how to find dynamic control

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ASP.NET

Dear all

I am trying to create dynmaic file upload control as many as i required....
when i am trying To read files in it then it is getting failed

I did as follows

in declaration section

Dim TxtboxImageF As New FileUpload

creating controls in following way in loop as per my requirement and adding it in dynamic table and finally after ending loop adding table in placeholder

TxtboxImageF = New FileUpload
TxtboxImageF.ID = CreateControlid("DOC", increamentvariable)
CellDocumentFrame.Controls.Add(TxtboxImageF)
RowDocumentFrame.Cells.Add(CellDocumentFrame)
TblDocumentFrame.Rows.Add(RowDocumentFrame)

after ending loop addin table in placeholder

 PHobservations.Controls.Add(TblDocumentFrame)

Private Function CreateControlid(ByVal ctltype As String, ByVal docid As Integer) As String
        Dim Var_strbuildvalue As New StringBuilder
        Var_strbuildvalue.Length = 0
        Var_strbuildvalue.Append(ctltype)
        Var_strbuildvalue.Append("^")
        Var_strbuildvalue.Append(docid)
        CreateControlid = Var_strbuildvalue.ToString
    End Function

when i try to find the control it is getting failed in following way

Dim i As Integer = "1"
TxtboxImageF = Me.FindControl("DOC^" & i)

please help