How can I use one webpage to show several reports once a time?

I am migratting a vb6 application to a asp.net application, in the original app I used a simple form with CRViewer control and it receives the data to show and also the report name.
I need to traslate it to asp.net, but I don't find the way to use only one webpage with Crystal Viewer control.
I am trying to  handle the PreviousPage property to get the dataset and the report name, but I have lost the previouspage property, I tryied to use tye viewstate("prevpage"), but when I try to make a select case of the previouspage to identify the data and the report name I got an Error:
This is the code where I am trying to have reference to previous page:
:
If PreviousPage IsNot Nothing Then
            If ViewState("pagPrevia") Is Nothing Then
                ViewState("pagPrevia") = PreviousPage.ToString.Trim
            Else
                pagPrevia = ViewState("pagPrevia")
            End If
            Select Case ViewState("pagPrevia").ToString
                Case Is = "ASP.paginstalaciones_aspx"
                    pagRegresar = "pagInstalaciones.aspx"
                Case Is = "ASP.plantas_aspx"
                    pagRegresar = "pagPlantas.aspx"
            End Select
:
The exception gotten is invalidcastexception:
It cann't convert an object type 'ASP.pagplantas_aspx' to type 'ASP.paginstalaciones_aspx'.

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