
}

The Problem that occurs is that the OnApplyTemplate-Method is called after the Focus-Method. So you don’t have the TextBox defined in your Template. The Solution is very easy. Just hide the Focus-Method from the base-class and set a flag. But keep in mind that this won’t work if you store your SimpleControl in a variable of type Control, cause then no polymorphic call is made to the Focus-Method defined in SimpleControl. Here a sample implementation:

public class SimpleControl : Control
{
  private bool _applyFocusToPartTextBox;
  private TextBox _partTextBox;
  public SimpleControl()
  {
    this.DefaultStyleKey = typeof(SimpleControl);
  }
  public override void OnApplyTemplate()
  {
    base.OnApplyTemplate();
    _partTextBox = this.GetTemplateChild("PART_Text") as TextBox;
    if (_partTextBox != null)
    {
      if (_applyFocusToPartTextBox)
        _partTextBox.Focus();
    }
    _applyFocusToPartTextBox = false;
  }
  public new bool Focus()
  {
    if (_partTextBox == null)
    {
      _applyFocusToPartTextBox = true;
      return true;
    }
    return base.Focus();
  }
}