Question

this works:

    //delegate
    Parallel.For(1023456789, 1033456789, delegate(long i)
            {
                if (i % 10000000 == 0) Console.WriteLine("{0:N0}", i);
                if (IsPanDigital(i))
                {
                    list.Add(i);
                }
            }
        );

    //lambda expression
    Parallel.For(1023456789, 1033456789, i =>
            {
                if (i%10000000 == 0) Console.WriteLine("{0:N0}", i);
                if (IsPanDigital(i))
                {
                    list.Add(i);
                }
            }
        );

Is it possible to rewrite this logic using a Func? I’ve tried here..doesn’t compile.

    var list = new List<long>();
    Parallel.For(1023456789, 1033456789, Blah(i, ref list));

public static Func<long> Blah(long i, ref List<long> list)
{
    if (i % 10000000 == 0) Console.WriteLine("{0:N0}", i);
    if (IsPanDigital(i))
    {
        list.Add(i);
    }
}

Am trying to see if it can be done.

Answer 1

You nearly had it:

public static void Blah(long i, ref List<long> list)
{
    if (i % 10000000 == 0) Console.WriteLine("{0:N0}", i);
    if (IsPanDigital(i))
    {
        list.Add(i);
    }
}

var list = new List<long>();
Parallel.For(1023456789, 1033456789, i => Blah(i, ref list));

(I changed Blah‘s return type to void, and add i => to wrap it in a lambda so it can match Action<long>)

Edit: Or change Blah to Action<long> and do some other minor refactoring:

public static Action<long> Blah(List<long> list)
{
    return i =>
    {
        if (i % 10000000 == 0) Console.WriteLine("{0:N0}", i);
        if (IsPanDigital(i))
        {
            list.Add(i);
        }
    };
}

var list = new List<long>();
Parallel.For(1023456789, 1033456789, Blah(list));

I think this second example is closer to what you were trying to achieve.

As @lee pointed out, the ref parameter, at least in the code you’ve shown, is not needed. ref parameters also can’t be used inside lambdas, which caused a compiler error, so I removed it. If you really need to use ref, go for the first example.

The reason you can’t use Func is because a Func<T> is something that returns a T value, and Parallel is looking for an Action<T>, which returns void.

@dtb pointed out something very important: “Note that adding to a list from multiple threads in parallel is not thread-safe.” You might be able to fix this by putting a lock around your list.Add method, locking to a private static object. I would guess the vast majority of your work is in calculating IsPanDigital, so I think this makes sense.