FileContentGenerator.cs

public static class FileContentResultGenerator
{
    public static byte[] ToCsv(this DataSet ds)
    {
        var sb = new StringBuilder();
        for (var i = 0; i < ds.Tables[0].Columns.Count; i++)
        {
            sb.Append(ds.Tables[0].Columns[i]);
            if (i < ds.Tables[0].Columns.Count - 1)
            {
                sb.Append(",");
            }
        }

        sb.AppendLine();
        foreach (DataRow dr in ds.Tables[0].Rows)
        {
            for (var i = 0; i < ds.Tables[0].Columns.Count; i++)
            {
                /* for debugging - turns out I wasn't crazy */
                //Console.WriteLine($@"{ds.Tables[0].Columns[i]} :: {dr[i]}");
                if (!Convert.IsDBNull(dr[i]))
                {
                    var value = dr[i].ToString().ToCsvCell();
                    sb.Append(value);
                }

                if (i < ds.Tables[0].Columns.Count - 1)
                {
                    sb.Append(",");
                }
            }

            sb.AppendLine();
        }

        return Encoding.UTF8.GetBytes(sb.ToString());
    }

    public static byte[] ToTsv(this DataSet ds)
    {
        var sb = new StringBuilder();
        for (var i = 0; i < ds.Tables[0].Columns.Count; i++)
        {
            sb.Append(ds.Tables[0].Columns[i]);
            if (i < ds.Tables[0].Columns.Count - 1)
            {
                sb.Append("\t");
            }
        }

        sb.AppendLine();
        foreach (DataRow dr in ds.Tables[0].Rows)
        {
            for (var i = 0; i < ds.Tables[0].Columns.Count; i++)
            {
                if (!Convert.IsDBNull(dr[i]))
                {
                    var value = dr[i].ToString().Contains("\t")
                        ? $"{dr[i].ToString().Replace("\t", " ")}"
                        : dr[i];
                    sb.Append(value);
                }

                if (i < ds.Tables[0].Columns.Count - 1)
                {
                    sb.Append("\t");
                }
            }

            sb.AppendLine();
        }

        return Encoding.UTF8.GetBytes(sb.ToString());
    }

    public static byte[] ToPdf(this DataSet ds, string identifier = "Report")
    {
        var dataTable = ds.Tables[0];

        var pdfDoc = new Document(PageSize.A4, 25, 25, 25, 50);
        if (dataTable.Columns.Count > 4) //<-- change to landscape if big enough
        {
            pdfDoc = new Document(PageSize.A4.Rotate(), 25, 25, 25, 50);
        }

        var mStream = new MemoryStream();
        int pageModOnData = 100;
        var writer = PdfWriter.GetInstance(pdfDoc, mStream);
        var cols = dataTable.Columns.Count;
        var rows = dataTable.Rows.Count;
        var pageEventHandler = new TwoColumnHeaderFooter();
        writer.PageEvent = pageEventHandler;
        pdfDoc.Open();
        var para = new Paragraph($"{identifier} as of: " + DateTime.UtcNow.ToString("MM/dd/yyyy"),
            new Font(Font.HELVETICA, 12, Font.BOLD))
        {
            Alignment = Element.ALIGN_CENTER
        };
        pdfDoc.Add(para);
        var pdfTable = new Table(cols, rows)
        {
            BorderWidth = 1,
            Width = 100,
            Padding = 1,
            Spacing = 1
        };
        //creating table headers
        for (int i = 0; i < cols; i++)
        {
            Cell cellCols = new Cell();
            Font colFont = FontFactory.GetFont(FontFactory.HELVETICA, 8, Font.BOLD);
            Chunk chunkCols = new Chunk(dataTable.Columns[i].ColumnName, colFont);
            cellCols.Add(chunkCols);
            cellCols.Width = 250;
            pdfTable.AddCell(cellCols);
        }

        Int64 rowCount = 0;

        for (int k = 0; k < rows; k++)
        {
            if (rowCount % pageModOnData == 0 && rowCount != 0)
            {
                rowCount = 0;
                pdfDoc.Add(pdfTable);
                pdfTable.DeleteAllRows();
            }


            for (int j = 0; j < cols; j++)
            {
                Cell cellRows = new Cell();
                Font rowFont = FontFactory.GetFont(FontFactory.HELVETICA, 8);
                Chunk chunkRows = new Chunk(dataTable.Rows[k][j].ToString(), rowFont);
                cellRows.Add(chunkRows);
                pdfTable.AddCell(cellRows);
            }

            rowCount++;
        }

        pdfDoc.Add(pdfTable);
        pdfDoc.Close();
        return mStream.ToArray();
    }

    public static byte[] ToXlsx(this DataSet ds)
    {
        var excelApp = OfficeOpenXML.GetInstance();
        return excelApp.GetExcelStream(ds)?.ToArray() ?? Array.Empty<byte>();
    }

    public static byte[] GetRequestedBytes(this DataSet ds, enumDownloadType downloadType, string identifier = "")
    {
        switch (downloadType)
        {
            case enumDownloadType.CSV:
                return ds.ToCsv();
            case enumDownloadType.PDF:
                return ds.ToPdf(identifier);
            case enumDownloadType.XLS:
            case enumDownloadType.XLSX:
                return ds.ToXlsx();
            case enumDownloadType.TSV:
                return ds.ToTsv();
            default:
                throw new ArgumentOutOfRangeException(nameof(downloadType), downloadType, null);
        }
    }

    public static FileContentResult Generate(byte[] data,
        string filePrefix = "",
        enumDownloadType downloadType = enumDownloadType.XLSX)
    {
        switch (downloadType)
        {
            case enumDownloadType.CSV:
                return new FileContentResult(data, "text/csv")
                {
                    FileDownloadName = $@"{filePrefix}-{DateTime.Now.ToUniversalIso8601()}.csv",
                };
            case enumDownloadType.PDF:
                return new FileContentResult(data, "application/pdf")
                {
                    FileDownloadName = $@"{filePrefix}-{DateTime.Now.ToUniversalIso8601()}.pdf",
                };
            case enumDownloadType.XLS:
            case enumDownloadType.XLSX:
                return new FileContentResult(data,
                    "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet")
                {
                    FileDownloadName =
                        $@"{filePrefix}-{DateTime.Now.ToUniversalIso8601()}{(downloadType == enumDownloadType.XLSX
                            ? ".xlsx"
                            : ".xls")}",
                };
            case enumDownloadType.TSV:
                return new FileContentResult(data, "text/tab-separated-values")
                {
                    FileDownloadName = $@"{filePrefix}-{DateTime.Now.ToUniversalIso8601()}.tsv",
                };
            default:
                throw new ArgumentOutOfRangeException(nameof(downloadType), downloadType, null);
        }
    }

    public static bool IsValidExcelFile(this HttpPostedFileBase file)
    {
        /* technically all of these file formats can be understood by the reader */
        if (file != null)
        {
            switch (file.ContentType)
            {
                /* xlsx */
                case "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet":
                /* xls */
                case "application/vnd.ms-excel":
                /* xlst */
                case "application/vnd.openxmlformats-officedocument.spreadsheetml.template":
                /* xlsm */
                case "application/vnd.ms-excel.sheet.macroEnabled.12":
                /* xltm */
                case "application/vnd.ms-excel.template.macroEnabled.12":
                /* xlsb */
                case "application/vnd.ms-excel.sheet.binary.macroEnabled.12":
                    return file.ContentLength > 0;
            }
        }

        return false;
    }

    private static string ToCsvCell(this string column)
    {
        /* Additional handling required for 'importing' csv into something like excel format*/
        var mustQuote = (column.Contains(",") || column.Contains("\"") || column.Contains("\r") ||
                         column.Contains("\n"));
        if (mustQuote)
        {
            var sb = new StringBuilder();
            sb.Append("\"");
            foreach (char nextChar in column)
            {
                sb.Append(nextChar);
                if (nextChar == '"')
                    sb.Append("\"");
            }

            sb.Append("\"");
            return sb.ToString();
        }

        return column;
    }
}