Which UserControl? do u mean Content Page of the master page or something else?
If UserControl is Content Page then,
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First thing is ArrayList is not an Control. So, u can't access it using Findcontrol method.
However, It's very easy to get arrayList value. Just define one public variable arrayList in ChildControl (Content Page).
Suppose u have one Master Page 'Main.master' and and a Content Page 'WebForm1.aspx' the following code can give u some idea abt it.
Code:
Public Partial Class WebForm1
Inherits System.Web.UI.Page
Public arrList As ArrayList
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
End Sub
End Class
Code:
Public Partial Class Main
Inherits System.Web.UI.MasterPage
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
End Sub
Protected Sub btnCheckArrayList_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCheckArrayList.Click
If DirectCast(Me.Parent, WebForm1) IsNot Nothing Then
Dim objClassOfWebForm1 As WebForm1 = DirectCast(Me.Parent, WebForm1)
objClassOfWebForm1.arrList = New ArrayList()
Response.Write("created")
End If
End Sub
End Class