Can I use Parallel.For with sql Commands?

Question

I have a class of range

public class avl_range
{
    public long start { get; set; }
    public long end { get; set; }
}

If I use a normal

Answer 1

Even if you could get it to work with MARS, connection objects are almost never thread safe anyway, you need to have a connection per thread. Parallel.ForEach has overloads to make this easy which have functions that run at the start of a thread and at the end.

var numbers = new List();

Func localInit => () => 
{
    var conn = new NpgsqlConnection(strConnection);
    conn.Open();
};

Action localFinally = (conn) => conn.Dispose();

Func forEachLoop = (number, loopState, conn) => //Begin definition of forLoop
{
     // only the console write line works ok
    Console.WriteLine(number.start + " - " + number.end);

    using (var cmd = new NpgsqlCommand())
    {
        cmd.Connection = conn;                            
        cmd.CommandText = String.Format( "SELECT * FROM avl_db.process_near_link({0}, {1});"
                                         , number.start
                                         , number.end);
        // here cause the error.
        using (var reader = cmd.ExecuteReader())
        {
            while (reader.Read())
            {
                Console.WriteLine(reader.GetString(0));
            }
        }
    }
    return conn;
};

Parallel.ForEach(numbers, localInit, forEachLoop, localFinally);

That being said, most of the time doing concurrent connections to a database is not the right idea, the bottleneck is likely elsewhere and you should use a profiler to see what is really slowing your program down and focus your efforts there.

---

Sample code for comments:

var numbers = GetDataForNumbers();
List results = new List();

Func> localInit => () => new List();

Func, List> forEachLoop = (number, loopState, localList) => //Begin definition of forLoop
{
    using (var conn = new NpgsqlConnection(strConnection))
    {
        conn.Open();

        //This line is going to slow your program down a lot, so i commented it out.
        //Console.WriteLine(number.start + " - " + number.end);

        using (var cmd = new NpgsqlCommand())
        {
            cmd.Connection = conn;                            
            cmd.CommandText = String.Format( "SELECT * FROM avl_db.process_near_link({0}, {1});"
                                             , number.start
                                             , number.end);
            using (var reader = cmd.ExecuteReader())
            {
                while (reader.Read())
                {
                    //Add a object to the thread local list, we don't need to lock here because we are the only thread with access to it.
                    localList.Add(reader.GetString(0));
                }
            }
        }
    }
    return localList;
};

Action> localFinally = localList => 
{
    //Combine the local list to the main results, we need to lock here as more than one thread could be merging at once.
    lock(results)
    {
        results.AddRange(localList);
    }
};

Parallel.ForEach(numbers, localInit, forEachLoop, localFinally);

//results now contains strings from all the threads here.