Access return value from Thread.Start()'s delegate function

Question

I've got a program that executes a method through a Thread.Start. The method has a return value that I'd like to get access to. Is there a way to do this? Here's a sampling...

var someValue = "";
Thread t = new Thread(delegate() { someValue = someObj.methodCall(); });

t.Start();

while (t.isAlive) Thread.Sleep(1000);

// Check the value of someValue

So once the while loop ends, the someValue should be set - but because it's executed in another thread it doesn't get set. Is there a simple way to get access to it?

Answer 1

When the caller and the threaded method share a variable, you already have access to it - once the thread has completed, you just check someValue.

Of course, you have to know when the threaded method is complete for this to be useful. At the bottom, there are two ways to do this:

• Send a callback into the threaded method that it can execute when it's finished. You can pass your callback method someValue. You can use this technique if you don't care when the callback executes.

• Use a WaitHandle of some kind (or Thread.Join). These tell you when a resource is ready or an event has completed. This technique is useful if you want to start a thread, do something else, then wait until the thread completes before proceeding. (In other words, it's useful if you want to sync back up with the thread, just not right away.)

Answer 2

I can't recreate your issue, I've got the same code and I'm seeing the expected result. If you're just going to sleep the current thread until it's complete you could just call .Join() on the thread and wait to be sure it's done executing.

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    string someValue = "";

    private void Form1_Load(object sender, EventArgs e)
    {

        Thread t = new Thread(delegate() { someValue = "asdf"; });

        t.Start();
        t.Join();

        //while (t.IsAlive) Thread.Sleep(1000);

        System.Diagnostics.Debug.Print(someValue);

    }
}

Answer 3

One of possible methods to return a value from a Thread is to use a context class as a parameter object. It can be used to pass parameters and retrieve the result as well.

If on the other hand you could use a BackgroundWorker class, it has already a dedicated Result object - that works the same way. But BackgroundWorker cannot be used for some purposes (for instance, it doesn't support STA Apartment State).

Keep in mind that you shouldn't read from ctx.Result until the thread is finished (i.e. t.IsAlive == false).

    void runThread()        
    {
        ThreadContext ctx = new ThreadContext();
        ctx.Value = 8;

        Thread t = new Thread(new ParameterizedThreadStart(MyThread));
        //t.SetApartmentState(ApartmentState.STA); // required for some purposes
        t.Start(ctx);

        // ...
        t.Join();

        Console.WriteLine(ctx.Result);
    }

    private static void MyThread(object threadParam)
    {
        ThreadContext context = (ThreadContext)threadParam;

        context.Result = context.Value * 4; // compute result
    }

    class ThreadContext
    {
        public int Value { get; set; }
        public int Result { get; set; }
    }

Answer 4

You can retrieve data from Thread function using delegate callback. The delegate can serve as a bridge between thread and the caller. For example:

public delegate void DelReturnValue(string value);
public class SayHello
{
    private string _name;
    private DelReturnValue _delReturnValue;

    public SayHello(string name, DelReturnValue delReturnValue)
    {
        _name = name;
        _delReturnValue = delReturnValue;
    }

    public void SayHelloMethod()
    {
        _delReturnValue(_name);
    }
}

public class Caller
{
    private static string _returnedValue;
    public static void ReturnValue(string value)
    {
        _returnedValue = value;
    }

    public static void Main()
    {
        DelReturnValue delReturnValue=new DelReturnValue(ReturnValue);
        SayHello sayHello = new SayHello("test", delReturnValue);
        Thread newThread = new Thread(new ThreadStart(sayHello.SayHelloMethod));
        newThread.Start();
        Thread.Sleep(1000);
        Console.WriteLine("value is returned: " + _returnedValue);
    }
}