Question

I have a List of objects and I’d like to loop over that list and start a new thread, passing in the current object.

I’ve written an example of what I thought should do this, but it’s not working. Specifically, it seems like the threads are getting overwritten on each iteration. This doesn’t really make sense to me though because I’m making a new Thread object each time.

This is the test code I wrote

class Program
{
    static void Main(string[] args)
    {
        TestClass t = new TestClass();
        t.ThreadingMethod();
    }
}

class TestClass
{
    public void ThreadingMethod()
    {
        var myList = new List<MyClass> { new MyClass("test1"), new MyClass("test2") };

        foreach(MyClass myObj in myList)
        {
            Thread myThread = new Thread(() => this.MyMethod(myObj));
            myThread.Start();
        }
    }

    public void MyMethod(MyClass myObj) { Console.WriteLine(myObj.prop1); }
}

class MyClass
{
    public string prop1 { get; set; }

    public MyClass(string input) { this.prop1 = input; }
}

The output on my machine is

test2
test2

but I expected it to be

test1
test2

I tried changing the thread lines to

ThreadPool.QueueUserWorkItem(x => this.MyMethod(myObj));

but none of the threads started.

I think I just have a misunderstanding about how threads are supposed to work. Can someone point me in the right direction and tell me what I’m doing wrong?

Answer 1

This is because you’re closing over a variable in the wrong scope. The solution here is to use a temporary in your foreach loop:

    foreach(MyClass myObj in myList)
    {
        MyClass tmp = myObj; // Make temporary
        Thread myThread = new Thread(() => this.MyMethod(tmp));
        myThread.Start();
    }

For details, I recommend reading Eric Lippert’s post on this exact subject: Closing over the loop variable considered harmful