I would simply inherit Stream and implement the required methods,  
including a field that holds a reference to an internal Stream to which  
you forward those methods, which is set to a MemoryStream until some point  
in time that you decide (for example, the MemoryStream becomes too large),  
at which point you copy the MemoryStream to a new FileStream, and then  
replace your internal Stream reference with the new FileStream.

That said, you should think carefully as to whether you really want to do  
this.  If you have a specific design requirement that the data wind up on  
the disk in a file accessible to the user once it gets too large, then  
perhaps your goal makes sense.  But if you are simply trying to do some  
sort of caching or something, in a way hidden to the user, you are likely  
to perform _worse_ trying to implement it yourself rather than just  
letting the Windows memory manager deal with it for you.

In other words, just use a MemoryStream, and let Windows decide what is  
"too large" as well as let Windows copy your data to the disk as necessary  
(via the normal virtual memory management strategies).

Pete