How Define Counter Inside Parallel.Foreach And Stop That Loop In A Specific Number

Question

How Can I Define Counter Inside Parallel.Foreach And Stop That Loot In A Specific Number?
I asked this Question because that counter inside Parallel.ForEach does not work in a regular action.
please see this little example :

static void Main(string[] args)
{
    int Count_Step = -1;
    string[] lines = new string[] 
    { 
        "0",
        "1",
        "2",
        "3",
        "4",
        "5",
        "6",
        "7",
        "8",
        "9",
        "10",
        "11",
        "12",
        "13",
        "14",
        "15",
        "16",
        "17",
        "18",
        "19"
    };

    List<string> list_lines = new List<string>(lines);

    ParallelOptions parallelOptions = new ParallelOptions();
    parallelOptions.MaxDegreeOfParallelism = 3;

    Parallel.ForEach(list_lines, parallelOptions, (line, state, index) =>
    {
        if (Count_Step == 10)
            state.Stop();
        Count_Step++;
        Console.WriteLine(index + " : " + line + " : " + Count_Step);
        //Thread.Sleep(5000);
    });

    Console.ReadLine();
}

i want 10 lines in output, not more!
how can i do that?

thanks in advance

Answer 1

If you only want to output 10 lines do this instead,

static void Main(string[] args)
{
    var lines = new List<string> 
    { 
        "0",
        "1",
        "2",
        "3",
        "4",
        "5",
        "6",
        "7",
        "8",
        "9",
        ...
    };

    var parallelOptions = new ParallelOptions
        {        
            MaxDegreeOfParallelism = 3
        };

    Parallel.ForEach(lines.Take(10), parallelOptions, (line, index) =>
    {
        Console.WriteLine("{0} : {1}", index, line);
        ////Thread.Sleep(5000);
    });

    Console.ReadLine();
}

alternatively

static void Main(string[] args)
{
    var lines = new List<string> 
    { 
        "0",
        "1",
        "2",
        "3",
        "4",
        "5",
        "6",
        "7",
        "8",
        "9",
        ...
    };

    var parallelOptions = new ParallelOptions
        {        
            MaxDegreeOfParallelism = 3
        };

    Parallel.For(0, 9, parallelOptions, i =>
    {
        Console.WriteLine("{0} : {1}", i, lines[i]);
        ////Thread.Sleep(5000);
    });

    Console.ReadLine();
}

or even

static void Main(string[] args)
{
    var lines = new List<string> 
    { 
        "0",
        "1",
        "2",
        "3",
        "4",
        "5",
        "6",
        "7",
        "8",
        "9",
        ...
    };

    Enumerable.Range(0, 10).AsParallel().WithDegreeOfParallelism(3).ForAll(i =>
    {
        Console.WriteLine("{0} : {1}", i, lines[i]);
        ////Thread.Sleep(5000);
    }

    Console.ReadLine();
}

---

If you really want your parallel iterations to update some value outside the loop, then you will need to ensure all updates and reads of the changing variable are thread-safe.

If you are incrementing an integer you could do it like this.

var stepCount = 0;

Enumerable.Range(0, 10).AsParallel().WithDegreeOfParallelism(3).ForAll(i =>
    {
        var thisCount = Interlocked.Increment(ref stepCount);
        Console.WriteLine("{0} : {1} : {2}", i, lines[i], thisCount);
        ////Thread.Sleep(5000);
    }

Console.ReadLine();

However, the output of thisCount is not garaunteed to be contiguous in the console window, there could be a thread switch between the two lines.

If you want to do somthing more sophisticated, like cancelling the processing part way through you should take a look at BlockingCollection.