Question

It is clear that SqlConnections are pooled, so the using keyword seems perfect and this is the approach I always used and seen used.

For example…

public List<string> MyQuery()
{
    List<string> list = new List<string>();
    using (SqlConnection conn = new SqlConnection(connectionString))
    {
        conn.Open();
        // do some stuff with connection, process the resultset and fill the list.
    }
    return list;
}

The question is : with several queries for a webpage, is better to create the connection each time or to share the connection between queries, sharing it for an unit of work?

The performances will be much greater or is just a premature unnecessary microoptimization?

Is really important to close every connection as soon as possible or is better to try to pack the queries all together in an unit of work?

An example of an unit of work could be for example…

List<string> list1, list2;
string myvalue1, string myvalue2;

using (SqlConnection conn = new SqlConnection(connection))
{
    list1 = MyQuery1(conn);
    list2 = MyQuery2(conn);
    myvalue1 = MyQuery3(conn);
    myvalue2 = MyQuery4(conn);
}

This can happens during page load where multiple data should be get from database.

Answer 1

Note that with connection pooling, you actually are not creating new connections with each ‘new’… you are generally just getting one back from the pool.

Therefore, you should always open connections as late as you can, and close them as early as you can.

If a set of commands needs to be part of one transaction, then it needs to be done together. But beyond that, I think you should always close when you are done, and get a new one.

Even if you have no other code running between those commands… what if a thread context switch happens in between? Your connection object is needlessly open, doing nothing, waiting to get control back.

If you had closed it, it could have gone back to the pool.