Watermark / hint text / placeholder TextBox

Question

How can I put some text into a TextBox which is removed automatically when user types something in it?

Answer 1

I saw John Myczek's solution, and its comments about Compatibility to ComboBox and PasswordBox, so I improved John Myczek's solution, and here it is:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;
using System.Windows.Documents;

/// 

/// Class that provides the Watermark attached property
/// 
public static class WatermarkService
{
    /// 

    /// Watermark Attached Dependency Property
    /// 
    public static readonly DependencyProperty WatermarkProperty = DependencyProperty.RegisterAttached(
       "Watermark",
       typeof(object),
       typeof(WatermarkService),
       new FrameworkPropertyMetadata((object)null, new PropertyChangedCallback(OnWatermarkChanged)));

    #region Private Fields

    /// 

    /// Dictionary of ItemsControls
    /// 
    private static readonly Dictionary itemsControls = new Dictionary();

    #endregion

    /// 

    /// Gets the Watermark property.  This dependency property indicates the watermark for the control.
    /// 
    ///  to get the property from
    /// The value of the Watermark property
    public static object GetWatermark(DependencyObject d)
    {
        return (object)d.GetValue(WatermarkProperty);
    }

    /// 

    /// Sets the Watermark property.  This dependency property indicates the watermark for the control.
    /// 
    ///  to set the property on
    /// value of the property
    public static void SetWatermark(DependencyObject d, object value)
    {
        d.SetValue(WatermarkProperty, value);
    }

    /// 

    /// Handles changes to the Watermark property.
    /// 
    ///  that fired the event
    /// A  that contains the event data.
    private static void OnWatermarkChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        Control control = (Control)d;
        control.Loaded += Control_Loaded;

        if (d is TextBox || d is PasswordBox)
        {
            control.GotKeyboardFocus += Control_GotKeyboardFocus;
            control.LostKeyboardFocus += Control_Loaded;
        }
        else if (d is ComboBox)
        {
            control.GotKeyboardFocus += Control_GotKeyboardFocus;
            control.LostKeyboardFocus += Control_Loaded;
            (d as ComboBox).SelectionChanged += new SelectionChangedEventHandler(SelectionChanged);
        }
        else if (d is ItemsControl)
        {
            ItemsControl i = (ItemsControl)d;

            // for Items property  
            i.ItemContainerGenerator.ItemsChanged += ItemsChanged;
            itemsControls.Add(i.ItemContainerGenerator, i);

            // for ItemsSource property  
            DependencyPropertyDescriptor prop = DependencyPropertyDescriptor.FromProperty(ItemsControl.ItemsSourceProperty, i.GetType());
            prop.AddValueChanged(i, ItemsSourceChanged);
        }
    }

    /// 

    /// Event handler for the selection changed event
    /// 
    /// The source of the event.
    /// A  that contains the event data.
    private static void SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        Control control = (Control)sender;
        if (ShouldShowWatermark(control))
        {
            ShowWatermark(control);
        }
        else
        {
            RemoveWatermark(control);
        }
    }

    #region Event Handlers

    /// 

    /// Handle the GotFocus event on the control
    /// 
    /// The source of the event.
    /// A  that contains the event data.
    private static void Control_GotKeyboardFocus(object sender, RoutedEventArgs e)
    {
        Control c = (Control)sender;
        if (ShouldShowWatermark(c))
        {
            RemoveWatermark(c);
        }
    }

    /// 

    /// Handle the Loaded and LostFocus event on the control
    /// 
    /// The source of the event.
    /// A  that contains the event data.
    private static void Control_Loaded(object sender, RoutedEventArgs e)
    {
        Control control = (Control)sender;
        if (ShouldShowWatermark(control))
        {
            ShowWatermark(control);
        }
    }

    /// 

    /// Event handler for the items source changed event
    /// 
    /// The source of the event.
    /// A  that contains the event data.
    private static void ItemsSourceChanged(object sender, EventArgs e)
    {
        ItemsControl c = (ItemsControl)sender;
        if (c.ItemsSource != null)
        {
            if (ShouldShowWatermark(c))
            {
                ShowWatermark(c);
            }
            else
            {
                RemoveWatermark(c);
            }
        }
        else
        {
            ShowWatermark(c);
        }
    }

    /// 

    /// Event handler for the items changed event
    /// 
    /// The source of the event.
    /// A  that contains the event data.
    private static void ItemsChanged(object sender, ItemsChangedEventArgs e)
    {
        ItemsControl control;
        if (itemsControls.TryGetValue(sender, out control))
        {
            if (ShouldShowWatermark(control))
            {
                ShowWatermark(control);
            }
            else
            {
                RemoveWatermark(control);
            }
        }
    }

    #endregion

    #region Helper Methods

    /// 

    /// Remove the watermark from the specified element
    /// 
    /// Element to remove the watermark from
    private static void RemoveWatermark(UIElement control)
    {
        AdornerLayer layer = AdornerLayer.GetAdornerLayer(control);

        // layer could be null if control is no longer in the visual tree
        if (layer != null)
        {
            Adorner[] adorners = layer.GetAdorners(control);
            if (adorners == null)
            {
                return;
            }

            foreach (Adorner adorner in adorners)
            {
                if (adorner is WatermarkAdorner)
                {
                    adorner.Visibility = Visibility.Hidden;
                    layer.Remove(adorner);
                }
            }
        }
    }

    /// 

    /// Show the watermark on the specified control
    /// 
    /// Control to show the watermark on
    private static void ShowWatermark(Control control)
    {
        AdornerLayer layer = AdornerLayer.GetAdornerLayer(control);

        // layer could be null if control is no longer in the visual tree
        if (layer != null)
        {
            layer.Add(new WatermarkAdorner(control, GetWatermark(control)));
        }
    }

    /// 

    /// Indicates whether or not the watermark should be shown on the specified control
    /// 
    ///  to test
    /// true if the watermark should be shown; false otherwise
    private static bool ShouldShowWatermark(Control c)
    {
        if (c is ComboBox)
        {
            return (c as ComboBox).SelectedItem == null;
            //return (c as ComboBox).Text == string.Empty;
        }
        else if (c is TextBoxBase)
        {
            return (c as TextBox).Text == string.Empty;
        }
        else if (c is PasswordBox)
        {
            return (c as PasswordBox).Password == string.Empty;
        }
        else if (c is ItemsControl)
        {
            return (c as ItemsControl).Items.Count == 0;
        }
        else
        {
            return false;
        }
    }

    #endregion
}

Now, a ComboBox can be also Editable, and PasswordBox can add a watermark too. Don't forget to use JoanComasFdz's comment above to solve the margin problem.

And, of course, All the credit goes to John Myczek.