Silverlight Convert PlatformKeyCode to Character Code - silverlight

Question

Question:
Is there a good way in Silverlight to intercept undesirable characters from being entered into a textbox?
Background:
I have a textbox which allows a user to enter a filename. I would like to exclude invalid file characters from being entered into the textbox. A few of these characters are:
'?'
'\'
'<'
'>'
Although the Silverlight TextBox class does not support a KeyPress event, it does have a KeyDown and a KeyUp event that can be used to retrieve character information when a key is entered into the textbox. It exposes these as a member of the Key enumeration or it can return an int for the PlatformKeyCode.
Of course the range of keys is larger/different from the range of characters - "F keys" are an example of this. However the presence of something like a KeyPress event in Windows Forms is indicative of the usefulness of being able to extract specific character information.
To do a proof of concept that things could work I hardcoded the PlatformKeyCode values for the undesired characters for my platform into the event handler and everything worked... but of course this is just my platform. I need to make sure this implementation is platform agnostic. Here is the code to demonstrate how I would like it to work:
private void theText_KeyDown(object sender, KeyEventArgs e)
{
int[] illegals = { 191, 188, 190, 220, 186, 222, 191, 56, 186};
if (illegals.Any(i => i == e.PlatformKeyCode)) e.Handled = true;
}

Answer 1

Both responses (in comments) from Henrik and Johannes contain the best answer for the scenario. Rather than thinking like a Windows Forms programmer and capturing the specific key event, the canonical approach in Silverlight is to use the TextChanged event to remove undesirable characters from a TextBox. A similar question on allowing only numeric input to a TextBox is what prompted the solution that was used.
The following code samples the approach I took after reading the comments and it works quite well to remove characters inappropriate for entry:
private void fileNameTextBox_TextChanged(object sender, TextChangedEventArgs e)
{
string illegalChars = #"?<>:""\/*|";
fileNameTextBox.Text = String.Join("", fileNameTextBox.Text.Split(illegalChars.ToCharArray()));
fileNameTextBox.SelectionStart = fileNameTextBox.Text.Length;
}

Answer 2

using System;
using System.Linq;
using System.Text.RegularExpressions;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Interactivity;
public class FilterTextBoxBehavior : Behavior<TextBox>
{
public readonly static DependencyProperty AllowAlphaCharactersProperty = DependencyProperty.Register("AllowAlphaCharacters", typeof(bool), typeof(FilterTextBoxBehavior), new PropertyMetadata(true));
public bool AllowAlphaCharacters
{
get { return (bool)GetValue(AllowAlphaCharactersProperty); }
set { SetValue(AllowAlphaCharactersProperty, value); }
}
public readonly static DependencyProperty AllowNumericCharactersProperty = DependencyProperty.Register("AllowNumericCharacters", typeof(bool), typeof(FilterTextBoxBehavior), new PropertyMetadata(true));
public bool AllowNumericCharacters
{
get { return (bool)GetValue(AllowNumericCharactersProperty); }
set { SetValue(AllowNumericCharactersProperty, value); }
}
public readonly static DependencyProperty AllowSpecialCharactersProperty = DependencyProperty.Register("AllowSpecialCharacters", typeof(bool), typeof(FilterTextBoxBehavior), new PropertyMetadata(true));
public bool AllowSpecialCharacters
{
get { return (bool)GetValue(AllowSpecialCharactersProperty); }
set { SetValue(AllowSpecialCharactersProperty, value); }
}
public readonly static DependencyProperty DoNotFilterProperty = DependencyProperty.Register("DoNotFilter", typeof(string), typeof(FilterTextBoxBehavior), new PropertyMetadata(default(string)));
public string DoNotFilter
{
get { return (string)GetValue(DoNotFilterProperty); }
set { SetValue(DoNotFilterProperty, value); }
}
protected override void OnAttached()
{
base.OnAttached();
if (AssociatedObject == null) { return; }
FilterAssociatedObject();
AssociatedObject.TextChanged += OnTextChanged;
}
protected override void OnDetaching()
{
base.OnDetaching();
if (AssociatedObject == null) { return; }
FilterAssociatedObject();
AssociatedObject.TextChanged -= OnTextChanged;
}
private void OnTextChanged(object sender, TextChangedEventArgs e) { FilterAssociatedObject(); }
private void FilterAssociatedObject()
{
int cursorLocation = AssociatedObject.SelectionStart;
for (int i = AssociatedObject.Text.Length - 1; i >= 0; i--)
{
char c = AssociatedObject.Text[i];
if (ValidChar(c)) { continue; }
AssociatedObject.Text = AssociatedObject.Text.Remove(i, 1);
cursorLocation--;
}
AssociatedObject.SelectionStart = Math.Min(AssociatedObject.Text.Length, Math.Max(0, cursorLocation));
}
private bool ValidChar(char c)
{
if (!string.IsNullOrEmpty(DoNotFilter) && DoNotFilter.Contains(c)) { return true; }
if (!AllowAlphaCharacters && char.IsLetter(c)) { return false; }
if (!AllowNumericCharacters && char.IsNumber(c)) { return false; }
if (!AllowSpecialCharacters && Regex.IsMatch(c.ToString(), #"[\W|_]")) { return false; }
return true;
}
}