Question

Is there an easy way to have a text box display a format in a text box and remove it when editing ?

Example : I want to ask for a money value. When the user finished entering the value 5.5, it displays $ 5.00 . When the user reclick on the textbox, it should switch back to 5.5 .

Thanks you.

Edit : What I am trying to do would be the equivalent of a converter except the textbox does not have a binding set to it.

Answer 1

There’s a couple of solutions for this. Perhaps the easiest would be to implement a custom Behavior (you’ll need to reference System.Windows.Interactivity.dll) & hook onto some events TextBox exposes, like GotFocus and LostFocus. Here’s a sample I’ve done for selecting all of a TextBox’s text when it gets focused:

using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
using System.Windows.Interactivity;

namespace TextBoxUtility
{
    public class TextBoxSelectAllOnFocusBehavior : Behavior<TextBox>
    {
        protected override void OnAttached()
        {
            base.OnAttached();

            this.AssociatedObject.PreviewMouseLeftButtonDown += OnPreviewMouseLeftButtonDown;
            this.AssociatedObject.GotKeyboardFocus += OnSelectAll;
            this.AssociatedObject.MouseDoubleClick += OnSelectAll;
        }

        protected override void OnDetaching()
        {
            this.AssociatedObject.PreviewMouseLeftButtonDown -= OnPreviewMouseLeftButtonDown;
            this.AssociatedObject.GotKeyboardFocus -= OnSelectAll;
            this.AssociatedObject.MouseDoubleClick -= OnSelectAll;
            base.OnDetaching();
        }

        private static void OnPreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            var textbox = sender as TextBox;

            if (textbox != null && !textbox.IsKeyboardFocusWithin)
            {
                textbox.Focus();
                e.Handled = true;
            }
        }

        private static void OnSelectAll(object sender, RoutedEventArgs e)
        {
            var textbox = sender as TextBox;
            if (textbox != null)
            {
                textbox.SelectAll();
            }
        }

Edit: Forgot to show how to hook the behavior up to the TextBox.
To attach the Behavior, in your XAML you’ll need to add a namespace reference to your Behavior’s namespace, and then attach it as a child of the TextBox markup like so:

<TextBox Text="{Binding ....}">
    <i:Interaction.Behaviors>
        <utility:TextBoxSelectAllOnFocusBehavior />
    </i:Interaction.Behaviors>
</TextBox>

You could do something similar, though with yours you’d want to hook up to the LostFocus-related events and adjust the contents of the TextBox’s Text property.