How do I create an Autoscrolling TextBox

Question

I have a WPF application that contains a multiline TextBox that is being used to display debugging text output.

How can I set the TextBox so that as text is appended to

Answer 1

This solution is inspired by Scott Ferguson's solution with the attached property, but avoids storing an internal dictionary of associations and thereby has somewhat shorter code:

    using System;
    using System.Windows;
    using System.Windows.Controls;

    namespace AttachedPropertyTest
    {
        public static class TextBoxUtilities
        {
            public static readonly DependencyProperty AlwaysScrollToEndProperty = DependencyProperty.RegisterAttached("AlwaysScrollToEnd",
                                                                                                                      typeof(bool),
                                                                                                                      typeof(TextBoxUtilities),
                                                                                                                      new PropertyMetadata(false, AlwaysScrollToEndChanged));

            private static void AlwaysScrollToEndChanged(object sender, DependencyPropertyChangedEventArgs e)
            {
                TextBox tb = sender as TextBox;
                if (tb != null) {
                    bool alwaysScrollToEnd = (e.NewValue != null) && (bool)e.NewValue;
                    if (alwaysScrollToEnd) {
                        tb.ScrollToEnd();
                        tb.TextChanged += TextChanged;
                    } else {
                        tb.TextChanged -= TextChanged;
                    }
                } else {
                    throw new InvalidOperationException("The attached AlwaysScrollToEnd property can only be applied to TextBox instances.");
                }
            }

            public static bool GetAlwaysScrollToEnd(TextBox textBox)
            {
                if (textBox == null) {
                    throw new ArgumentNullException("textBox");
                }

                return (bool)textBox.GetValue(AlwaysScrollToEndProperty);
            }

            public static void SetAlwaysScrollToEnd(TextBox textBox, bool alwaysScrollToEnd)
            {
                if (textBox == null) {
                    throw new ArgumentNullException("textBox");
                }

                textBox.SetValue(AlwaysScrollToEndProperty, alwaysScrollToEnd);
            }

            private static void TextChanged(object sender, TextChangedEventArgs e)
            {
                ((TextBox)sender).ScrollToEnd();
            }
        }
    }

As far as I can tell, it behaves exactly as desired. Here's a test case with several text boxes in a window that allows the attached AlwaysScrollToEnd property to be set in various ways (hard-coded, with a CheckBox.IsChecked binding and in code-behind):

Xaml:

Code-Behind:

    using System;
    using System.Windows;
    using System.Windows.Controls;

    namespace AttachedPropertyTest
    {
        public partial class Window1 : Window
        {
            public Window1()
            {
                InitializeComponent();
            }

            void Button_Click(object sender, RoutedEventArgs e)
            {
                TextBoxUtilities.SetAlwaysScrollToEnd(tb3, true);
            }
        }
    }