Send DataSet as xml file.

// Create MemeoryStream
System.IO.MemoryStream ms = new System.IO.MemoryStream();

// Write MemeoryStream

MyDataset.WriteXml(ms,System.Data.XmlWriteMode.IgnoreSchema);

Response.Clear();

// filename & attachment

Response.AddHeader("Content-Disposition", "attachment; filename=Acounts.xml");

// size of the file,to show PRocess of downloading

Response.AddHeader("Content-Length", ms.Length.ToString());

// mode:download

Response.ContentType = "application/octet-stream";

// sending to client

byte[] b = ms.ToArray();

Response.OutputStream.Write(b,0,b.Length);

Response.End();

}