限制多行文本框中的行长

Question

I need to limit the number of characters on a single line that a user can enter into a multiline textbox. I have a function that can do that for data typed in, but not for data cut and pasted in.

I've tried reading the textbox into an array, using substring, and copying back to the text string, but this code (posted) throws an exception.

private void LongLine_TextChanged(object sender, TextChangedEventArgs e)
    {
        int lineCount = 
 ((System.Windows.Controls.TextBox)sender).LineCount;
        //string newText = "";
        for (int i = 0; i < lineCount; i++)
        {
            if 
    (((System.Windows.Controls.TextBox)sender).GetLineLength(i) > 20)
            {
                string textString = ((System.Windows.Controls.TextBox)sender).Text;
                string[] textArray = Regex.Split(textString, "\r\n");
                textString = "";
                for (int k =0; k < textArray.Length; k++)
                {
                    String textSubstring = textArray[k].Substring(0, 20);
                    textString += textSubstring;
                }
                ((System.Windows.Controls.TextBox)sender).Text = textString;
            }
        }
        e.Handled = true;
    }

Answer 1

This does what you seem to be asking for:

It truncates the end of any line that is longer than 20 as you type and when you paste or otherwise change the text.

private void TextBox_TextChanged(object sender, TextChangedEventArgs e)
{
    const int MAX_LINE_LENGTH = 20;
    var textbox = sender as TextBox;
    var exceedsLength = false;
    // First test if we need to modify the text
    for (int i = 0; i < textbox.LineCount; i++)
    {
        if (textbox.GetLineLength(i) > MAX_LINE_LENGTH)
        {
            exceedsLength = true;
            break;
        }
    }
    if (exceedsLength)
    {
        // Split the text into lines
        string[] oldTextArray = textbox.Text.Split(new[] { Environment.NewLine }, StringSplitOptions.None);
        var newTextLines = new List<string>(textbox.LineCount);
        for (int k = 0; k < oldTextArray.Length; k++)
        {
            // truncate each line
            newTextLines.Add(string.Concat(oldTextArray[k].Take(MAX_LINE_LENGTH)));
        }
        // Save the cursor position
        var cursorPos = textbox.SelectionStart;
        // To avoid the text change calling back into this event, detach the event while setting the Text property
        textbox.TextChanged -= TextBox_TextChanged;
        // Set the new text
        textbox.Text = string.Join(Environment.NewLine, newTextLines);
        textbox.TextChanged += TextBox_TextChanged;
        // Restore the cursor position
        textbox.SelectionStart = cursorPos;
        // if at the end of the line, the position will advance automatically to the next line, supress that
        if (textbox.SelectionStart != cursorPos)
        {
            textbox.SelectionStart = cursorPos - 1;
        }
    }
    e.Handled = true;
}