Compress serialized class data

Question

I have this class to encode and decode a serialized class to a string and back.

Imports System.IO
Imports System.Runtime.Serialization.Formatters.Binary
Imports System.Runtime.Serialization
Imports System.Text


Public Class FormData


  Public Function Encode(fdi As FormDataItem) As String
    Dim formatter As New BinaryFormatter
    Dim ms As New MemoryStream

    formatter.Serialize(ms, fdi)

    ms.Position = 0

    Dim output As New StringBuilder
    Dim waarde As String = ""

    Do While ms.Position < ms.Length
      waarde = Hex(ms.ReadByte)
      If waarde.Length = 1 Then waarde = "0" + waarde

      output.Append(waarde)

    Loop

    Return output.ToString
  End Function

  Public Function Decode(text As String) As FormDataItem
    Dim ms As New MemoryStream()
    Dim waarde As Byte
    Dim getal As String

    For ab = 0 To text.Length - 1 Step 2
      getal = text.Substring(ab, 2)
      waarde = CByte(Val("&h" + getal))

      ms.WriteByte(waarde)
    Next

    ms.Position = 0

    Dim formatter As New BinaryFormatter
    Dim fdi As New FormDataItem

    ms.Position = 0

    Try
      fdi = DirectCast(formatter.Deserialize(ms), FormDataItem)
    Catch ex As Exception
      Return Nothing
    End Try

    Return fdi
  End Function
End Class

Now I would like to compress the string in the encode function and decompress in the decode function because this data is used in URLs.

I'm a bit stuck on where and how to compress the data. Do I do that right after serialization or compress just the string?

any help is appreciated!

rg. Eric