How to call webmethod in Asp.net C#

There are quite a few elements of the $.Ajax() that can cause issues if they are not defined correctly. I would suggest rewritting your javascript in its most basic form, you will most likely find that it works fine.

Script example:

$.ajax({
    type: "POST",
    url: '/Default.aspx/TestMethod',
    data: '{message: "HAI" }',
    contentType: "application/json; charset=utf-8",
    success: function (data) {
        console.log(data);
    },
    failure: function (response) {
        alert(response.d);
    }
});

WebMethod example:

[WebMethod]
public static string TestMethod(string message)
{
     return "The message" + message;
}

This is a bit late, but I just stumbled on this problem, trying to resolve my own problem of this kind. I then realized that I had this line in the ajax post wrong:

data: "{'quantity' : " + total_qty + ",'itemId':" + itemId + "}",

It should be:

data: "{quantity : '" + total_qty + "',itemId: '" + itemId + "'}",

As well as the WebMethod to:

public static string AddTo_Cart(string quantity, string itemId)

And this resolved my problem.

Hope it may be of help to someone else as well.

Necro'ing this Question ;)

You need to change the data being sent as Stringified JSON, that way you can modularize the Ajax call into a single supportable function.

First Step: Extract data construction

/***
 * This helper is used to call WebMethods from the page WebMethods.aspx
 * 
 * @method - String value; the name of the Web Method to execute
 * @data - JSON Object; the JSON structure data to pass, it will be Stringified
 *      before sending
 * @beforeSend - Function(xhr, sett)
 * @success - Function(data, status, xhr)
 * @error - Function(xhr, status, err)
 */
function AddToCartAjax(method, data, beforeSend, success, error) {
    $.ajax({
        url: 'AddToCart.aspx/', + method,
        data: JSON.stringify(data),
        type: "POST",
        dataType: "json",
        contentType: "application/json; charset=utf-8",
        beforeSend: beforeSend,
        success: success,
        error: error
    })
}

Second Step: Generalize WebMethod

[WebMethod]
public static string AddTo_Cart ( object items ) {
    var js = new JavaScriptSerializer();
    var json = js.ConvertToType<Dictionary<string , int>>( items );

    SpiritsShared.ShoppingCart.AddItem(json["itemId"], json["quantity"]);      
    return "Add";
}

Third Step: Call it where you need it

This can be called just about anywhere, JS-file, HTML-file, or Server-side construction.

var items = { "quantity": total_qty, "itemId": itemId };

AddToCartAjax("AddTo_Cart", items,
    function (xhr, sett) {  // @beforeSend
        alert("Start!!!");
    }, function (data, status, xhr) {   // @success
        alert("a");
    }, function(xhr, status, err){  // @error
        alert("Sorry!!!");
    });

The problem is at [System.Web.Services.WebMethod], add [WebMethod(EnableSession = false)] and you could get rid of page life cycle, by default EnableSession is true in Page and making page to come in life though life cycle events..

Please refer below page for more details http://msdn.microsoft.com/en-us/library/system.web.configuration.pagessection.enablesessionstate.aspx

One problem here is that your method expects int values while you are passing string from ajax call. Try to change it to string and parse inside the webmethod if necessary :

[System.Web.Services.WebMethod]
public static string AddTo_Cart(string quantity, string itemId)
{
    //parse parameters here
    SpiritsShared.ShoppingCart.AddItem(itemId, quantity);      
    return "Add";
}

Edit : or Pass int parameters from ajax call.

I'm not sure why that isn't working, It works fine on my test. But here is an alternative technique that might help.

Instead of calling the method in the AJAX url, just use the page .aspx url, and add the method as a parameter in the data object. Then when it calls page_load, your data will be in the Request.Form variable.

jQuery

jQuery.ajax({
    url: 'AddToCart.aspx',
    type: "POST",
    data: {
        method: 'AddTo_Cart', quantity: total_qty, itemId: itemId
    },
    dataType: "json",
    beforeSend: function () {
        alert("Start!!! ");
    },
    success: function (data) {
        alert("a");
    },
    failure: function (msg) { alert("Sorry!!! "); }
});

C# Page Load:

if (!Page.IsPostBack)
{
    if (Request.Form["method"] == "AddTo_Cart")
    {
        int q, id;
        int.TryParse(Request.Form["quantity"], out q);
        int.TryParse(Request.Form["itemId"], out id);
        AddTo_Cart(q,id);
    }
}

you need to JSON.stringify the data parameter before sending it.

Here is your answer. use

                   jquery.json-2.2.min.js 
                      and
                   jquery-1.8.3.min.js

Javascript :

function CallAddToCart(eitemId, equantity) {
   var itemId = Number(eitemId);
   var quantity = equantity;
   var dataValue = "{itemId:'" + itemId+ "', quantity :'"+ quantity "'}" ;
    $.ajax({
           url: "AddToCart.aspx/AddTo_Cart",
           type: "POST",
           dataType: "json",
           data: dataValue,
           contentType: "application/json; charset=utf-8",
           success: function (msg) {
                alert("Success");
           },
           error: function () { alert(arguments[2]); }      
        });
 }

and your C# web method should be

[WebMethod]
[ScriptMethod(ResponseFormat = ResponseFormat.Json)]
public static string AddTo_Cart(int itemId, string quantity)
{
   SpiritsShared.ShoppingCart.AddItem(itemId, quantity);      
  return "Item Added Successfully";
}

From any of the button click or any other html control event you can call to the javascript method with the parameter which in turn calls to the webmethod to get the value in json format.