c# - How to prevent postback when button is clicked -

i have code save file in database. have function in code behind:

protected void attachfile(object sender, eventargs e)     {         if (txtfile.hasfile)         {             string filename = path.getfilename(txtfile.postedfile.filename);             string contenttype = txtfile.postedfile.contenttype;              using (stream fs = txtfile.postedfile.inputstream)             {                 using (binaryreader br = new binaryreader(fs))                 {                     random ran = new random();                      byte[] bytes = br.readbytes((int32)fs.length);                     newrequestservice newrequest = new newrequestservice();                      string temp_id = "";                     temp_id = ran.next(100000, 999999).tostring();                      saveattachfile(filename, contenttype, bytes, temp_id);                     hdnattachid.value = temp_id;                     lblattach.innerhtml = msg;                 }             }          }         else { lblattach.innerhtml = "\"please browse file attach.\""; }      } 

and here's html:

<asp:button id="btnattached" runat="server" text="attach file" onclick="attachfile" /> <asp:fileupload id="txtfile" size="46" runat="server" /> 

by clicking "attach file" button, call code behind function , save file browse user. work fine when refresh page goes function again , repeat saving process ending duplicate of records. how can avoid happend?

some tried are:

1. adding if (!ispostback) {} condition.
2. adding onclientclick="return false" attribute in button.

but did not solve problem. thank in advance.

use technique prevent double trigger on control events after refreshing:

1. add code code behind page prerender event

   protected void page_prerender(object sender, eventargs e) {     viewstate["checkrefresh"] = session["checkrefresh"]; } 

2. add line on page_load event.

   protected void page_load(object sender, eventargs e) {   if (!ispostback)   {     session["checkrefresh"] = server.urldecode(system.datetime.now.tostring());     // rest of code here ...   } } 

3. on controls event, button click or other control want prevent on page refresh check these condition first.

   protected void attachfile(object sender, eventargs e) {     if (session["checkrefresh"].tostring() == viewstate["checkrefresh"].tostring())     {        // code starts here ..         if (txtfile.hasfile)        {           //...        }         // code ends here ..         session["checkrefresh"] = server.urldecode(system.datetime.now.tostring());     } } 

cheers