What is the default ADO.NET isolation level?
You may not want to rely on defaults if you need to be absolute about your application data ops. These things can change between framework versions.
Highly recommend being explicit about isolation levels and session settings on all data calls - either via TransactionScope (which may mean escalation to DTC dependant on circumstances) or explicitly within the target stored proc call (if thats a route taken).
More details on DTC / TransactionScope: https://stackoverflow.com/a/9075800/1568341
TL;DR
A: Read committed for SQLS but don't assume a default