Retrieving an accurate string array from a RichTextBox control

You have to do a few tricks to achieve this which have to do with querying for the position of the actual lines according to the character index. The following program shows one way of doing this. You might have to harden it a bit, but it should get you started:

public partial class Form1 : Form
{        
    public Form1()
    {
        InitializeComponent();                        
    }        

    private void richTextBox1_TextChanged(object sender, EventArgs e)
    {
        if (richTextBox1.Text == "")
            return; 

        // Lines corresponding to the first and last characters:
        int firstLine = richTextBox1.GetLineFromCharIndex(0); 
        int lastLine = richTextBox1.GetLineFromCharIndex(richTextBox1.Text.Length);

        // Get array of lines:
        List<string> lines = new List<string>();
        for (int i = firstLine; i <= lastLine; i++)
        {
            int firstIndexFromLine = richTextBox1.GetFirstCharIndexFromLine(i);
            int firstIndexFromNextLine = richTextBox1.GetFirstCharIndexFromLine(i + 1);

            if (firstIndexFromNextLine == -1)
            {
                // Get character index of last character in this line:
                Point pt = new Point(richTextBox1.ClientRectangle.Width, richTextBox1.GetPositionFromCharIndex(firstIndexFromLine).Y);
                firstIndexFromNextLine = richTextBox1.GetCharIndexFromPosition(pt);
                firstIndexFromNextLine += 1;
            }

            lines.Add(richTextBox1.Text.Substring(firstIndexFromLine, firstIndexFromNextLine - firstIndexFromLine));
        }

        // Print to richTextBox2 while debugging:
        richTextBox2.Text = "";
        foreach (string line in lines)
        {
            richTextBox2.AppendText(">> " + line + Environment.NewLine);
        }                        
    }
}