VB.net JavaScriptSerializer how do I make this Json string?

Question

I'm trying to create this JSON string but having problem creating it. I'm getting System.NullReferenceException error.

Here is the json string that I would like to create

{"id":["1","2","3","4"]},"sid":["s1","s2","s3","s4"]} 

Here is the code I have so far.

Imports System.Web.Services
Imports System.Data

Partial Class test_sidetab
    Inherits System.Web.UI.Page

    <WebMethod()> Public Shared Function getfav() As String
        Dim fav As New Favorites()
        Dim js As New System.Web.Script.Serialization.JavaScriptSerializer()
            Dim i As Integer = 1
            While i < 10
                fav.id.Add(i.ToString)
                fav.sid.Add("s"+i.ToString)
                i += 1
            End While
        Dim str As String = js.Serialize(fav)
        Return str
    End Function
End Class
Public Class Favorites

    Public id As ArrayList, sid As ArrayList

End Class

Answer 1

fav.id is null. You need to put an ArrayList into it by writing Public id As New ArrayList (or, better yet, List(Of String) )