ASP.NET: Unable to cast IO.StreamWriter to IO.MemoryStream

Question

I have a complex class module that make a lot of processing and return a object of type IO.StreamWriter, but I need cast this object to IO.MemoryStream, furthemore to Byte(). Finally it converts Byte() to Base64String that ajax consumes and generate a CSV file.

I made this with DynamicPDF.Document, but this library has a method that returns binary.

Dim objSW As IO.StreamWriter = EXP.ExportarBinario(objCommand, HttpContext.Current.Response.OutputStream, 1)
Dim objMS As New MemoryStream()

objSW.BaseStream.CopyTo(objMS)

Dim objArrayByte As Byte() = objMS.ToArray()

Dim strArrayByte64 As String = String.Format("data:{0};base64,{1}", "application/vnd.excel", Convert.ToBase64String(objArrayByte))

Return strArrayByte64