unable to call custom code in ssrs 2008 r2

i have placed the following code in code section

Public Function ChangeWord(ByVal s As String) As String
   Dim strBuilder As New System.Text.StringBuilder(s)
   If s.Contains("com") Then
      strBuilder.Replace("com", "in")
      Return strBuilder.ToString()
      Else : Return s
   End If
End Function

i have column called empmail which contains employee mail ids. In data field of empmail column i have added the expression

=Code.ChangeWord(Fields!empmail.Value)
its saying ChangeWord is an unrecognized keyword

thank you