I recommend you look my previous answer for the close questions How do I build a JSON object to send to an AJAX WebService? and Can I return JSON from an .asmx Web Service if the ContentType is not JSON?
The correct code should looks like following
[WebMethod]
[ScriptMethod (ResponseFormat = ResponseFormat.Json)]
public EntityLayer.TestPage1 GetData(int id)
{
TestPage1 test = TestPage1.GetData(id).SingleOrDefault();
return test;
}
and
var myData = 5;
$.ajax({
type: "POST",
contentType: "application/json; charset=utf-8",
url: "WebService.asmx/GetData",
//data: {id:JSON.stringify(myData)},
data: JSON.stringify({id:myData}),
dataType: "json",
success: function(response){
alert("UserName=" + response.d.UserName +
", FirstName=" + response.d.FirstName +
", MiddleName=" + response.d.MiddleName+
", LastName=" + response.d.LastName);
}
})
where JSON.stringify is a function from the script json2.js which you can download from http://www.json.org/js.html.
If the id values are integer JSON.stringify(myData) are the same as myData, but for all more complex examples I strictly recommend you to use this function.
How you can also see from the code the all results of the web method will be saved in the property d, so you should use for example response.d.FirstName syntax to access the first name.
UPDATED: In case of HTTP GET the data parameter should be {id:JSON.stringify(myData)}. In case of HTTP POST: JSON.stringify({id:myData})