ASP.net unzip not working

Public Shared Function DecompressorZippedFile(ByVal filename As String) As Stream
        msg = "Test decompression on file " & filename
        MsgBox(msg)
 
        Dim infile As FileStream
        Try
            ' Open the file as a FileStream object.
            infile = New FileStream(filename, FileMode.Open, FileAccess.Read, FileShare.Read)
            Dim buffer(infile.Length - 1) As Byte
            ' Read the file to ensure it is readable.
            Dim count As Integer = infile.Read(buffer, 0, buffer.Length)
            If count <> buffer.Length Then
                infile.Close()
                msg = "Test Failed: Unable to read data from file"
                MsgBox(msg)
                Return Nothing
            End If
 
            Dim zipStream As New GZipStream(infile, CompressionMode.Decompress)
            'MsgBox("zip stream length " & zipStream.Length)    'System.NotSupportedException
            Dim decompressedBuffer(buffer.Length + buffer_size) As Byte
            ' Use the ReadAllBytesFromStream to read the stream.
            Dim totalCount As Integer = ReadAllBytesFromStream(zipStream, decompressedBuffer)
            msg = "Decompressed " & totalCount & " bytes"
            MsgBox(msg)
 
            If Not CompareData(buffer, buffer.Length, decompressedBuffer, totalCount) Then
                msg = "Error. The two buffers did not compare."
                MsgBox(msg)
 
            End If
            infile.Close()
 
            Return infile
        Catch e As Exception
            msg = "Error: The file being read contains invalid data."
            MsgBox(msg)
            Return Nothing
        End Try
    End Function