Returning a value from thread?

How do I return a value from a thread?

One of the easiest ways to get a return value from a thread is to use closures. Create a variable that will hold the return value from the thread and then capture it in a lambda expression. Assign the "return" value to this variable from the worker thread and then once that thread ends you can use it from the parent thread.

void Main()
{
  object value = null; // Used to store the return value
  var thread = new Thread(
    () =>
    {
      value = "Hello World"; // Publish the return value
    });
  thread.Start();
  thread.Join();
  Console.WriteLine(value); // Use the return value here
}

It depends on how do you want to create the thread and available .NET version:

.NET 2.0+:

A) You can create the Thread object directly. In this case you could use "closure" - declare variable and capture it using lambda-expression:

object result = null;
Thread thread = new System.Threading.Thread(() => { 
    //Some work...
    result = 42; });
thread.Start();
thread.Join();
Console.WriteLine(result);

B) You can use delegates and IAsyncResult and return value from EndInvoke() method:

delegate object MyFunc();
...
MyFunc x = new MyFunc(() => { 
    //Some work...
    return 42; });
IAsyncResult asyncResult = x.BeginInvoke(null, null);
object result = x.EndInvoke(asyncResult);

C) You can use BackgroundWorker class. In this case you could use captured variable (like with Thread object) or handle RunWorkerCompleted event:

BackgroundWorker worker = new BackgroundWorker();
worker.DoWork += (s, e) => {
    //Some work...
    e.Result = 42;
};
worker.RunWorkerCompleted += (s, e) => {
    //e.Result "returned" from thread
    Console.WriteLine(e.Result);
};
worker.RunWorkerAsync();

.NET 4.0+:

Starting with .NET 4.0 you could use Task Parallel Library and Task class to start your threads. Generic class Task<TResult> allows you to get return value from Result property:

//Main thread will be blocked until task thread finishes
//(because of obtaining the value of the Result property)
int result = Task.Factory.StartNew(() => {
    //Some work...
    return 42;}).Result;

.NET 4.5+:

Starting with .NET 4.5 you could also use async/await keywords to return value from task directly instead of obtaining Result property:

int result = await Task.Run(() => {
    //Some work...
    return 42; });

Note: method, which contains the code above shoud be marked with asynckeyword.

For many reasons using of Task Parallel Library is preferable way of working with threads.

I would use the BackgroundWorker approach and return the result in e.Result.

EDIT:

This is commonly associated with WinForms and WPF, but can be used by any type of .NET application. Here's sample code for a console app that uses BackgroundWorker:

using System;
using System.Threading;
using System.ComponentModel;
using System.Collections.Generic;
using System.Text;

namespace BGWorker
{
    class Program
    {
        static bool done = false;

        static void Main(string[] args)
        {
            BackgroundWorker bg = new BackgroundWorker();
            bg.DoWork += new DoWorkEventHandler(bg_DoWork);
            bg.RunWorkerCompleted += new RunWorkerCompletedEventHandler(bg_RunWorkerCompleted);
            bg.RunWorkerAsync();

            while (!done)
            {
                Console.WriteLine("Waiting in Main, tid " + Thread.CurrentThread.ManagedThreadId);
                Thread.Sleep(100);
            }
        }

        static void bg_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            Console.WriteLine("Completed, tid " + Thread.CurrentThread.ManagedThreadId);
            done = true;
        }

        static void bg_DoWork(object sender, DoWorkEventArgs e)
        {
            for (int i = 1; i <= 5; i++)
            {
                Console.WriteLine("Work Line: " + i + ", tid " + Thread.CurrentThread.ManagedThreadId);
                Thread.Sleep(500);
            }
        }
    }
}

Output:

Waiting in Main, tid 10
Work Line: 1, tid 6
Waiting in Main, tid 10
Waiting in Main, tid 10
Waiting in Main, tid 10
Waiting in Main, tid 10
Waiting in Main, tid 10
Work Line: 2, tid 6
Waiting in Main, tid 10
Waiting in Main, tid 10
Waiting in Main, tid 10
Waiting in Main, tid 10
Waiting in Main, tid 10
Work Line: 3, tid 6
Waiting in Main, tid 10
Waiting in Main, tid 10
Waiting in Main, tid 10
Waiting in Main, tid 10
Waiting in Main, tid 10
Work Line: 4, tid 6
Waiting in Main, tid 10
Waiting in Main, tid 10
Waiting in Main, tid 10
Waiting in Main, tid 10
Work Line: 5, tid 6
Waiting in Main, tid 10
Waiting in Main, tid 10
Waiting in Main, tid 10
Waiting in Main, tid 10
Waiting in Main, tid 10
Waiting in Main, tid 10
Completed, tid 6

2014 UPDATE

See @Roger's answer below.

https://stackoverflow.com/a/24916747/141172

He points out that you can use a Task that returns a Task<T>, and check Task<T>.Result.

Returning a value from thread?

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