char c1 = 'A';
c1++; // c1 is 'B' now

If Reached_Z Then Add_Another_A

public static IEnumerable<string> GetColumns()
{
   string s = null;
   for (char c2 = 'A'; c2 <= 'Z' + 1; c2++)
   {
      for (char c = 'A'; c <= 'Z'; c++)
      {
         yield return s + c;
      }
      s = c2.ToString ();
   }
}

string currentCell = "A1";
int currentRow = int.Parse(Regex.Match(currentCell, @"d+").Value);
string currentCol = Regex.Match(currentCell, @"[A-Z]+").Value;
foreach (string column in GetColumns().Where (c => c >= currentCol && currentCol <= "AA"))
{
   Console.WriteLine (column + currentRow);
}

int columnsToAdd = 26;
currentCell = "C5";
currentRow = int.Parse(Regex.Match(currentCell, @"d+").Value);
currentCol = Regex.Match(currentCell, @"[A-Z]+").Value;
foreach (string column in GetColumns().Where (c => c >= currentCol))
{
   if (columnsToAdd--) == 0)
      break;
   Console.WriteLine (column + currentRow);
}

public static string IncrementXLColumn(string Address)
    {
        var parts = System.Text.RegularExpressions.Regex.Matches(Address,  @"([A-Z]+)|(d+)");
        if (parts.Count != 2) return null;
        return incCol(parts[0].Value) + parts[1].Value;
    }

    private static string incCol(string col)
    {
        if (col == "") return "A";
        string fPart = col.Substring(0, col.Length - 1);
        char lChar = col[col.Length - 1];
        if (lChar == 'Z') return incCol(fPart) + "A";
        return fPart + ++lChar;
    }