Question

I have this custom textbox that I am working on and I can use it in xaml, but when I run my app I cannot select it or type in it. Here is my code:

public class ModdedTextBox : TextBox
{
    private bool selectionStartChangeFromUI;
    private bool selectionLengthChangeFromUI;
    private bool selectedTextChangeFromUI;

    static ModdedTextBox()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(ModdedTextBox), new FrameworkPropertyMetadata(typeof(ModdedTextBox)));
        //this.SelectionChanged += this.OnSelectionChanged;
        //PropertyDescriptor VerticalOffsetProperty = TypeDescriptor.GetProperties(typeof(ModdedTextBox))["VerticalOffset"];
        //VerticalOffsetProperty.AddValueChanged(this, this.OnVerticalOffsetChanged);
    }
    public static readonly DependencyProperty BindableSelectionStartProperty =
    DependencyProperty.Register(
    "BindableSelectionStart",
    typeof(int),
    typeof(ModdedTextBox),
    new PropertyMetadata(OnBindableSelectionStartChanged));

    public static readonly DependencyProperty BindableSelectionLengthProperty =
        DependencyProperty.Register(
        "BindableSelectionLength",
        typeof(int),
        typeof(ModdedTextBox),
        new PropertyMetadata(OnBindableSelectionLengthChanged));

    public static readonly DependencyProperty BindableSelectedTextProperty =
        DependencyProperty.Register(
        "BindableSelectedText",
        typeof(string),
        typeof(ModdedTextBox),
        new PropertyMetadata(OnBindableSelectedTextChanged));
    public static readonly DependencyProperty DelayedTextProperty =
        DependencyProperty.Register(
        "DelayedText",
        typeof(string),
        typeof(ModdedTextBox),
        new PropertyMetadata(OnDelayedTextChanged));

    public int BindableSelectionStart
    {
        get
        {
            return (int)this.GetValue(BindableSelectionStartProperty);
        }

        set
        {
            this.SetValue(BindableSelectionStartProperty, value);
        }
    }

    public int BindableSelectionLength
    {
        get
        {
            return (int)this.GetValue(BindableSelectionLengthProperty);
        }

        set
        {
            this.SetValue(BindableSelectionLengthProperty, value);
        }
    }
    public string BindableSelectedText
    {
        get
        {
            return (string)this.GetValue(BindableSelectedTextProperty);
        }

        private set
        {
            this.SetValue(BindableSelectedTextProperty, value);
        }
    }
    public string DelayedText
    {
        get
        {
            return (string)this.GetValue(DelayedTextProperty);
        }

        private set
        {
            this.SetValue(DelayedTextProperty, value);
        }
    }

    private static void OnBindableSelectionStartChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs args)
    {
        var textBox = dependencyObject as ModdedTextBox;

        if (!textBox.selectionStartChangeFromUI)
        {
            int newValue = (int)args.NewValue;
            textBox.SelectionStart = newValue;
        }
        else
        {
            textBox.selectionStartChangeFromUI = false;
        }
    }

    private static void OnBindableSelectionLengthChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs args)
    {
        var textBox = dependencyObject as ModdedTextBox;

        if (!textBox.selectionLengthChangeFromUI)
        {
            int newValue = (int)args.NewValue;
            textBox.SelectionLength = newValue;
        }
        else
        {
            textBox.selectionLengthChangeFromUI = false;
        }
    }
    private static void OnBindableSelectedTextChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs args)
    {
        var textBox = dependencyObject as ModdedTextBox;

        if (!textBox.selectedTextChangeFromUI)
        {
            string newValue = (string)args.NewValue;
            textBox.BindableSelectedText = newValue;
        }
        else
        {
            textBox.selectedTextChangeFromUI = false;
        }
    }
    private static void OnDelayedTextChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs args)
    {
    }
    private void OnSelectionChanged(object sender, RoutedEventArgs e)
    {
        if (this.BindableSelectionStart != this.SelectionStart)
        {
            this.selectionStartChangeFromUI = true;
            this.BindableSelectionStart = this.SelectionStart;
        }

        if (this.BindableSelectionLength != this.SelectionLength)
        {
            this.selectionLengthChangeFromUI = true;
            this.BindableSelectionLength = this.SelectionLength;
        }
        if (this.BindableSelectedText != this.SelectedText)
        {
            this.selectedTextChangeFromUI = true;
            this.BindableSelectedText = this.SelectedText;
        }
    }
    private void OnVerticalOffsetChanged(object sender, EventArgs e)
    {
        MessageBox.Show("hello the vertical offset works");
    }
}

Answer 1

your control needs a style to display itself.
comment out this one line from the constructor, to use default style

DefaultStyleKeyProperty.OverrideMetadata(typeof(ModdedTextBox), new FrameworkPropertyMetadata(typeof(ModdedTextBox)));

Done!

edit: Alternatively this will explicitly make your control use TextBox style

DefaultStyleKeyProperty.OverrideMetadata(
    typeof(ModdedTextBox),
    new FrameworkPropertyMetadata(typeof(TextBox)));