Can't set focus to a child of UserControl

Question

I have a UserControl which contains a TextBox. When my main window loads I want to set the focus to this textbox so I added Focusable=\"True

Answer 1

“When setting initial focus at application startup, the element to receive focus must be connected to a PresentationSource and the element must have Focusable and IsVisible set to true. The recommended place to set initial focus is in the Loaded event handler" (MSDN)

Simply add a "Loaded" event handler in the constructor of your Window (or Control), and in that event handler call the Focus() method on the target control.

    public MyWindow() {
        InitializeComponent();
        this.Loaded += new RoutedEventHandler(MyWindow_Loaded);
    }

    void MyWindow_Loaded(object sender, RoutedEventArgs e) {
        textBox.Focus();
    }