Column Numbers to Letters

[Costas]

' src - https://bettersolutions.com/excel/rows-columns/vba-column-numbers-letters.htm

Public Function Col_Letter(ByVal iColNo As Integer) As String 
Dim sstartletter As String 
   On Error GoTo AnError 
   Col_Letter = Left(Cells(1, iColNo).Address(False, False), _ 
                  Len(Cells(1, iColNo).Address(False, False)) - 1) 
   Exit Function 
AnError: 
   Call MsgBox(Err.Number & " - " & Err.Description) 
End Function

---

// src - https://stackoverflow.com/a/2652855

public static string ColumnAdress(int col)
{
  if (col <= 26) { 
    return Convert.ToChar(col + 64).ToString();
  }
  int div = col / 26;
  int mod = col % 26;
  if (mod == 0) {mod = 26;div--;}
  return ColumnAdress(div) + ColumnAdress(mod);
}

public static int ColumnNumber(string colAdress)
{
  int[] digits = new int[colAdress.Length];
  for (int i = 0; i < colAdress.Length; ++i)
  {
    digits[i] = Convert.ToInt32(colAdress[i]) - 64;
  }
  int mul=1;int res=0;
  for (int pos = digits.Length - 1; pos >= 0; --pos)
  {
    res += digits[pos] * mul;
    mul *= 26;
  }
  return res;
}

ref - How to Convert Column Number (e.g. 28) to Column Letter (e.g. AB) in Google Sheets