c# - Get exception in web method when trying to assign value in page attributes -

when i'm trying assign value in textbox through webmethod on page got exception

my html code

<form id="form1" runat="server"><br /><br /> <asp:scriptmanager id="src" runat="server" enablepagemethods="true"></asp:scriptmanager> <asp:updatepanel id="up1" runat="server">     <contenttemplate>                    <div class="row">             <div class="col-md-6">                 <asp:button id="btnchangemake" runat="server" text="change make" onclientclick="changemake()"/><br />                 <asp:textbox id="txtmake" runat="server"></asp:textbox>             </div>         </div>     </contenttemplate> </asp:updatepanel> 

javascript code

function changemake() { pagemethods.changemaketext("devil", onsucess, onerror); function onsucess(result) {     alert(result); } function onerror(result) {     alert(result); } 

}

c# code

[webmethod] public static void changemaketext(string datavalue) {     if (httpcontext.current != null)     {         page page = (page)httpcontext.current.handler;         textbox txtmake = (textbox)page.findcontrol("txtmake");         txtmake.text = datavalue;     } } 

exception @ 'txtmake.text = datavalue;'

an exception of type 'system.nullreferenceexception' occurred in app_web_0lr1r3rs.dll not handled in user code

additional information: object reference not set instance of object.

you not access page controls in web method rather can send value calling method in javascript , assign txtmake.

c# code

public static string changemaketext(string datavalue) {     //your code         return datavalue; } 

javascript code

function changemake() {      pagemethods.changemaketext("devil", onsucess, onerror);      function onsucess(result) {           document.getelementbyid("<%=txtmake.clientid%>").value = result;      } } 

you have used ajax updatepanel bind click event button btnchangemake (by double click in desginer) , change value in server side click handler.

void btnchangemake_click(object sender, eventargs e) {      txtmake.text = datavalue; }