Question

When I am building my typing tutor application the errors user made I want to show it in RED into his textbox. I created a LIST to have the index values stored but cannot figure it out how to retieve it and make them display as RED COLOR in UserTexbox. Anyways here is my code:

void ShowErrors()
{
    try
    {
        List<int> lst = new List<int>();
        string sample, user;
        sample = TBox_Sample.Text; //Sample Text Given to the user
        user = TBox_User.Text;     //User input string
        for (int i = 0; i < sample.Length; i++)
        {
            if (sample[i] != user[i])
            {
                lst.Add(i);   //Made this list which contains indexes of errors positioned.

            }

        }

        string user_new = TBox_User.Text.ToString();

        for (int j = 0; j <= lst.Count; j++)
        {
            ??? I WANT TO SHOW IN 'TBox_User' ALL ERRORS MARKED WITH RED WITH THE HELP OF MY LIST OBJECT: LST !!! 
        }
    }
    catch (IndexOutOfRangeException)
    {
        MessageBox.Show("There is no input from the user!");
        //int ijj = 0;
    }
    catch (Exception)
    {
        MessageBox.Show("Unknown Error!");
    }

Answer 1

You should be using a RichTextBox instead of a TextBox, that way you can either change the SelectionStart, SelectionLength, and SelectionColor, or modify the Rtf directly. Otherwise with a normal textbox you would be looking at overriding WndProc and doing some custom painting in the control which is quite a bit of work. Here is a quick example of using a RichTextBox to do this (all you would have to do is update your TextBox to RichTextBox and change your for-loop to the following:

foreach (int index in lst) {
    richTextBox.SelectionStart = index;
    richTextBox.SelectionLength = 1;
    richTextBox.SelectionColor = Color.Red;
}

Let me know how it works for you.