add aother control on my page then i looses old one control on place holder

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ASP.NET4

this is my code,


Public Class tempformmanager
Inherits System.Web.UI.Page


Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load


End Sub

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs) Handles Button1.Click
Dim btn1 As New System.Web.UI.WebControls.Button
btn1.Text = "this is button"
PlaceHolder1.Controls.Add(btn1)


End Sub

Protected Sub Button2_Click(ByVal sender As Object, ByVal e As EventArgs) Handles Button2.Click
PlaceHolder1.Visible = True

Dim radio1 As New System.Web.UI.WebControls.RadioButton
radio1.Text = " radio button"
PlaceHolder2.Controls.Add(radio1)
End Sub

Protected Sub Button3_Click(ByVal sender As Object, ByVal e As EventArgs) Handles Button3.Click
Dim chkbox1 As New System.Web.UI.WebControls.CheckBox
chkbox1.Text = " checkbox "
PlaceHolder3.Controls.Add(chkbox1)
End Sub

Protected Sub Button4_Click(ByVal sender As Object, ByVal e As EventArgs) Handles Button4.Click
Dim txtbox1 As New System.Web.UI.WebControls.TextBox
PlaceHolder4.Controls.Add(txtbox1)
End Sub

Protected Sub Button5_Click(ByVal sender As Object, ByVal e As EventArgs) Handles Button5.Click
Dim lbl1 As New System.Web.UI.WebControls.Label
lbl1.Text = "this is label"
PlaceHolder5.Controls.Add(lbl1)
End Sub

Private Sub tempformmanager_PreInit(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.PreInit
PlaceHolder1.Visible = True

End Sub
End Class