Eccolo qui , è ripreso da un esempio in rete :

codice:
Public Class GridViewExportUtil

    Public Shared Sub Export(ByVal fileName As String, ByVal gv As GridView)
        HttpContext.Current.Response.Clear()
        HttpContext.Current.Response.AddHeader("content-disposition", String.Format("attachment; filename={0}", fileName))
        HttpContext.Current.Response.ContentType = "application/ms-excel"
        Dim sw As StringWriter = New StringWriter
        Dim htw As HtmlTextWriter = New HtmlTextWriter(sw)
        '  Create a form to contain the grid
        Dim table As Table = New Table
        table.GridLines = gv.GridLines
        '  add the header row to the table
        If (Not (gv.HeaderRow) Is Nothing) Then
            GridViewExportUtil.PrepareControlForExport(gv.HeaderRow)
            table.Rows.Add(gv.HeaderRow)
        End If
        '  add each of the data rows to the table
        For Each row As GridViewRow In gv.Rows
            GridViewExportUtil.PrepareControlForExport(row)
            table.Rows.Add(row)
        Next
        '  add the footer row to the table
        If (Not (gv.FooterRow) Is Nothing) Then
            GridViewExportUtil.PrepareControlForExport(gv.FooterRow)
            table.Rows.Add(gv.FooterRow)
        End If
        '  render the table into the htmlwriter
        table.RenderControl(htw)
        '  render the htmlwriter into the response
        HttpContext.Current.Response.Write(sw.ToString)
        HttpContext.Current.Response.End()
    End Sub

    ' Replace any of the contained controls with literals
    Private Shared Sub PrepareControlForExport(ByVal control As Control)
        Dim i As Integer = 0
        Do While (i < control.Controls.Count)
            Dim current As Control = control.Controls(i)
            If (TypeOf current Is LinkButton) Then
                control.Controls.Remove(current)
                control.Controls.AddAt(i, New LiteralControl(CType(current, LinkButton).Text))
            ElseIf (TypeOf current Is ImageButton) Then
                control.Controls.Remove(current)
                control.Controls.AddAt(i, New LiteralControl(CType(current, ImageButton).AlternateText))
            ElseIf (TypeOf current Is HyperLink) Then
                control.Controls.Remove(current)
                control.Controls.AddAt(i, New LiteralControl(CType(current, HyperLink).Text))
            ElseIf (TypeOf current Is DropDownList) Then
                control.Controls.Remove(current)
                control.Controls.AddAt(i, New LiteralControl(CType(current, DropDownList).SelectedItem.Text))
            ElseIf (TypeOf current Is CheckBox) Then
                control.Controls.Remove(current)
                control.Controls.AddAt(i, New LiteralControl(CType(current, CheckBox).Checked))
                'TODO: Warning!!!, inline IF is not supported ?
            End If
            If current.HasControls Then
                GridViewExportUtil.PrepareControlForExport(current)
            End If
            i = (i + 1)
        Loop
    End Sub