Webservice method not found by JQuery Ajax-Request error 500

Question

JS:

function showComments(couponId) {
            var jsontxt = JSON.stringify({ couponId });
            $.ajax({
                type: "POST",
                url: "<% =Page.ResolveUrl("~/Admin/UserCoupons.aspx/GetComments") %>",
                data: jsontxt,
                contentType: 'application/json; charset=utf-8',
                dataType: 'json',
                //async:true,
                success: function (result) {
                    alert("We returned: " + result.d);
                },
                failure: function (response) {
                    alert(response.d);
                }
            });

        }

C#:

[WebMethod]
        [ScriptMethod(ResponseFormat = ResponseFormat.Json)]
        public static string GetComments(string couponId)
        {
            //will return precomipled generated html for display in div
            return "Success";

        }

The C# code is in page's code behind .cs file i have solved this all before using stack over flow thanks to experts here. but this time there is some thing else

i am getting my debug pointer in page_load method then in master page's page load method. but debug doesn't enters to the GetComments

and getting exception

Unknown web method GetComments. Parameter name: methodName

Below is the stackTrace

   at System.Web.Script.Services.WebServiceData.GetMethodData(String methodName)
   at System.Web.Script.Services.RestHandler.CreateHandler(WebServiceData webServiceData, String methodName)
   at System.Web.Script.Services.RestHandler.CreateHandler(HttpContext context)
   at System.Web.Script.Services.RestHandlerFactory.GetHandler(HttpContext context, String requestType, String url, String pathTranslated)
   at System.Web.Script.Services.ScriptHandlerFactory.GetHandler(HttpContext context, String requestType, String url, String pathTranslated)
   at System.Web.HttpApplication.MaterializeHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Answer 1

Your data in ajax should be JSON.stringify({ couponId:'123' }) //parameter name & value.

function showComments(couponId) {
            var jsontxt = JSON.stringify({ couponId:'123' });
            $.ajax({
                type: "POST",
                url: "<% =Page.ResolveUrl("~/Admin/UserCoupons.aspx/GetComments") %>",
                data: jsontxt,
                contentType: 'application/json; charset=utf-8',
                dataType: 'json',
                //async:true,
                success: function (result) {
                    alert("We returned: " + result.d);
                },
                failure: function (response) {
                    alert(response.d);
                }
            });

        }

Answer 2

after a long google and all others and applying every possible solutions. finally, i found a solution. and that's edit the file at ~/App_start/RouteConfig.cs

replace

settings.AutoRedirectMode = RedirectMode.Permanent;

with

settings.AutoRedirectMode = RedirectMode.Off;

this solved my problem. any ways thanks to all who took interest and showed their generosity.