passing complex object to web service method in VB.NET

I am developing an applocation which reads mails in outlook inbox alongwith 
attachments and all these informmation is stored in a collection which i nee
d to pass it to a web service, which will perform certain operations on that
 collection and then return that modified collection to the client.
but when i am trying to pass a simple collection of strings to my web servic
e it gives me an error "You must implement Add(System.Object) as it inherits
 from IColletion".
Can someone tell me the exact way as to how i can pass collection to and fro
m a web service in VB.NET

Thanks in advance