c# - How to call a .net method in AJAX on fileupload change? -

i'm trying upload file using ajax problem i'm rookie in .net , don't know if did right or not appreciate or suggestions.

here html code:

<asp:fileupload id="fileupload1" onchange="show()" runat="server" /> 

my method should called when users selects file

this script found in msdn:

<script type="text/javascript">     function show() {         var file = document.getelementbyid("fileupload1");         alert("test")         $.ajax({             type: "post",             url: "sendsms.aspx/staticupdate",             data: '{name:"test" }',             contenttype: "application/json; charset=utf-8",             datatype: "json",             success: onsuccess,             failure: function (response) {                 alert("test1");             }         });     }     function onsuccess(response) {         alert("test2");     } </script> 

and here asp method:

    [system.web.services.webmethod] public static void staticupdate() {     sendsms upload = new sendsms();     upload.upload(); }  public void upload() {     string filename = system.datetime.now.tostring("ddmmyyhhmmsss") + fileupload1.filename;     if (ispostback)     {         boolean fileok = false;         string path = server.mappath("~/upload/");         if (fileupload1.hasfile)         {             string fileextension =                 system.io.path.getextension(filename).tolower();             string[] allowedextensions = { ".txt" };             (int = 0; < allowedextensions.length; i++)             {                 if (fileextension == allowedextensions[i])                 {                     fileok = true;                 }             }         }         if (fileok)         {             try             {                 fileupload1.postedfile.saveas(path                     + filename);                 system.io.streamreader myfile = new system.io.streamreader("c:\\inetpub\\wwwroot\\viber_bulk_ui\\upload\\" + filename + "");                 textbox2.text = myfile.readtoend();                 myfile.close();                 string path = "c:\\inetpub\\wwwroot\\viber_bulk_ui\\upload\\" + filename + "";                 system.io.file.delete(path);             }             catch (exception ex)             {                 label1.text = "file not uploaded.";             }         }         else         {             label1.text = "cannot accept files of type.";         }     }  } 

the error this:

nullreferenceexception unhandled user code. object reference not set instance of object.

thanks in advance

 string filename = system.io.path.getfilename(fileupload1.filename);  string ext = system.io.path.getextension(fileupload1.filename);  if (fileextension == ".txt"){             using (system.io.filestream fs = new system.io.filestream(server.mappath(("c:\\inetpub\\wwwroot\\viber_bulk_ui\\upload\\" + filename), system.io.filemode.append, system.io.fileaccess.write, system.io.fileshare.read, 8, system.io.fileoptions.none))         {             byte[] data = system.text.encoding.ascii.getbytes(filename);             fs.write(data, 0, data.length);             fs.flush();             fs.close();         }   }