Can I save an EXCEL worksheet as CSV via ClosedXML?

Other answers will fail to generate a valid CSV if cells have the separator so here is a better way

var lastCellAddress = worksheet.RangeUsed().LastCell().Address;
File.WriteAllLines(csvFileName, worksheet.Rows(1, lastCellAddress.RowNumber)
    .Select(r => string.Join(",", r.Cells(1, lastCellAddress.ColumnNumber)
            .Select(cell =>
        {
            var cellValue = cell.GetValue<string>();
            return cellValue.Contains(",") ? $"\"{cellValue}\"" : cellValue;
        }))));

This is based on @Extragorey's answer

No, it is not possible directly in ClosedXML. You have to use loops or LINQ to build your own CSV file.

For example:

System.IO.File.WriteAllLines(csvFileName,
    worksheet.RowsUsed().Select(row =>
        string.Join(";", row.Cells(1, row.LastCellUsed(false).Address.ColumnNumber)
                            .Select(cell => cell.GetValue<string>()))
 ));

As @Emanuele points out, @Raidri's answer won't generate a proper CSV format, and it also omits blank rows entirely. To fix this:

var lastCellAddress = worksheet.RangeUsed().LastCell().Address;
System.IO.File.WriteAllLines(csvFileName, worksheet.Rows(1, lastCellAddress.RowNumber)
    .Select(row => String.Join(",", row.Cells(1, lastCellAddress.ColumnNumber)
        .Select(cell => cell.GetValue<string>()))
));