Using 'HasFile' to check whether a file exists before uploading

I'm writing an aspx form which gathers form data and uploads a file from the user's PC
In its simplest implementation, it seems to upload a file even if there is no file to upload!
In the destination directory, I end up with a properly named file with zero size.

To get around this, I'm trying to use the 'HasFile' property described below
http://msdn2.microsoft.com/en-us/library/system.web.ui.webcontrols.fileupload.hasfile(VS.80).aspx
(In practice, I'm using an If... Then... statement to check whether there is a file to upload, upload it and add it to a mail message as an attachment.)

However, since adding 'HasFile' to my code, I keep getting the following error message:
Compiler Error Message: BC30456: 'HasFile' is not a member of 'System.Web.UI.HtmlControls.HtmlInputFile'.

I understand the HasFile property is new in .net Framework Version 2.0 but am 99% certain that I have this version of dotNet - I've just downloaded the latest dotnetfx.exe file and ran a repair of my .net Framework installation in which it states it is version 2.0.

ANY IDEAS HOW TO FIX MY CODE SO THIS WORKS AS INTENDED?

----------Sample Code------------

<% @ Import Namespace="System.Data"       %>
<% @ Import Namespace="System.Data.OleDb" %>
<% @ Import Namespace="System.Web.Mail" %>
<% @ Import Namespace="System.Web.UI.WebControls" %>

Sub Button_Click( s As Object, e As EventArgs )
  Dim strDestinationFilePath As string
  Dim strUploadFileExtension As string
  If (inpFileUp.HasFile) Then
   strUploadFileExtension = Mid(inpFileUp.PostedFile.FileName, InStrRev(inpFileUp.PostedFile.FileName, ".") + 1)
   strDestinationFilePath = strPath & strFilename & strUploadFileExtension    'strPath and strFilename are declared/defined eslewhere
   inpFileUp.PostedFile.SaveAs( strUploadFilePath )
  End If
End Sub
...

...
<input id="inpFileUp" type="file" runat="server" />
 <asp:Button
   Text="submit"
   OnClick="Button_Click"
   runat="server" />

----------------------End of sample code----------------