Nested Using statements affects performance

Question

     using (var db1 = new DataBase1Entities())
        {
           using (var db2 = new DataBase2Entities())
           {
                var list = (from obj in db2.Table1
                            where !db1.Table2.Any(i => i.Table2Col == obj.Table1Col)
                            select obj).ToList();
           }
        }

does anyone know how to retrieve value from one database table and compare it with another database table ? If the above code is correct, then will it cause performance issue ?

Nested using is common pattern to use multiple IDisposables. — Sinatr
– Sinatr, Commented Nov 9, 2017 at 12:43
It's not using but resource allocation/freeing that affect the performance; another resource consuming process is the code inside using - "code to retrieve data from db1 and compare it with db2" — Dmitrii Bychenko
– Dmitrii Bychenko, Commented Nov 9, 2017 at 12:45
"affect performance" relative to what? Obviously having code will take time to execute. What exactly is the question here? — Lasse V. Karlsen
– Lasse V. Karlsen, Commented Nov 9, 2017 at 12:48
"affect performance" relative to what Based on the context, presumably compared to not using nested using. i.e. have the second using outside of the first. — mjwills
– mjwills, Commented Nov 9, 2017 at 12:50
What type is db1? Or: which ORM is this? For the rest: race your horses. — Gert Arnold
– Gert Arnold, Commented Nov 9, 2017 at 13:09

MakePeaceGreatAgain · Accepted Answer · 2017-11-09 13:01:45Z

5

You seem not to know what a using-statement does. Effectively it´s nothing but a try-finally-block where within the finally there´s a call to Dispose. So your code is translated to the following:

DataBase1Entities db1 = null;
try 
{
    db1 = new DataBase1Entities();
    DataBase1Entities db2 = null;
    try 
    {
        db2 = new DataBase2Entities()
        // do something with db2
    }
    finally 
    {
        if(db2 != null) db2.Dispose(); }
    }
}
finally
{
    if(db1 != null) db1.Dispose();
}

So it´s not the using that may or may not have an effect on performance, but the Dispose. However you shouldn't bother for that at all, as you have to call Dispose. Doing not so is a really bad idea - in particular if it´s done because of any performance-issues. Dispose will release any unmanaged resources, e.g. file handlers. If you´re not calling it there´s no way to release those resources at all which will probably produce memory-leaks.

edited Nov 9, 2017 at 13:01

answered Nov 9, 2017 at 12:45

MakePeaceGreatAgain

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7 Comments

ProgrammingLlama Over a year ago

I don't know, but +1 from me.

M. Schena Over a year ago

don't know neither, but because of that +1 from me too.

Dmitrii Bychenko Over a year ago

if you check for null the code should be DataBase1Entities db1 = null; try {db1 = new DataBase1Entities(); ... } finally {if(db1 != null) db1.Dispose()};. Local variables (db1, db2) are not initialized with null, that's why if(db1 != null) doesn't sence in the current code

MakePeaceGreatAgain Over a year ago

@DmitryBychenko You´re right. Corrected appropriately.

Gert Arnold Over a year ago

I didn't downvote, but neither do I think that because of an unexplained downvote people should upvote. Try to understand the downvote instead. Like: this may be a nice explanation, but it doesn't answer the question(s).

|

mjwills · Accepted Answer · 2017-11-09 13:09:41Z

1

You could certainly remove the nesting if you wanted to. Something like (untested):

HashSet<YourType> bob;

using (var db1 = new DataBase1Entities())
{
    bob = new HashSet<YourType>(db1.Table2.Select(z => z.Table2Col);
}

using (var db2 = new DataBase2Entities())
{
     var list = (from obj in db2.Table1
                 where !bob.Contains(obj.Table1Col)
                 select obj).ToList();
}

You'd need to profile it to check whether it was any faster. Much of the speed benefit, if any, would likely be due to the use of the HashSet rather than the removal of the nested using.

answered Nov 9, 2017 at 13:09

mjwills

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