******************* Este es el evento del boton*******************
 Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim clase As New class1
        clase.showreport(llenardatos(lblerror.Text), Server.MapPath("reportelista.rpt"), Me.CrystalReportViewer1, lblcontador.Text)
        Me.CrystalReportViewer1.Visible = True
        If Me.lblerror.Text = "False" Then
            Me.lblerror.Text = ""
        End If
    End Sub
****************Con esta "Hago mi consulta para llenar el reporte"*********
    Private Function llenardatos(ByRef strerror As String) As DataSet
        Dim strcon As String
        Dim clase As New class1
        strcon = "select nombre_alumno,apellidos,curso,lugar_curso,mes,año from tb_listas where mes'" & ddlmes.SelectedValue & "' and curso='" & ddlcurso.SelectedItem.Text & "' and lugar_curso='" & ddlsede.SelectedItem.Text & "'"
        Dim ds As New DataSet
        ds = clase.consulta(strcon, strerror)
        If strerror = "False" Then
            Return ds
        Else
            lblerror.Text = strerror
        End If
    End Function
******** Este es supuestamente el que debe mostrar el reporte **************
   Public Sub showreport(ByVal ds As DataSet, ByVal rpt As String, ByVal crv As CrystalReportViewer, ByRef registros As String)
        Dim oRpt As New ReportDocument
        oRpt.Load(rpt)
        oRpt.Database.Tables(0).SetDataSource(ds.Tables(0))
        crv.ReportSource = oRpt
        crv.DataBind()
        registros = "registros = " & ds.Tables("table").Rows.Count
    End Sub

********** Evento boton **********************************
Protected Sub btnexportar_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnexportar.Click
        exportreport(llenardatos(Me.lblerror.Text))
    End Sub

******Funcion de exportacion********************************
    Private Sub exportreport(ByVal dsdatos As DataSet)

        Dim crd As New ReportDocument
        crd.Load(Server.MapPath("reportelista.rpt"))
        nombreReporte += "Reporte Listas"
        crd.Database.Tables(0).SetDataSource(dsdatos.Tables("table"))
        Dim rptStream As New System.IO.MemoryStream
        Try
            rptStream = CType(crd.ExportToStream(CInt(Me.ddlopciones.SelectedValue)), System.IO.MemoryStream)
        Catch ex As Exception
            Me.lblerror.Text = ex.Message & " en " & ex.Source
        End Try
        Response.Clear()
        Response.Buffer = True

        'Le indicamos el tipo de documento que vamos a exportar
        Response.ContentType = FormatoDocumento()

        'Automaticamente se descarga el archivo
        Response.AddHeader("Content-Disposition", "attachment;filename=" + Me.nombreReporte)

        'Se escribe el archivo
        Response.BinaryWrite(rptStream.ToArray())
        Response.End()

    End Sub