SqlConnection.ClearAllPools();
var conn = new SqlConnection(new SqlConnectionStringBuilder { DataSource = @".\sqlexpress", IntegratedSecurity = true }.ConnectionString);

Action printIsolationLevel = () =>
{
	var cmd = conn.CreateCommand();
	cmd.CommandText = @"SELECT CASE transaction_isolation_level 
						WHEN 0 THEN 'Unspecified' 
						WHEN 1 THEN 'ReadUncommitted' 
						WHEN 2 THEN 'ReadCommitted' 
						WHEN 3 THEN 'Repeatable' 
						WHEN 4 THEN 'Serializable' 
						WHEN 5 THEN 'Snapshot' END AS TRANSACTION_ISOLATION_LEVEL 
						FROM sys.dm_exec_sessions 
						where session_id = @@SPID";
	Console.WriteLine(cmd.ExecuteScalar());
};

conn.Open();
printIsolationLevel(); // "ReadCommitted"
conn.Close();

using (var scope = new TransactionScope(
					TransactionScopeOption.RequiresNew,
					new TransactionOptions { IsolationLevel = System.Transactions.IsolationLevel.Serializable }))
{
	conn.Open();
	printIsolationLevel(); // "Serializable"
	conn.Close();
}

conn.Open();
printIsolationLevel(); // "Serializable" !!?
conn.Close();