Unknown web method in asp.net - asp.net

Question

In the below code I have a textbox .My aim is when I focus on textbox it will call the server side code through ajax.But I got a error unknown web method txtField_GotFocus parameter name method name.Please help me to rectify the error.
design code:
$(document).ready(function () {
$("#<%=txtField.ClientID%>").bind("focus", function () {
$.ajax({
type: "POST",
url: "<%=Request.FilePath%>/txtField_GotFocus",
data: "{}",
contentType: "application/json",
dataType: "json",
success: function (msg) {
//alert("success message here if you want to debug");
},
error: function (xhr) {
var rawHTML = xhr.responseText;
var strBegin = "<" + "title>";
var strEnd = "</" + "title>";
var index1 = rawHTML.indexOf(strBegin);
var index2 = rawHTML.indexOf(strEnd);
if (index2 > index1) {
var msg = rawHTML.substr(index1 + strBegin.length, index2 - (index1 + strEnd.length - 1));
alert("error: " + msg);
} else {
alert("General error, check connection");
}
}
});
});
});
<asp:TextBox ID="txtField" runat="server" AutoPostBack="true" OnTextChanged="txtField_TextChanged" ClientIDMode="Static"></asp:TextBox>
field.ascx:
public static void txtField_GotFocus()
{
string foo = HttpContext.Current.Request["foo"];
//code...
}

Answer 1

You are missing [WebMethod] annotation. Also i have modified your code
[WebMethod]
public static string txtField_GotFocus()
{
string foo = HttpContext.Current.Request["foo"];//oops i got my super data
//code...
return "awesome, it works!";
}
Check this Article
Your refactored ajax code
$(document).ready(function () {
$("#<%=txtField.ClientID%>").bind("focus", function () {
$.ajax({
type: "POST",
url: "<%=Request.FilePath%>/txtField_GotFocus",
data: "{foo:'whatever'}",
success: function (msg) {
alert(msg);//awesome, it works!
},
error: function (xhr) {
}
});
});
});

Answer 2

Data returned form Server is stored in msg.d field. If you return a single value, you should get it using msg.d. If you return a JSON serialized object you have to parse it using JSON.parse(msg.d)
$(document).ready(function () {
$("#<%=txtField.ClientID%>").bind("focus", function () {
$.ajax({
type: "POST",
url: "<%=Request.FilePath%>/txtField_GotFocus",
data: "{foo:'whatever'}",
success: function (msg) {
alert(msg.d);//awesome, it works!
},
error: function (xhr) {
}
});
});
});