ExcelORM/ExcelORM/ExcelORM/ExcelDynamicWriter.cs

using ClosedXML.Excel;
using ExcelORM.Models;

namespace ExcelORM;

public class ExcelDynamicWriter
{
    private readonly IXLWorkbook xlWorkbook;
    public ExcelDynamicWriter(string? path = null)
    {
        xlWorkbook = File.Exists(path) ? new XLWorkbook(path) : new XLWorkbook();
    }

    private static int GenerateHeader(IXLWorksheet worksheet, IEnumerable<DynamicCell> firstRow)
    {
        var rowIndex = 1;
        foreach (var item in firstRow)
            worksheet.Cell(rowIndex, item.Position).Value = item.Header;

        return ++rowIndex;
    }

    public void Write(IEnumerable<List<DynamicCell>>? values, string? worksheetName = null, bool append = false)
    {
        if (values == null) return;

        var xlWorksheet = xlWorkbook.Worksheets.FirstOrDefault(x => x.Name.Equals(worksheetName, StringComparison.InvariantCultureIgnoreCase));
        
        xlWorksheet ??= !string.IsNullOrWhiteSpace(worksheetName) ?
            xlWorkbook.AddWorksheet(worksheetName)
            : xlWorkbook.Worksheets.Count == 0 ? xlWorkbook.AddWorksheet() : xlWorkbook.Worksheets.First();

        Write(values, xlWorksheet, append);
    }

    private static void Write(IEnumerable<List<DynamicCell>> values, IXLWorksheet worksheet, bool append)
    {
        var rowIndex = append switch
        {
            true => worksheet.LastRowUsed().RowNumber() + 1,
            false => GenerateHeader(worksheet, values.First()),
        };

        foreach (var row in values)
        {
            foreach (var cell in row)
            {
                if (cell.Value == null) continue;
                
                worksheet.Cell(rowIndex, cell.Position).Value = XLCellValue.FromObject(cell.Value);
            }

            rowIndex++;
        }
    }

    public void SaveAs(string path, IExcelConverter? converter = null)
    {
        xlWorkbook.SaveAs(path);
        converter?.MakeCompatible(path);
    } 
}