Does a C# using statement perform try/finally?

Question

Suppose that I have the following code:

private void UpdateDB(QuoteDataSet dataSet, Strint tableName)
{
    using(SQLiteConnection conn = new SQLiteConnection(_connectionString))
    {
        conn.Open();
        using (SQLiteTransaction transaction = conn.BeginTransaction())
        {
            using (SQLiteCommand cmd = new SQLiteCommand("SELECT * FROM " + tableName, conn))
            {
                using (SQLiteDataAdapter sqliteAdapter = new SQLiteDataAdapter())
                {
                    sqliteAdapter.Update(dataSet, tableName);
                }
            }
            transaction.Commit();
        }
    }
}

The C# documentation states that with a using statement the object within the scope will be disposed and I've seen several places where it's suggested that we don't need to use try/finally clause.

I usually surround my connections with a try/finally, and I always close the connection in the finally clause. Given the above code, is it reasonable to assume that the connection will be closed if there is an exception?

Similar to http://stackoverflow.com/questions/278902/using-statement-vs-try-finally — Mark Byers, Apr 25 '10 at 21:37
Not an answer to your question. Please read about sql injection attacks. — Raj Kaimal, Apr 25 '10 at 21:39

score 20 · Accepted Answer · answered Apr 25 '10 at 21:31

20

You are correct; the using statement compiles to a try / finally block.

The compiler transforms using(resource) statement; into the following code:

{
   ResourceType resource = expression;
   try {
      statement;
   }
   finally {
      if (resource != null) ((IDisposable)resource).Dispose();
   }
}

(The cast to IDisposable is in case ResourceType implements IDisposable explicitly.

answered Apr 25 '10 at 21:31

SLaks

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what is best to release a resource which can not be instantiated within using statement or being reused or being passed as out param? – bjan May 11 '12 at 05:17
@bjan: It depends on the situation. – SLaks May 11 '12 at 12:49

score 10 · Answer 2 · edited Aug 26 '22 at 17:25

Yes, you either need to use a try/finally or a using statement. You don't need both.

A using statement is almost the same as a try/finally except that in C# 3 you can't reassign to the variable inside the using block.

using (IDisposable d = foo())
{
     d = null; // Error:  Cannot assign to 'd' because it is a 'using variable'
}

Previously you could reassign but the original object would still be disposed, not the newly assigned object and you would also get this compile warning:

Possibly incorrect assignment to local 'd' which is the argument to a using or lock statement. The Dispose call or unlocking will happen on the original value of the local.

what is best to release a resource which can not be instantiated within using statement or being reused or being passed as out param? — bjan, May 11 '12 at 05:18

LukeH · Answer 3 · 2010-04-25T21:41:04.533

Yes, the using statement is pretty much just shorthand for a try ... finally block.

For example, this code...

using (MyDisposableType foo = new MyDisposableType())
{
    foo.DoSomething();
}

...would equate to the following...

{
    MyDisposableType foo = new MyDisposableType();
    try
    {
        foo.DoSomething();
    }
    finally
    {
        if (foo != null)
            ((IDisposable)foo).Dispose();
    }
}

score 1 · Answer 4 · answered Apr 25 '10 at 21:27

1

You can assume that the connection will be closed if you get an exception.

answered Apr 25 '10 at 21:27

MadBoy

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score 1 · Answer 5 · answered Apr 25 '10 at 21:30

Using() ensures that the item instantiated within the parameters will be disposed of regardless of that happens within the associated code block. This includes closing the database connection assuming that SQLiteConnection handles its disposal correctly.

Does a C# using statement perform try/finally?

5 Answers5

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