modify Text in TextBox before Text is displayed

Question

I want to modify the Text, that is typed in the TextBox, before it is displayed, without looking on the Text, that is already typed.

Example: xaml:

<TextBox x:Name="tb" TextChanged="tb_TextChanged">
    my long text
</TextBox>

c#:

private void tb_TextChanged(object sender, TextChangedEventArgs e)
{
    TextBox tb = sender as TextBox;

    if (tb != null)
    {
        //save caretIndex
        int caretIndex = tb.CaretIndex;

        //modify Text //can be any modification
        //vvvvvvvvvvv Lines that I'm talking about

        tb.Text.ToLower(); //or: tb.Text.ToLower(); etc.

        char[] notAllowedChars = new char[] {'/t', '~', '#'}; //any other
        foreach (char c in notAllowedChars)
        {
            tb.Text = tb.Text.Replace(c, '_'); //replace unwanted characters
        }

        //^^^^^^^^^^^^ these lines modify the whole text

        //restore caretIndex
        tb.CaretIndex = caretIndex;
    }
}

In that example, the whole Text is going to be modified. But all of the other characters are already modified. So I don't want to go through them again. I want only to modify the changed text before it gets inserted.

I'm talking of 100.000+ Characters. That means, that the looking up all characters causes an unwanted performance issue.

Is there any solution or is it an impossible demand.

Answer 1

You can look at the TextChangedEventArgs to see the changes, and then modify the text again...here is some starting code:

private bool m_bInTextChanged;
private void tb_TextChanged(object sender, TextChangedEventArgs e)
{
    if (m_bInTextChanged)
        return;

    m_bInTextChanged = true;

    TextBox tb = sender as TextBox;

    if (tb != null)
    {
        //save caretIndex
        int caretIndex = tb.CaretIndex;

        if (e.Changes.Any())
        {
            var addedchanges = e.Changes.Where(tc => tc.AddedLength > 0);

            foreach (var added in addedchanges)
            {
                string stringchanged = tb.Text.Substring(added.Offset, added.AddedLength);

                tb.Select(added.Offset, added.AddedLength);

                tb.SelectedText = stringchanged.ToLower();
            }
        }

        //restore caretIndex
        tb.CaretIndex = caretIndex;
    }

    m_bInTextChanged = false;
}