Add a preview text input event. Like so: <TextBox PreviewTextInput="PreviewTextInput" />.
Then inside that set the e.Handled if the text isn't allowed. e.Handled = !IsTextAllowed(e.Text);
I use a simple regex in IsTextAllowed method to see if I should allow what they've typed. In my case I only want to allow numbers, dots and dashes.
private static readonly Regex _regex = new Regex("[^0-9.-]+"); //regex that matches disallowed text
private static bool IsTextAllowed(string text)
{
return !_regex.IsMatch(text);
}
If you want to prevent pasting of incorrect data hook up the DataObject.Pasting event DataObject.Pasting="TextBoxPasting" as shown here (code excerpted):
// Use the DataObject.Pasting Handler
private void TextBoxPasting(object sender, DataObjectPastingEventArgs e)
{
if (e.DataObject.GetDataPresent(typeof(String)))
{
String text = (String)e.DataObject.GetData(typeof(String));
if (!IsTextAllowed(text))
{
e.CancelCommand();
}
}
else
{
e.CancelCommand();
}
}