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
SELECT @@Identity
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();