Question

I am doing this:

    private static void Main(string[] args)
    {
        var dict1 = new Dictionary<int, string>();
        var dict2 = new Dictionary<int, string>();
        DateTime t1 = DateTime.Now;
        for (int i = 1; i < 1000000; i++)
        {
            Parallel.Invoke(
                  () => dict1.Add(i, "Test" + i), 
                  () => dict2.Add(i, "Test" + i) );
        }
        TimeSpan t2 = DateTime.Now.Subtract(t1);

        Console.WriteLine(t2.TotalMilliseconds);

        Console.ReadLine();
    }

So doing a for loop 1 million time and adding items to two different dictionaries.
The problem is that it takes 11 secs which is more than 5 time the normal sequential method (without tasks/threads) which takes only 2 sec.
Don’t know why.

Answer 1

Like others have said or implied, parallel code is not always faster due to the overhead of parallelization.

That being said, your code is adding an item to 2 dictionaries in parallel 1M times while you should be adding 1M items to 2 dictionaries in parallel. The difference is subtle but the end result is code that is ~10% faster (on my machine) than your sequential case.

Parallel.Invoke(() => FillDictionary(dict1, 1000000), () => FillDictionary(dict2, 1000000));

...

private static void FillDictionary(Dictionary<int, string> toFill, int itemCount)
{
    for(int i = 0 ; i < itemCount; i++)
        toFill.Add(i, "test" + i);
}