WPF TextBox. How to define some restriction?

Question

How can I restrict TextBox to accept only capital letters, or for example digits, or forbid to put any special character?

Sure it's a piece of cake to catch TextInput event and handle the text here, but is it the proper way to do this?

Answer 1

I've done this in the past with an attached behavior, which can be used like this:

<TextBox b:Masking.Mask="^\p{Lu}*$"/>

The attached behavior code looks like this:

/// <summary>
/// Provides masking behavior for any <see cref="TextBox"/>.
/// </summary>
public static class Masking
{
 private static readonly DependencyPropertyKey _maskExpressionPropertyKey = DependencyProperty.RegisterAttachedReadOnly("MaskExpression",
  typeof(Regex),
  typeof(Masking),
  new FrameworkPropertyMetadata());

 /// <summary>
 /// Identifies the <see cref="Mask"/> dependency property.
 /// </summary>
 public static readonly DependencyProperty MaskProperty = DependencyProperty.RegisterAttached("Mask",
  typeof(string),
  typeof(Masking),
  new FrameworkPropertyMetadata(OnMaskChanged));

 /// <summary>
 /// Identifies the <see cref="MaskExpression"/> dependency property.
 /// </summary>
 public static readonly DependencyProperty MaskExpressionProperty = _maskExpressionPropertyKey.DependencyProperty;

 /// <summary>
 /// Gets the mask for a given <see cref="TextBox"/>.
 /// </summary>
 /// <param name="textBox">
 /// The <see cref="TextBox"/> whose mask is to be retrieved.
 /// </param>
 /// <returns>
 /// The mask, or <see langword="null"/> if no mask has been set.
 /// </returns>
 public static string GetMask(TextBox textBox)
 {
  if (textBox == null)
  {
   throw new ArgumentNullException("textBox");
  }

  return textBox.GetValue(MaskProperty) as string;
 }

 /// <summary>
 /// Sets the mask for a given <see cref="TextBox"/>.
 /// </summary>
 /// <param name="textBox">
 /// The <see cref="TextBox"/> whose mask is to be set.
 /// </param>
 /// <param name="mask">
 /// The mask to set, or <see langword="null"/> to remove any existing mask from <paramref name="textBox"/>.
 /// </param>
 public static void SetMask(TextBox textBox, string mask)
 {
  if (textBox == null)
  {
   throw new ArgumentNullException("textBox");
  }

  textBox.SetValue(MaskProperty, mask);
 }

 /// <summary>
 /// Gets the mask expression for the <see cref="TextBox"/>.
 /// </summary>
 /// <remarks>
 /// This method can be used to retrieve the actual <see cref="Regex"/> instance created as a result of setting the mask on a <see cref="TextBox"/>.
 /// </remarks>
 /// <param name="textBox">
 /// The <see cref="TextBox"/> whose mask expression is to be retrieved.
 /// </param>
 /// <returns>
 /// The mask expression as an instance of <see cref="Regex"/>, or <see langword="null"/> if no mask has been applied to <paramref name="textBox"/>.
 /// </returns>
 public static Regex GetMaskExpression(TextBox textBox)
 {
  if (textBox == null)
  {
   throw new ArgumentNullException("textBox");
  } 

  return textBox.GetValue(MaskExpressionProperty) as Regex;
 }

 private static void SetMaskExpression(TextBox textBox, Regex regex)
 {
  textBox.SetValue(_maskExpressionPropertyKey, regex);
 }

 private static void OnMaskChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs e)
 {
  var textBox = dependencyObject as TextBox;
  var mask = e.NewValue as string;
  textBox.PreviewTextInput -= textBox_PreviewTextInput;
  textBox.PreviewKeyDown -= textBox_PreviewKeyDown;
  DataObject.RemovePastingHandler(textBox, Pasting);

  if (mask == null)
  {
   textBox.ClearValue(MaskProperty);
   textBox.ClearValue(MaskExpressionProperty);
  }
  else
  {
   textBox.SetValue(MaskProperty, mask);
   SetMaskExpression(textBox, new Regex(mask, RegexOptions.Compiled | RegexOptions.IgnorePatternWhitespace));
   textBox.PreviewTextInput += textBox_PreviewTextInput;
   textBox.PreviewKeyDown += textBox_PreviewKeyDown;
   DataObject.AddPastingHandler(textBox, Pasting);
  }
 }

 private static void textBox_PreviewTextInput(object sender, TextCompositionEventArgs e)
 {
  var textBox = sender as TextBox;
  var maskExpression = GetMaskExpression(textBox);

  if (maskExpression == null)
  {
   return;
  }

  var proposedText = GetProposedText(textBox, e.Text);

  if (!maskExpression.IsMatch(proposedText))
  {
   e.Handled = true;
  }
 }

 private static void textBox_PreviewKeyDown(object sender, KeyEventArgs e)
 {
  var textBox = sender as TextBox;
  var maskExpression = GetMaskExpression(textBox);

  if (maskExpression == null)
  {
   return;
  }

  //pressing space doesn't raise PreviewTextInput - no idea why, but we need to handle
  //explicitly here
  if (e.Key == Key.Space)
  {
   var proposedText = GetProposedText(textBox, " ");

   if (!maskExpression.IsMatch(proposedText))
   {
    e.Handled = true;
   }
  }
 }

 private static void Pasting(object sender, DataObjectPastingEventArgs e)
 {
  var textBox = sender as TextBox;
  var maskExpression = GetMaskExpression(textBox);

  if (maskExpression == null)
  {
   return;
  }

  if (e.DataObject.GetDataPresent(typeof(string)))
  {
   var pastedText = e.DataObject.GetData(typeof(string)) as string;
   var proposedText = GetProposedText(textBox, pastedText);

   if (!maskExpression.IsMatch(proposedText))
   {
    e.CancelCommand();
   }
  }
  else
  {
   e.CancelCommand();
  }
 }

 private static string GetProposedText(TextBox textBox, string newText)
 {
  var text = textBox.Text;

  if (textBox.SelectionStart != -1)
  {
   text = text.Remove(textBox.SelectionStart, textBox.SelectionLength);
  }

  text = text.Insert(textBox.CaretIndex, newText);

  return text;
 }
}