Incrementa el string tcCodigo Ej. string lastCodigo = IncrementXLColumn("AA"); -> Devolvera lastCodigo = "AB";

increaseString.cs

/// <summary>
/// Incrementa el string tcCodigo
/// Ej. string lastCodigo = IncrementXLColumn("AA"); -> Devolvera lastCodigo = "AB";
/// </summary>
/// <param name="tcCodigo"></param>
/// <returns></returns>
protected static string IncrementXLColumn(string tcCodigo)
{
    //var parts = System.Text.RegularExpressions.Regex.Matches(Address, @"([A-Z]+)|(\d+)");
    var lParts = System.Text.RegularExpressions.Regex.Matches(tcCodigo, @"([A-Z])|([A-Z])");
    if (lParts.Count != 2) return null;
    if (lParts[1].Value == "Z") return incCol(lParts[0].Value) + "A";
    else return lParts[0].Value + incCol(lParts[1].Value);
}
private static string incCol(string tcCol)
{
    if (tcCol == "") return "A";
    string fPart = tcCol.Substring(0, tcCol.Length - 1);
    char lChar = tcCol[tcCol.Length - 1];
    if (lChar == 'Z') return incCol(fPart) + "A";
    return fPart + ++lChar;
}