The JSON value could not be converted to System.Collections.Generic.IEnumerable`1[Newtonsoft.Json.Linq.JToken]

If you are getting the following error in your .Net Core Project
"The JSON value could not be converted to System.Collections.Generic.IEnumerable`1[Newtonsoft.Json.Linq.JToken]"
Then you have to install the "Microsoft.AspNetCore.Mvc.NewtonsoftJson" Nuget package.

Afterwards, in the startup class, update Startup.ConfigureServices to call AddNewtonsoftJson.
Example:

public void ConfigureServices(IServiceCollection services)
{
services.AddControllersWithViews().AddNewtonsoftJson();
  }

JavaScript Takeaways for .Net Developers

1) Capture server controls where ClientIDMode is not set to Static

document.getElementById("<%= txtRule.ClientID %>").value

2) Storing Session value in JavaScript variable

var userAttempts = '<%= Session["userAttemptSession"] %>';


3) Manipulating Hidden field values

/*Get value*/
var hitCountContainer = document.getElementById("hdnMyCount")
var hitCount = parseInt(hitCountContainer.value);
hitCount = hitCount + 1;

/*Set Value*/
hitCountContainer.value = hitCount.toString()

4) Invoking a post-back (without any parameteres)

__doPostBack('', '');

5) Invoking post-back (including parameter passing)

__doPostBack('hdnMyCount', hitCount.toString());

Accessing the post-back parameters in Page_Init

string controlName = Request.Params["__EVENTTARGET"];
string controlValue = Request.Params["__EVENTARGUMENT"];

This example may be useful in a scenario where you need the client-side updated values during the Page_Init. Reason is that the values are not accessible at this point of the ASP.Net Page Life Cycle. If you need to access hidden fields during Page_Load this is not necessary as the values are already accessible by that time (of the ASP.Net Page Life Cycle). Dynamically generating Controls in an application page on user request (eg: add new row having ASP.Net controls) would be an ideal scenario.

FYI:
There is a nice little image of the ASP.Net Page Life Cycle in this MSDN Blog. Adorable isn't it ?   ;) The very essence of ASP.Net forms programming !!
         
6) Sending multiple parameters in JavaScript postback

Here I send two JavaScript variables count & type.

__doPostBack('addNew', JSON.stringify({ count: count.toString(), type: selectedText}));

In the server side capture using the following

/*Using directive*/
using System.Web.Script.Serialization
...
...
/*Code logic*/
string receivedArgs = Request.Params["__EVENTARGUMENT"];
var arguments = new JavaScriptSerializer().Deserialize<Dictionary<string, object>>(receivedArgs);
var countValue = arguments["count"];
var typeValue = arguments["type"];

AJAX call from Visual Web Part - SharePoint

1)

In the JavaScript function create the JSON object. Here the stringify method is used to construct the object which contains the parameters to be sent. The function in the JavaScript method of the aspx page is as below:

    function CallMyPage(itemValue, paramTwo, paramThree, paramFour) {

        var final = { myItem: itemValue, parameterTwo: paramTwo, parameterThree: paramThree, siteUrl: paramFour };

        $.ajax({

            type: "POST",

            url: paramFour + "/_layouts/15/MyFeatureFolder/WebMethods.aspx/MyWebMethod",

            data: JSON.stringify(final),

            contentType: "application/json; charset=utf-8",

            dataType: "json",

            success: OnSuccess,

           failure: function (response) {

             alert(response.d);

          }

       });

    }

2)

In the code behind within a SPGridView I have a HyperLink control. I set the JavaScript method call in the code behind as:

editLink.NavigateUrl = "javascript:CallMyPage('" + itemGuid.Text + "','" + MyParamTwo + "' ,'" + MyParamThree + "', '" + SPContext.Current.Web.Url + "');"; 

3)

In my approach instead of a web service I am using an application page (WebMethods.aspx) to get the job done. In the application page servicing the AJAX call, I create the web method

        [System.Web.Services.WebMethod]

        public static string MyWebMethod(string myItem, string parameterTwo, string parameterThree, string siteUrl)

        {

            string result = string.Empty;

            try 

            {

                //TODO - Process request and return result

            }

            catch (Exception)

            {

                //Exception Handling

            }

            return result;

4)

On success

    function OnSuccess(response) {

        if (response.d) {           

            /* Handle  result */            

        }

    }

Note: I have used the jquery-1.11.2.min.js file in my solution