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Should context.SaveChanges() be called after adding all objects to context?

I am new to EF 4.1, i am trying to add list of object to sql table using EF and DB first approach.
1) Should i be calling context.SaveChanges() inside for loop after AddObject(Trans) or its okay to call it outside the loop?
2) Also, would all transactions be roll back in case there is an error?
I have read the Entity Framework already supports native database transactions, If any dirty ObjectStateEntry object cannot be persisted, then ObjectContext.SaveChanges will roll back the transaction and throw an exception.
3) What else can be done to increase the performance? I will be inserting around 1200 records in one go.
try
{
using (TransactionModel context = new TransactionModel())
{
try
{
foreach (var item in lstUnAllocatedTransaction)
{
Transaction Trans = new Transaction();
Trans.Amount = item.Amount;
Trans.Date = Datetime.Now;
Trans.DateAllocated = null;
context.Transactions.AddObject(Trans);
}
context.SaveChanges();
}
catch (Exception ex)
{
}
}
}
catch (Exception ex)
{
}
}

1) Should i be calling context.SaveChanges() inside for loop after AddObject(Trans) or its okay to call it outside the loop?
It's okay (and preferable) to call outside the loop if it's okay that the whole add operations are one transaction. If you need one transaction per item, then obviously you need to call it inside the loop.
2) Also, would all transactions be roll back in case there is an error? I have read the Entity Framework already supports native database transactions, If any dirty ObjectStateEntry object cannot be persisted, then ObjectContext.SaveChanges will roll back the transaction and throw an exception.
Only the last transaction will be rolled back. That's why 1) makes a difference. If you want to insert all-or-none then you need to add all and then call SaveChanges once.
3) What else can be done to increase the performance? I will be inserting around 1200 records in one go.
You could call AddRange instead of Add, otherwise I never had problems with a small number of records. If you think it's too slow, look into bulk inserts. There is nothing build into EF to do that, you will need an extension.

NHibernate rollback does not seem to work

My question is simple. I have the below code and user is still inserted. When I check the database just after SaveOrUpdate (and before the rollback) I see user is already inserted. It's like flush mode and transaction not working. Where am I going wrong?
using (var session = sessionFactory.OpenSession())
{
session.FlushMode = FlushMode.Never;
using (var tran = session.BeginTransaction())
{
var user = CreateUser();
session.SaveOrUpdate(user);
tran.Rollback();
}
}

If we check "just after SaveOrUpdate" ... we are still inside of one transaction. Lot of stuff could happen before final decision (commit or rollback).
And one of the operation would be to decide if object should be Created or Updated. So in case, that the ID generator is set to native/identity (e.g. SQL server) - NHibernate must execute the INSERT to get the ID. Lot of operation could be postponed, but to receive the ID - there is no way to wait.
So most likely your ID need to be obtained from DB.. and that's why INSERT happens. BUT other stuff won't be written into DB, until Flush() is called... So, I wouldn't mark described behaviour as something special.

Entity Framework - force commit without transaction

I have a loop that looks a bit like this:
CountsToReport = rep.Counts_Get().Where(x => x.Status == "Completed");
foreach (var count in CountsToReport.ToList())
{
//do some stuff
//send an email
count.Status = "Reported";
rep.SaveChanges();
}
Where "rep" is a repository wrapper around an EF context.
When this runs, the unfortunate email recipient gets deluged with spam because the SaveChanges call doesn't actually commit the changes - so the loop keep getting the same counts, emailing them, and marking them as "Reported" but doesn't actually save the change.
If you stop the loop, and re-start the code, the change saves successfully. You can confirm this scenario by stepping through the code: the EF object in C# changes its Status, but the underlying data in SQL doesn't change.
I'm presuming this is because SaveChanges doesn't actually commit the transaction - it just marks the data as having changed ready for the end of the transaction. But we're not using transactions anywhere else in the DB, and it'd be a bit of a pain to change the repository for this one use case.
Is there any other way I can force EF to commit this change and escape my endless loop of doom? Or am I mistaken about the cause?
EDIT: Putting this in the repository and calling it instead of SaveChanges his fixes it:
public void SaveWithTransaction()
{
using (var transaction = new System.Transactions.TransactionScope())
{
db.SaveChanges();
transaction.Complete();
}
}
But it seems ugly. Still interested to know if there's another way round.
EDIT: This is deceptive. It looks like it's just the old add/modified problem again. Marking the object as modified seems to help.

It is quite possible that entities in your CountsToReport are detached from context.
Thus not only ChangeTracker doesn't see any sort of change done over Count, it doesn't know anything about the entity at all.
To have the issue fixed:
iterate through collection of CountsToReport
attach each entity back to context, something like db.Counts.Attach(count)
modify the count.Status and call db.SaveChanges();
like this:
foreach (var count in CountsToReport.ToList())
{
//do some stuff
//send an email
rep.Counts.Attach(count);
count.Status = "Reported";
rep.SaveChanges();
}
Have a look at the following link for more information about entity states and how to deal with them:
http://msdn.microsoft.com/en-us/data/jj592676.aspx

transactions using C# to update a database

I'm doing a multi-layer MVVM WPF application, which is connected to an Oracle database. I need some explanations about TransactionScope. Consider the following example:
using (TransactionScope ts = new TransactionScope())
{
...
bank.setID(BankName, Branch);
check.addCheck(check);
...
ts.Complete();
}
This code is for explanation only: bank.setID() updates a record, while addCheck actually inserts a record. I couldn't figure out how to test this. I wanted to execute the update and shutdown of the database before inserting with the second method, and then check to see if the update is rolled back. Is this already right? Am I on the right track ? Is this the purpose of TransactionScope?
Thanks in advance

Edit: I wasn't sure at first if you understood DB transactions, so here's a very short description:
TransactionScope is designed to wrap a database transaction with a mechanism that is exception safe.
You use it to wrap a set of operations that should be atomic with a transaction, so if you fail on one update, all of the DB actions within that transaction get rolled back.
You use it in a using block so that if your code throws an exception, the transaction gets rolled back, instead of committed to the database.
I wanted to execute the update and shutdown the database before inserting with the second method, and then check to see if the update is rolled back ... Am I on the right track ?
Yep. Your code should already handle it:
If the DB is shut down before addCheck does an insert, then the insert should throw an exception
Since an exception gets thrown, Complete should never get called
The finally block should automatically roll back the transaction when the (hidden) finally block is reached, and Complete hasn't been called
I couldn't figure out how to test this
If you would like to write a unit test for your code, then I wouldn't suggest the "take the DB offline in the middle of execution" approach.
Instead, I'd suggest making your DB logic flexible enough to point at different DBs. Then, either remove the table or some of the columns that addCheck inserts to. Try to set up your DB so that setID succeeds, but addCheck fails.

TransactionScope is well documented. MSDN it.
to test how it works, no need to take database offline. use this snippet:
using (TransactionScope ts = new TransactionScope())
{
try
{
...
bank.setID(BankName, Branch);
throw new System.InvalidOperationException("sht happens");
check.addCheck(check);
...
ts.Complete();
}
catch
{
//catch the exception
//ts.Complete() is not called, thus update/insert rollbacks
}
}

How to clear the DataContext cache on Linq to Sql

I'm using Linq to Sql to query some database, i only use Linq to read data from the DB, and i make changes to it by other means. (This cannot be changed, this is a restriction from the App that we are extending, all updates must go trough its sdk).
This is fine, but I'm hitting some cache problems, basically, i query a row using Linq, then i delete it trough external means, and then i create a new row externally if i query that row again using linq i got the old (cached) data.
I cannot turn off Object Tracking because that seems to prevent the data context from auto loading associated propertys (Foreign Keys).
Is there any way to clear the DataContex cache?
I found a method sufring the net but it doesn't seem safe: http://blog.robustsoftware.co.uk/2008/11/clearing-cache-of-linq-to-sql.html
What do you think? what are my options?.

If you want to refresh a specific object, then the Refresh() method may be your best bet.
Like this:
Context.Refresh(RefreshMode.OverwriteCurrentValues, objectToRefresh);
You can also pass an array of objects or an IEnumerable as the 2nd argument if you need to refresh more than one object at a time.
Update
I see what you're talking about in comments, in reflector you see this happening inside .Refresh():
object objectByKey = context.Services.GetObjectByKey(trackedObject.Type, keyValues);
if (objectByKey == null)
{
throw Error.RefreshOfDeletedObject();
}
The method you linked seems to be your best option, the DataContext class doesn't provide any other way to clear a deleted row. The disposal checks and such are inside the ClearCache() method...it's really just checking for disposal and calling ResetServices() on the CommonDataServices underneath..the only ill-effect would be clearing any pending inserts, updates or deletes that you have queued.
There is one more option, can you fire up another DataContext for whatever operation you're doing? It wouldn't have any cache to it...but that does involve some computational cost, so if the pending insert, update and deletes aren't an issue, I'd stick with the ClearCache() approach.

I made this code to really CLEAR the "cached" entities, detaching it.
var entidades = Ctx.ObjectStateManager.GetObjectStateEntries(EntityState.Added | EntityState.Deleted | EntityState.Modified | EntityState.Unchanged);
foreach (var objectStateEntry in entidades)
Ctx.Detach(objectStateEntry.Entity);
Where Ctx are my Context.

You should be able to just requery the result sets that are using this objects. This would not pull a cached set, but would actually return the final results. I know that this may not be as easy or feasible depending on how you setup your app...
HTH.