_UserSize

The *_UserSize routine is called when sizing the RPC data buffer before the marshaling on the client or server side. The routine should work in terms of cumulative size. The StartingSize argument is the current buffer offset. The routine should return the cumulative size that includes the possible padding and then the data size. The starting size indicates the buffer offset for the user object and it may or may not be aligned properly. User's routine should account for all padding as necessary. In other words, the routine should return a new offset, after the user object. The sizing routine is not called if the wire size can be computed at the compile time. Note that for most unions, even if there are no pointers, the actual size of the wire representation may be determined only at the runtime.

This routine actually can return an over estimate as long as the marshaling routine does not use more than the sizing routine promised and so the marshaling buffer is not overwritten then or later (by subsequent objects).