Excel get column index from column name

colIndexToAlpha(27) -> return AA

alphaToColIndex(AA) -> return 27

// get cellReference’s column from column index
public string colIndexToAlpha(int colIndex) {
  List<string> colNameList = new List<string>();
  int num1 = Convert.ToInt32(colIndex) – 1;
  int num2 = 0;
  int num3 = 0;
  while (true) {
    num2 = num1 / 26;
    num3 = num1 – 26 * num2;
    colNameList.Insert(0, numToAlpha(num3));
    if (num2 < 26) {
      colNameList.Insert(0, numToAlpha(num2 – 1));
      break;
    }
    num1 = num2;
  }
  return String.Join(“”, colNameList.ToArray());
}

private String numToAlpha(int number) {
  if (number < 0) {
    return “”;
  }
  Char c = (Char)(65 + number);
  return c.ToString();
}

// get column index from cellReference’s column
public int alphaToColIndex(string name) {
  int index = 0;
  for (int i = 0; i < name.Length; i++) {
    index *= 26;
    index += alphaToNum(name[i]);
  }
  return index;
}

private int alphaToNum(char alpha) {
  return alpha – 64;// A -> 1 Z -> 26
}

private int alphaToNum(string alpha) {
  if (alpha.Length != 1) {
    return 0;
  }
  return alpha[0] – 64; // A -> 1 Z -> 26
}