When I try to invoke this command, it appears to be ignored. I've learned that this command can be very useful.
Consider the problem of a long da.fill operation. Normally you start it and just have to wait. Even if you run it in a background thread, it has to play out. With the SelectCommand.Cancel (which works for other providers), I can handle the datatable rowchanged event and look for a global "cancelled" variable I set elsewhere; if set I issue the SelectCommand.Cancel method and I'm good to go.
Strongly recommended.
P.S., the command works for OleDB and SQLServer, but gives a non-supported error in Firebird.