how to open save as dialog box in asp.net

If this really is ASP.NET, - i.e. a web site based program - then you can't, not in the way you mean. If you want to open a save as for the server file system, then you can open a OpenFileDialog, but it will appear on the Server, not the Client. If this is to do with downloading a file to the client and forcing a save dialog at his end, you can't do that either - the download is under the control of the browser and it and it alone decides if a save dialog is needed.

String FileName = "FileName.txt";
String FilePath = "C:/...."; //Replace this
System.Web.HttpResponse response = System.Web.HttpContext.Current.Response;
response.ClearContent();
response.Clear();
response.ContentType = "text/plain";
response.AddHeader("Content-Disposition", "attachment; filename=" + FileName + ";");
response.TransmitFile(FilePath);
response.Flush();
response.End();