What does .d in JSON mean?

Are you referring to the ADO.NET Data Services?

I remember hearing a presentation about the JSON returning this and I think its just a wrapper to ensure the payload is a JSON object as opposed to an array (which is the case of returning multiple entities).

Why 'd' specifically? I think I remember them saying something like 'well it had to be something'.

Based on this tutorial: JSON Web Service And jQuery with Visual Studio 2008

The Web Method returns a Product that is serialized in JSON format. Since there is not JSON type, the returned value is a String with JSON format.

On the client side, the ajax call returns a JSON.

The result looks like {d: 'returned-string-with-JSON-format'}

More exactly something like: {d:'{"ID":123,"Name":"Surface Pro 2"}'}

Note that 'returned-string-with-JSON-format' is a string not a JSON object so you cannot do result.d.ID.

Instead you need to convert it to JSON object by using JSON.parse(result.d) or eval(result.d)

At the end, what you really want is do this:

result = JSON.parse(result.d)

UPDATEAlso consider this demo, where I use a JSON in string format and convert it to JSON object:

ASPX Code Here:

<asp:Content runat="server" ID="BodyContent" ContentPlaceHolderID="MainContent">    <script type="text/javascript">        function GetData()        {            alert("I am called");                $.ajax({                    type: "POST",                    url: "Contact.aspx/GetProducts",                    data: "{}",                    contentType: "application/json; charset=utf-8",                    dataType: "json",                    success: function (result) {                       var data = JSON.parse(result.d)                       alert(data.Id);                    },                    error:function(ex)                    {                        alert("Test");                    }                });        }    </script>     <asp:TextBox ID="txtName" runat="server"></asp:TextBox>    <asp:Button ID="Button1" runat="server" Text="Button" OnClientClick="GetData();" /></asp:Content>

C# Code Here:

public partial class Contact : Page    {        protected void Page_Load(object sender, EventArgs e)        {            if (!IsPostBack)            {                BindList();            }            int[] arr1 = new int[] { 1, 2 };            ListBox1.SelectedValue = "1";            ListBox1.SelectedValue = "4";        }        void BindList()        {            List<Product> lst = new List<Product>()            {                new Product{Id=1,Name="Photo"},                new Product{Id=2,Name="Photo"},                new Product{Id=3,Name="Photo"},                new Product{Id=4,Name="Photo"}            };            ListBox1.DataSource = lst;            ListBox1.DataTextField = "Name";            ListBox1.DataValueField = "Id";            ListBox1.DataBind();        }        [WebMethod]        public static string GetProducts()        {            // instantiate a serializer            JavaScriptSerializer TheSerializer = new JavaScriptSerializer();            //optional: you can create your own custom converter           // TheSerializer.RegisterConverters(new JavaScriptConverter[] { new MyCustomJson() });            //var products = context.GetProducts().ToList();            Product products = new Product() { Id = 1, Name = "Testing Services" };            var TheJson = TheSerializer.Serialize(products);            return TheJson;        }    }