111

int lastLine = 0;
private void HighlightCurrentLine()
{
  // Save current selection
  int selectionStart = rtb.SelectionStart;
  int selectionLength = rtb.SelectionLength;

  // Get character positions for the current line
  int firstCharPosition = rtb.GetFirstCharIndexOfCurrentLine();
  int lineNumber = rtb.GetLineFromCharIndex(firstCharPosition);
  int lastCharPosition = rtb.GetFirstCharIndexFromLine(lineNumber + 1);
  if (lastCharPosition == -1)
    lastCharPosition = rtb.TextLength;

  // Clear any previous color
  if (lineNumber != lastLine)
  {
    int previousFirstCharPosition = rtb.GetFirstCharIndexFromLine(lastLine);
    int previousLastCharPosition = rtb.GetFirstCharIndexFromLine(lastLine + 1);
    if (previousLastCharPosition == -1)
      previousLastCharPosition = rtb.TextLength;

    rtb.SelectionStart = previousFirstCharPosition;
    rtb.SelectionLength = previousLastCharPosition - previousFirstCharPosition;
    rtb.SelectionBackColor = SystemColors.Window;
    lastLine = lineNumber;
  }

  // Set new color
  rtb.SelectionStart = firstCharPosition;
  rtb.SelectionLength = lastCharPosition - firstCharPosition;
  if (rtb.SelectionLength > 0)
    rtb.SelectionBackColor = Color.PaleTurquoise;

  // Reset selection
  rtb.SelectionStart = selectionStart;
  rtb.SelectionLength = selectionLength;
}