Following code demonstrate how to write Data in XML file from dataset:

 DataSet ds = new DataSet();
 da.Fill(ds, "products");
 DataTable dt = ds.Tables["products"];
dt.WriteXml(@"d:\rohit\File1.xml");

Following code demonstrate how to write Data in EXCEL file from

dataset:

DataSet ds = new DataSet();
da.Fill(ds, "products");
DataTable dt = ds.Tables["products"];
object missing = Type.Missing;
object misValue = System.Reflection.Missing.Value;
Microsoft.Office.Interop.Excel.Application excel = new Microsoft.Office.Interop.Excel.Application();
Microsoft.Office.Interop.Excel.Workbook wb = excel.Workbooks.Add();
Microsoft.Office.Interop.Excel.Worksheet ws = wb.Sheets[1] as Microsoft.Office.Interop.Excel.Worksheet;
int rowCount = 1;
foreach (DataRow dr in dt.Rows)
{
   rowCount += 1;
   for (int i = 1; i < dt.Columns.Count + 1; i++)
   {
        // Add the header the first time through 
        if (rowCount == 2)
        {
            ws.Cells[1, i] = dt.Columns[i - 1].ColumnName;
        }
        ws.Cells[rowCount, i] = dr[i - 1].ToString();
    }
 }
wb.SaveAs(@"d:\rohit\excelData.xls", Excel.XlFileFormat.xlWorkbookNormal, misValue, 
                                     misValue, misValue, misValue,  
                                     Excel.XlSaveAsAccessMode.xlExclusive, misValue, 
                                     misValue, misValue, misValue, misValue);
wb.Close(missing, missing, missing);
excel.Quit();

Following code demonstrate how to write Data in CSV file from dataset:

DataSet ds = new DataSet();
da.Fill(ds, "products");
DataTable dt = ds.Tables["products"];
StreamWriter sw = new StreamWriter(@"d:\rohit\csvData.csv", false);
// First we will write the headers.
int iColCount = dt.Columns.Count;
for (int i = 0; i < iColCount; i++)
{
     sw.Write(dt.Columns[i]);
     if (i < iColCount - 1)
     {
           sw.Write(",");
     }
}
sw.Write(sw.NewLine);
// Now write all the rows.
foreach (DataRow dr in dt.Rows)
{
      for (int i = 0; i < iColCount; i++)
      {
        if (!Convert.IsDBNull(dr[i]))
       sw.Write(dr[i].ToString());
        if (i < iColCount - 1)
              sw.Write(",");
       }
       sw.Write(sw.NewLine);
 }
 sw.Close();