And autonumber column is the same as any other column, except you can't write to it yourself.
So a normal SELECT will retrieve it. If you are trying to INSERT a new row and then get the number that was assigned, then
immediately after the INSERT will do it.
If you use ExecuteScalar instead of ExecuteReader or DataAdapter.Fill then you can just cast the result to an integer:
int lastNo = (int) myCommand.ExecuteScalar();