Question

When using textbox.Undo(); I get the following error:

Cannot Undo or Redo while undo unit is open.

Now I understand why this is the case (because it is an active undoable event) but through what event can I perform validation on a text box and undo the change if the user has typed an invalid character?

Answer 1

Instead of using Undo and TextChanged, you should use the PreviewTextInput and DataObject.Pasting events. In the PreviewTextInput event handler, set e.Handled to true if the typed text is an invalid character. In the Pasting event handler, call e.CancelCommand() if the pasted text is invalid.

Here is an example for a text box that accepts only the digits 0 and 1:

XAML:

<Window x:Class="BinaryTextBox.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="133" Width="329">
    <StackPanel>
        <TextBox x:Name="txtBinary" Width="100" Height="24"
                 PreviewTextInput="txtBinary_PreviewTextInput"
                 DataObject.Pasting="txtBinary_Pasting"/>
    </StackPanel>
</Window>

Code behind:

using System.Text.RegularExpressions;
using System.Windows;
using System.Windows.Input;

namespace BinaryTextBox
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void txtBinary_PreviewTextInput(object sender,
                 TextCompositionEventArgs e)
        {
            e.Handled = e.Text != "0" && e.Text != "1";
        }

        private void txtBinary_Pasting(object sender, DataObjectPastingEventArgs e)
        {
            if (!Regex.IsMatch(e.DataObject.GetData(typeof(string)).ToString(), "^[01]+$"))
            {
                e.CancelCommand();
            }
        }
    }
}