how can I cast a currentUser.ProviderUserKey to GUID type

 I've reading about this artical:
http://www.asp.net/learn/security/tutorial-08-vb.aspx
when I trying to write the code below in C#, I don't know how to cast the currentUser.ProviderUserKey to GUID Type.
here is the VB.CODE:
 
1 Protected Sub UserProfileDataSource_Selecting(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.SqlDataSourceSelectingEventArgs) Handles UserProfileDataSource.Selecting 2 ' Get a reference to the currently logged on user 3 Dim currentUser As MembershipUser = Membership.GetUser() 4 5 ' Determine the currently logged on ...