Prepare action for usage in another thread

Question

I want do define an action that sets a property with a value (calculated by a worker thread). That action should be called in another thread context (UI thread).

To simplify the problem - it boils down to the question: why does this code don't work and what do I have to do to make it work like intended:

    public struct Person
    {
        public string Name;
    }

    Person person;
    Action<Person> action;

    public void Update()
    {
        person = new Person();

        new Thread(() =>
        {
            action = new Action<Person>(c => c.Name = "Me");
        }).Start();

        Thread.Sleep(1000);
        action(person);

        Debug.WriteLine(person.Name ?? "null");
    }

Why is this giving me "null" instead of "Sami"?

struct keyword is not present in java. This doesnot look like java. — Subir Kumar Sao, May 11 '12 at 19:20
Is this specifically how to set a UI component item from a background thread? — Tejs, May 11 '12 at 19:26

score 4 · Accepted Answer · edited May 23 '17 at 10:34

4

The type Person is a struct. This means that when you pass person as an argument, a copy is made. So the action updates a copy, a not person itself. If you change Person to a class, you will see that your example works.

For more information about the difference between structs and classes, see what's the difference between struct and class in .Net?

edited May 23 '17 at 10:34

Community

1
1

answered May 11 '12 at 19:27

Elian Ebbing

18,779
5
48
56

Thanks, that was the missing link! – Samara Lindner May 12 '12 at 08:28

score 1 · Answer 2 · answered May 11 '12 at 19:34

To prove @ElianEbbing's answer:

   class Program {
      Person person;
      Action<Person> action;
      static void Main(string[] args) {
         Program p = new Program();
         p.Update();

         Console.ReadLine();
      }
      public void Update() {
         person = new Person();

         new Thread(() => {
            action = new Action<Person>(c => c.Name = "Sami");
         }).Start();

         Thread.Sleep(1000);
         action(person);

         Console.WriteLine(person.Name ?? "null");
      }
   }

   public class Person {
      public string Name;
   }

Because you declare Person as a struct, it is a value type - meaning a copy of Person is made. The Person object that has Name set to "Sami" is not the same Person object you are using to write the name.

score 1 · Answer 3 · answered May 11 '12 at 19:35

1

You are using threads and actions wrong. By creating a thread that (sets an action to set the person's name), you are effectively doing nothing special.

    new Thread(() =>
    {
        action = new Action<Person>(c => c.Name = "Me");
    }).Start();
    Thread.Sleep(1000);

is the same as

action = new Action<Person>(c => c.Name = "Me");
action(person);

What you wanted was something like this:

action = new Action<Person>(c => c.Name = "Me");
new Thread(action).Start();
Thread.Sleep(1000);

answered May 11 '12 at 19:35

Oblivion2000

616
4
9

I was thinking the same thing but I didn't consider making corrections there. Although you do point out the correct use of action and Thread, it won't solve the OP's issue because `Person` is a struct. ;) – IAbstract May 11 '12 at 21:06
Oh, I see. You are absolutely correct. Thanks at all posters, u guys really helped me out! – Samara Lindner May 12 '12 at 08:29
Thought about this again - in the example I'm doing nothing special, but the thread is thought of as a worker thread. It may calculate something apart from the ui thread which i can use in the code within the action. I think I still need another thread to prepare the action. – Samara Lindner May 12 '12 at 09:05

Prepare action for usage in another thread

3 Answers3