SQL Server transactions errors due to different Isolation Levels - sql-server

Question

I created the following stored procedure in order to examine the isolation level behavior in transaction:
CREATE PROCEDURE ReadCommittedIsolationLevel
AS
BEGIN
BEGIN TRANSACTION t1
BEGIN TRY
EXEC SnapShotIsolationLevel
COMMIT TRANSACTION
END TRY
BEGIN CATCH
PRINT ERROR_MESSAGE()
ROLLBACK TRANSACTION t1
END CATCH
END
CREATE PROCEDURE SnapShotIsolationLevel
AS
BEGIN
SET TRANSACTION ISOLATION LEVEL SNAPSHOT
BEGIN TRANSACTION t2
BEGIN TRY
SELECT TOP 20 *
FROM Orders
ORDER BY 1 DESC
COMMIT
END TRY
BEGIN CATCH
PRINT ERROR_MESSAGE()
ROLLBACK TRAN t2
END CATCH
END
And then I run this:
EXEC ReadCommittedIsolationLevel
I get this error:
Transaction failed in database 'MyDataBase' because the statement was run under snapshot isolation but the transaction did not start in snapshot isolation. You cannot change the isolation level of the transaction to snapshot after the transaction has started unless the transaction was originally started under snapshot isolation level.
Cannot roll back t2. No transaction or savepoint of that name was found.
If I remove the transactions and run it like plain stored procedures, it works fine.
Why is that?

Answer 1

The error gave you the correct explanation:
You cannot change the isolation level of the transaction to snapshot
after the transaction has started.
SQL Server does not have nested transactions, the only real transaction you have is the outer transaction started under Read Committed, the next begin tran does nothing except for incrementing ##trancount. You can read more on it here: A SQL Server DBA myth a day: (26/30) nested transactions are real by Paul Randal
Snapshot Isolation means consistent data at the transaction level, not at the statement level(RCSI), so you should change Isolation level to Snapshot at the beginning of the transaction or you cannot change it to Snapshot at all.