C#

public class NPOIWrapper {

    enum XLSType {XLS, XLSX}

    public string FileName { get; set;}
    private int _CurrentSheetIndex = 0;
    public int CurrentSheetIndex { get {return _CurrentSheetIndex; } set {_CurrentSheetIndex = value; } }

    public NPOIWrapper() { }
    public NPOIWrapper(string cFileName) {
        this.FileName = cFileName;
        ReadXLS();
    }

    private string[] _Sheet;
    public string[] Sheet {get { return _Sheet;} }

    private int _SheetCount = 0;
    public int SheetCount {get { return _SheetCount;} }

    XLSType xlsType;

    object wb;
    ISheet[]        sh;


    public bool ReadXLS(string XLSFileName = "") {
        bool Ret = true;
        string XLSName, Extension;
        StringBuilder Sheets = new StringBuilder();

        FileStream fs;
        XLSName = String.IsNullOrEmpty(XLSFileName)?this.FileName:XLSFileName;
        Extension = Path.GetExtension(XLSName).Substring(1).ToUpper();
        switch (Extension) {
            case "XLS":
                xlsType = XLSType.XLS;
                break;
            case "XLSX":
                xlsType = XLSType.XLSX;
                break;
            default:
                Ret = false;
                break;
        }
        if(Ret) {
            fs = new FileStream(XLSName, FileMode.Open, FileAccess.Read);
            if (fs==null) Ret = false;
            else {
                Type t = xlsType==XLSType.XLS?typeof(HSSFWorkbook):typeof(XSSFWorkbook);
                try {
                    wb = Activator.CreateInstance(t,  new object[] {fs});
                } catch (Exception ex) { MessageBox.Show("Problemi nella lettura del foglio\n" + ex.Message);}
                if (wb != null) {
                    _SheetCount = ((IWorkbook)wb).Count;
                    sh = new ISheet[_SheetCount];
                    for (int i = 0; i < _SheetCount; i++) {
                        sh[i] = ((IWorkbook)wb).GetSheetAt(i);
                        Sheets.Append("\n" + sh[i].SheetName);
                    }
                    this._Sheet = Sheets.ToString().Substring(1).Split('\n');
                }
                fs.Close();
            }
        }
        return Ret;
    }
    public ICell GetCell(int Row, int Col) {
        ICell Ret;
        Ret = GetCell(Row, Col, this.CurrentSheetIndex);
        return Ret;
    }

    public ICell GetCell(int Row, int Col, int Sheet) {
        ICell Ret;
        Ret = sh[Sheet].GetRow(Row).GetCell(Col);
        return Ret;
    }

    public String GetCellValue(int Row, int Col) {
        String Ret;
        Ret = GetCellValue(Row, Col, this.CurrentSheetIndex);
        return Ret;
    }

    public string GetCellValue(int Row, int Col, int Sheet) {
        string Ret;
        Ret = sh[Sheet].GetRow(Row).GetCell(Col).ToString();
        return Ret;
    }

    public IRow GetRow(int Row, int Sheet) {
        IRow Ret;
        Ret = sh[Sheet].GetRow(Row);
        return Ret;
    }

    public IRow GetRow(int Row) {
        IRow Ret;
        Ret = GetRow(Row, CurrentSheetIndex);
        return Ret;
    }
}

IWorkbook wb;
...
wb = Activator.CreateInstance( t, new object[] { fs } ) as IWorkbook;
...
_SheetCount = wb.Count;
...

DataSet ds = new DataSet();
            DataTable dt = new DataTable();
            // Specify connection options and open an connection
            NpgsqlConnection conn = new NpgsqlConnection("Server=localhost;User Id=xxx;" +
                                    "Password=xxx;Database=xxx;");


            conn.Open();

            string query = "SELECT xxxx";
            //// Define a query
            NpgsqlCommand cmd = new NpgsqlCommand(query, conn);

            //// Execute a query
            NpgsqlDataReader dr = cmd.ExecuteReader();


            //// Read all rows and output the first column in each row
            while (dr.Read())
                data.Add(new Lagerteile() { ID = (int)dr["ID"], Kunde = dr["Name"].ToString(), Liefertermin = (DateTime)dr["Liefertermin"], WANr = dr["WaNr"].ToString(), Name = dr["Bezeichnung"].ToString(), Pos = dr["Pos"].ToString(), Stk = (int)dr["Stk"] });



            conn.Close();

          dg123.DataContext = data;

1. Open the source file with a FileStream
2. Chain a DeflateStream onto the filestream (just pass in constructor)
3. Read from the deflate stream the number of bytes you want in your chunks (into byte array)
4. Write entire byte array to individual file.
5. Goto 3 (until end of filestream).