Why does List(of Int32) get JSON serialized as ArrayList?

Question

I am trying to serialize a List(Of Int32) using the JavaScriptSerializer. It seems to work until I try to Deserialize the object back to a List, at which point I get this error:

Unable to cast object of type 'System.Collections.ArrayList' to type 'System.Collections.Generic.List`1[System.Int32]'.

My code is below. Can anyone please explain why, even when I explicitly cast to the correct type, that the list is automatically saved as an ArrayList?

Thanks in advance.

Private Property _serialized As String
    Get
        Return ViewState("_serialized")
    End Get
    Set(value As String)
        ViewState("_serialized") = value
    End Set
End Property

Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load
    If Not Page.IsPostBack Then
        Dim l As New List(Of Int32) From {2, 4, 6, 8, 10}
        Dim d As New Dictionary(Of String, Object)
        d.Add("myList", l)
        Dim js As New Script.Serialization.JavaScriptSerializer
        Me._serialized = js.Serialize(d)
    End If
End Sub

Protected Sub btn_Click(sender As Object, e As EventArgs) Handles btn.Click
    Dim js As New Script.Serialization.JavaScriptSerializer
    Dim d As Dictionary(Of String, Object) = js.Deserialize(Of Dictionary(Of String, Object))(Me._serialized)
    Dim l As List(Of Int32) = CType(d.Item("myList"), List(Of Int32))
    For Each i As Int32 In l
        Trace.Warn(i.ToString)
    Next
End Sub

The error line is this:

Dim l As List(Of Int32) = CType(d.Item("myList"), List(Of Int32))

Answer 1

JSON doesn't store the server-side representation; it wouldn't know that it's a List, so it has to go generic. But you should be able to do this to get it back:

Ctype(d.Item("myList"), ArrayList).Cast(Of Integer)().ToList()