ciao, ho proceduto cosi:
private void WriteToLine(int lineNumber, string text)
{
int pos = richTextBox1.GetFirstCharIndexFromLine(lineNumber);
richTextBox1.Select(pos, richTextBox1.Lines[lineNumber].Length);
richTextBox1.SelectedText = text;
}
private void button1_Click(object sender, EventArgs e)
{
WriteToLine(2, "Marco");
}