Replacing for loop with Tasks - passing variable

Question

Let's have 2 classes:

class MyClass
{
    public string str = "something";
    public int ind = 5;

    public MyClass()
    {

    }

    public void MyMethod()
    {
        //do something here
    }
}

And second one:

class ResultClass
{
    private string resultstring;
    private int resultint;

    public ResultClass(string resultstring, int resultint)
    {
        this.resultstring = resultstring;
        this.resultint = resultint;
    }

}

Now in the main form, let's say on form load we do:

private void Form1_Load(object sender, EventArgs e)
{
    List<MyClass> listMyClass = new List<MyClass>();
    List<ResultClass> listResultClass = new List<ResultClass>();
    MyClass itemMyClass1 = new MyClass();
    MyClass itemMyClass2 = new MyClass();
    listMyClass.Add(itemMyClass1);
    listMyClass.Add(itemMyClass2);
    for(int i = 0; i < listMyClass.Count; i++)
    {
        string s = listMyClass[i].str;
        int ind = listMyClass[i].ind;
        ResultClass result = new ResultClass(s, ind);
        listResultClass.Add(result);
    }
}

The problem:

I want to replace the for loop with tasks, so it can calculate all members of listResultClass simultaneously.

The best method so far I found is trying it like that:

private void Form1_Load(object sender, EventArgs e)
    {
        List<MyClass> listMyClass = new List<MyClass>();
        List<ResultClass> listResultClass = new List<ResultClass>();
        MyClass itemMyClass1 = new MyClass();
        MyClass itemMyClass2 = new MyClass();
        listMyClass.Add(itemMyClass1);
        listMyClass.Add(itemMyClass2);


        var tasks = listMyClass.Select(t => Task<ResultClass>.Factory.StartNew(() => TestTask(listMyClass))).ToArray();
        Task.WaitAll(tasks);
        listResultClass.AddRange(tasks.Select(task => task.Result));
    }

    private ResultClass TestTask(List<MyClass> listMyClass)
    {
        string s = listMyClass[i].str; //problem here
        int ind = listMyClass[i].ind; //and here
        ResultClass result = new ResultClass(s, ind);
        return result;
    }

But I have no idea how to steer the [i] value. I tried declaring an int and passing it to the TestTask, incrementing it at the end of the function, but it doesn't seem to work (probably because it's fired multiple times in the same time, making the int always 0).

How can I properly replace that loop?

Patrick Hofman · Accepted Answer · 2017-02-07 12:07:46Z

1

It seems to me you just need to pass in t, since that is the item you want to process:

listMyClass.Select(t => Task.Run(() => TestTask(t)))

Or, you can get the index of the item in the Select and pass that one in the method:

listMyClass.Select
            ( (t, idx) => Task.Run
                          (() => TestTask(listMyClass, idx))
            )

edited Feb 7, 2017 at 12:07

answered Feb 7, 2017 at 12:05

Patrick Hofman

158k23 gold badges270 silver badges343 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Janushoff Over a year ago

The second one with index worked perfectly for me, thanks!

score 1 · Accepted Answer · 2017-02-07 12:47:29Z

1

Besides using the appropriate override of the Select method with an index, as already shown in the accepted answer, let me also suggest that you should not block the form's load event. Make it async

    private async void Form1_Load(object sender, EventArgs e)
    {

and add the await keyword inside

        var tasks = listMyClass.Select((t, i) => Task.Run(() => 
            TestTask(listMyClass, i)));
        listResultClass.AddRange(await Task.WhenAll(tasks));
    }

edited Feb 7, 2017 at 12:47

answered Feb 7, 2017 at 12:37

user6996876

1 Comment

Janushoff Over a year ago

Thanks for improving the answer!

Collectives™ on Stack Overflow

Replacing for loop with Tasks - passing variable

2 Answers 2

1 Comment

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related