HLib/IO/Excel/ExcelColumnHelperUxer.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace KssSmaPlaLib.IO.Excel
{
    public static class ExcelColumnHelperUxer
    {
        public static string GetExcelColumnLetter(int columnNumber)
        {
            if (columnNumber < 1)
                throw new ArgumentOutOfRangeException(nameof(columnNumber), "Column number must be >= 1");

            string columnLetter = string.Empty;
            while (columnNumber > 0)
            {
                columnNumber--; // Adjust for 0-indexed calculation
                columnLetter = (char)('A' + (columnNumber % 26)) + columnLetter;
                columnNumber /= 26;
            }
            return columnLetter;
        }
    }
}