从线程返回一个值？

Question

如何从线程返回值？

Answer 1

从线程获取返回值的最简单方法之一是使用闭包。创建一个变量来保存线程的返回值，然后在 lambda 表达式中捕获它。从工作线程将“返回”值分配给该变量，然后一旦该线程结束，您就可以从父线程中使用它。

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
}

Answer 2

这取决于你想如何创建线程和可用的 .NET 版本：

.NET 2.0+：

A) 您可以直接创建Thread 对象。在这种情况下，您可以使用“闭包” - 声明变量并使用 lambda 表达式捕获它：

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

B) 您可以使用委托和IAsyncResult 并从EndInvoke() 方法返回值：

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

C) 你可以使用BackgroundWorker 类。在这种情况下，您可以使用捕获的变量（如 Thread 对象）或处理 RunWorkerCompleted 事件：

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+：

从 .NET 4.0 开始，您可以使用 Task Parallel Library 和 Task 类来启动您的线程。泛型类Task<TResult> 允许您从Result 属性中获取返回值：

//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+：

从 .NET 4.5 开始，您还可以使用 async/await 关键字直接从任务返回值，而不是获取 Result 属性：

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

注意：方法，包含上面的代码应该用asynckeyword标记。

出于多种原因，使用任务并行库是处理线程的首选方式。

Answer 3

我会使用BackgroundWorker 方法并在 e.Result 中返回结果。

编辑：

这通常与 WinForms 和 WPF 相关联，但可以由任何类型的 .NET 应用程序使用。以下是使用 BackgroundWorker 的控制台应用的示例代码：

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);
            }
        }
    }
}

输出：

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 年更新

请参阅下面@Roger 的回答。

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

他指出您可以使用返回Task<T> 的Task，并检查Task<T>.Result。

Answer 4

线程不是方法——你通常不会“返回”一个值。

但是，如果您尝试从某些处理的结果中取回一个值，您有很多选择，其中两个主要是：

• 您可以同步共享数据，并进行适当的设置。
• 您还可以通过某种形式的回调将数据传回。

这实际上取决于您创建线程的方式、您希望如何使用它，以及您使用的语言/框架/工具。

Answer 5

我最喜欢的类，只需 2 行代码即可在另一个线程上运行任何方法。

class ThreadedExecuter<T> where T : class
{
    public delegate void CallBackDelegate(T returnValue);
    public delegate T MethodDelegate();
    private CallBackDelegate callback;
    private MethodDelegate method;

    private Thread t;

    public ThreadedExecuter(MethodDelegate method, CallBackDelegate callback)
    {
        this.method = method;
        this.callback = callback;
        t = new Thread(this.Process);
    }
    public void Start()
    {
        t.Start();
    }
    public void Abort()
    {
        t.Abort();
        callback(null); //can be left out depending on your needs
    }
    private void Process()
    {
        T stuffReturned = method();
        callback(stuffReturned);
    }
}

用法

    void startthework()
    {
        ThreadedExecuter<string> executer = new ThreadedExecuter<string>(someLongFunction, longFunctionComplete);
        executer.Start();
    }
    string someLongFunction()
    {
        while(!workComplete)
            WorkWork();
        return resultOfWork;
    }
    void longFunctionComplete(string s)
    {
        PrintWorkComplete(s);
    }

请注意 longFunctionComplete 不会与 startthework 在同一线程上执行。

对于带参数的方法，您始终可以使用闭包，或扩展类。

Answer 6

只需使用委托方法。

int val;
Thread thread = new Thread(() => { val = Multiply(1, 2); });
thread.Start();

现在制作可以在另一个线程上工作的乘法函数：

int Multiply(int x, int y)
{
    return x * y;
}

Answer 7

这是一个使用委托的简单示例...

void Main()
{
   DoIt d1 = Doer.DoThatThang;
   DoIt d2 = Doer.DoThatThang;

   IAsyncResult r1 = d1.BeginInvoke( 5, null, null );
   IAsyncResult r2 = d2.BeginInvoke( 10, null, null );

   Thread.Sleep( 1000 );

   var s1 = d1.EndInvoke( r1 );
   var s2 = d2.EndInvoke( r2 );

   s1.Dump(); // You told me 5
   s2.Dump(); // You told me 10
}

public delegate string DoIt( int x );

public class Doer
{
  public static string DoThatThang( int x  )
  {
    return "You told me " + x.ToString();
  }
}

Threading in C# 上有一个很棒的线程系列。

Answer 8

使用最新的 .NET Framework，可以使用任务从单独的线程返回值，其中 Result 属性会阻塞调用线程，直到任务完成：

  Task<MyClass> task = Task<MyClass>.Factory.StartNew(() =>
  {
      string s = "my message";
      double d = 3.14159;
      return new MyClass { Name = s, Number = d };
  });
  MyClass test = task.Result;

详情请见http://msdn.microsoft.com/en-us/library/dd537613(v=vs.110).aspx

Answer 9

我在尝试获取在线程中执行的方法的返回值时遇到了这个线程。我想我会发布我的解决方案。

此解决方案使用一个类来存储要（间接）执行的方法并存储返回值。该类可用于任何函数和任何返回类型。您只需使用返回值类型实例化对象，然后通过 lambda（或委托）传递函数以调用。

---

C# 3.0 实现

---

public class ThreadedMethod<T>
{

    private T mResult;
    public T Result 
    {
        get { return mResult; }
        private set { mResult = value; }
    }

    public ThreadedMethod()
    {
    }

    //If supporting .net 3.5
    public void ExecuteMethod(Func<T> func)
    {
        Result = func.Invoke();
    }

    //If supporting only 2.0 use this and 
    //comment out the other overload
    public void ExecuteMethod(Delegate d)
    {
        Result = (T)d.DynamicInvoke();
    }
}

要使用此代码，您可以使用 Lambda（或委托）。这是使用 lambdas 的示例：

ThreadedMethod<bool> threadedMethod = new ThreadedMethod<bool>();
Thread workerThread = new Thread((unused) => 
                            threadedMethod.ExecuteMethod(() => 
                                SomeMethod()));
workerThread.Start();
workerThread.Join();
if (threadedMethod.Result == false) 
{
    //do something about it...
}

---

VB.NET 2008 实现

---

任何使用 VB.NET 2008 的人都不能使用带有非值返回方法的 lambda。这会影响ThreadedMethod 类，因此我们将让ExecuteMethod 返回函数的值。这不会伤害任何东西。

Public Class ThreadedMethod(Of T)

    Private mResult As T
    Public Property Result() As T
        Get
            Return mResult
        End Get
        Private Set(ByVal value As T)
            mResult = value
        End Set
    End Property

    Sub New()
    End Sub

    'If supporting .net 3.5'
    Function ExecuteMethod(ByVal func As Func(Of T)) As T
        Result = func.Invoke()
        Return Result
    End Function

    'If supporting only 2.0 use this and' 
    'comment out the other overload'
    Function ExecuteMethod(ByVal d As [Delegate]) As T
        Result = DirectCast(d.DynamicInvoke(), T)
        Return Result
    End Function

End Class

Answer 10

C# 中用于启动线程的 ThreadStart 委托具有返回类型“void”。

如果您希望从线程获取“返回值”，您应该写入共享位置（以适当的线程安全方式）并在线程完成执行时从该位置读取。

Answer 11

如果您不想使用 BackgroundWorker，而只是使用常规线程，那么您可以触发事件以返回如下数据：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Threading;

namespace ThreadWithDataReturnExample
{
    public partial class Form1 : Form
    {
        private Thread thread1 = null;

        public Form1()
        {
            InitializeComponent();

            thread1 = new Thread(new ThreadStart(this.threadEntryPoint));
            Thread1Completed += new AsyncCompletedEventHandler(thread1_Thread1Completed);
        }

        private void startButton_Click(object sender, EventArgs e)
        {
            thread1.Start();
            //Alternatively, you could pass some object
            //in such as Start(someObject);
            //With apprioriate locking, or protocol where
            //no other threads access the object until
            //an event signals when the thread is complete,
            //any other class with a reference to the object 
            //would be able to access that data.
            //But instead, I'm going to use AsyncCompletedEventArgs 
            //in an event that signals completion
        }

        void thread1_Thread1Completed(object sender, AsyncCompletedEventArgs e)
        {
            if (this.InvokeRequired)
            {//marshal the call if we are not on the GUI thread                
                BeginInvoke(new AsyncCompletedEventHandler(thread1_Thread1Completed),
                  new object[] { sender, e });
            }
            else
            {
                //display error if error occurred
                //if no error occurred, process data
                if (e.Error == null)
                {//then success

                    MessageBox.Show("Worker thread completed successfully");
                    DataYouWantToReturn someData = e.UserState as DataYouWantToReturn;
                    MessageBox.Show("Your data my lord: " + someData.someProperty);

                }
                else//error
                {
                    MessageBox.Show("The following error occurred:" + Environment.NewLine + e.Error.ToString());
                }
            }
        }

        #region I would actually move all of this into it's own class
            private void threadEntryPoint()
            {
                //do a bunch of stuff

                //when you are done:
                //initialize object with data that you want to return
                DataYouWantToReturn dataYouWantToReturn = new DataYouWantToReturn();
                dataYouWantToReturn.someProperty = "more data";

                //signal completion by firing an event
                OnThread1Completed(new AsyncCompletedEventArgs(null, false, dataYouWantToReturn));
            }

            /// <summary>
            /// Occurs when processing has finished or an error occurred.
            /// </summary>
            public event AsyncCompletedEventHandler Thread1Completed;
            protected virtual void OnThread1Completed(AsyncCompletedEventArgs e)
            {
                //copy locally
                AsyncCompletedEventHandler handler = Thread1Completed;
                if (handler != null)
                {
                    handler(this, e);
                }
            }
        #endregion

    }
}

Answer 12

线程实际上没有返回值。但是，如果您创建一个委托，您可以通过 BeginInvoke 方法异步调用它。这将在线程池线程上执行该方法。您可以通过EndInvoke 获取任何返回值，例如调用。

例子：

static int GetAnswer() {
   return 42;
}

...

Func<int> method = GetAnswer;
var res = method.BeginInvoke(null, null); // provide args as needed
var answer = method.EndInvoke(res);

GetAnswer 将在线程池线程上执行，完成后您可以通过EndInvoke 检索答案，如图所示。

Answer 13

BackgroundWorker 非常适合为 Windows 窗体进行开发。

假设你想来回传递一个简单的类：

class Anything {
    // Number and Text are for instructional purposes only
    public int Number { get; set; }
    public string Text { get; set; }
    // Data can be any object - even another class
    public object Data { get; set; }
}

我编写了一个简短的类，它执行以下操作：

• 创建或清除列表
• 开始循环
• 在循环中，为列表创建一个新项目
• 在循环中，创建一个线程
• 在循环中，将项目作为参数发送到线程
• 在循环中，启动线程
• 在循环中，将线程添加到要观看的列表中
• 循环后，加入每个线程
• 所有连接都完成后，显示结果

从线程例程内部：

• 调用 lock 以便一次只有 1 个线程可以进入此例程（其他线程必须等待）
• 发布有关该项目的信息。
• 修改项目。
• 线程完成后，数据会显示在控制台上。

添加delegate 有助于将数据直接发送回主线程，但如果某些数据项不是线程安全的，则可能需要使用Invoke。

class AnyTask {

    private object m_lock;

    public AnyTask() {
        m_lock = new object();
    }
    // Something to use the delegate
    public event MainDelegate OnUpdate;

    public void Test_Function(int count) {
        var list = new List<Thread>(count);
        for (var i = 0; i < count; i++) {
            var thread = new Thread(new ParameterizedThreadStart(Thread_Task));
            var item = new Anything() {
                Number = i,
                Text = String.Format("Test_Function #{0}", i)
            };
            thread.Start(item);
            list.Add(thread);
        }
        foreach (var thread in list) {
            thread.Join();
        }
    }

    private void MainUpdate(Anything item, bool original) {
        if (OnUpdate != null) {
            OnUpdate(item, original);
        }
    }

    private void Thread_Task(object parameter) {
        lock (m_lock) {
            var item = (Anything)parameter;
            MainUpdate(item, true);
            item.Text = String.Format("{0}; Thread_Task #{1}", item.Text, item.Number);
            item.Number = 0;
            MainUpdate(item, false);
        }
    }

}

要对此进行测试，请创建一个小控制台应用程序，并将其放入 Program.cs 文件中：

// A delegate makes life simpler
delegate void MainDelegate(Anything sender, bool original);

class Program {

    private const int COUNT = 15;
    private static List<Anything> m_list;

    static void Main(string[] args) {
        m_list = new List<Anything>(COUNT);
        var obj = new AnyTask();
        obj.OnUpdate += new MainDelegate(ThreadMessages);
        obj.Test_Function(COUNT);
        Console.WriteLine();
        foreach (var item in m_list) {
            Console.WriteLine("[Complete]:" + item.Text);
        }
        Console.WriteLine("Press any key to exit.");
        Console.ReadKey();
    }

    private static void ThreadMessages(Anything item, bool original) {
        if (original) {
            Console.WriteLine("[main method]:" + item.Text);
        } else {
            m_list.Add(item);
        }
    }

}

这是我得到的截图：

我希望其他人能理解我试图解释的内容。

我喜欢处理线程和使用委托。它们让 C# 变得很有趣。

附录：适用于 VB 编码人员

我想看看将上面的代码编写为 VB 控制台应用程序所涉及的内容。转换涉及到一些我没想到的事情，所以我将在此处更新此线程以供那些想知道如何在 VB 中线程化的人使用。

Imports System.Threading

Delegate Sub MainDelegate(sender As Anything, original As Boolean)

Class Main

    Private Const COUNT As Integer = 15
    Private Shared m_list As List(Of Anything)

    Public Shared Sub Main(args As String())
        m_list = New List(Of Anything)(COUNT)
        Dim obj As New AnyTask()
        AddHandler obj.OnUpdate, New MainDelegate(AddressOf ThreadMessages)
        obj.Test_Function(COUNT)
        Console.WriteLine()
        For Each item As Anything In m_list
            Console.WriteLine("[Complete]:" + item.Text)
        Next
        Console.WriteLine("Press any key to exit.")
        Console.ReadKey()
    End Sub

    Private Shared Sub ThreadMessages(item As Anything, original As Boolean)
        If original Then
            Console.WriteLine("[main method]:" + item.Text)
        Else
            m_list.Add(item)
        End If
    End Sub

End Class

Class AnyTask

    Private m_lock As Object

    Public Sub New()
        m_lock = New Object()
    End Sub
    ' Something to use the delegate
    Public Event OnUpdate As MainDelegate

    Public Sub Test_Function(count As Integer)
        Dim list As New List(Of Thread)(count)
        For i As Int32 = 0 To count - 1
            Dim thread As New Thread(New ParameterizedThreadStart(AddressOf Thread_Task))
            Dim item As New Anything()
            item.Number = i
            item.Text = String.Format("Test_Function #{0}", i)
            thread.Start(item)
            list.Add(thread)
        Next
        For Each thread As Thread In list
            thread.Join()
        Next
    End Sub

    Private Sub MainUpdate(item As Anything, original As Boolean)
        RaiseEvent OnUpdate(item, original)
    End Sub

    Private Sub Thread_Task(parameter As Object)
        SyncLock m_lock
            Dim item As Anything = DirectCast(parameter, Anything)
            MainUpdate(item, True)
            item.Text = [String].Format("{0}; Thread_Task #{1}", item.Text, item.Number)
            item.Number = 0
            MainUpdate(item, False)
        End SyncLock
    End Sub

End Class


Class Anything
    ' Number and Text are for instructional purposes only
    Public Property Number() As Integer
        Get
            Return m_Number
        End Get
        Set(value As Integer)
            m_Number = value
        End Set
    End Property
    Private m_Number As Integer
    Public Property Text() As String
        Get
            Return m_Text
        End Get
        Set(value As String)
            m_Text = value
        End Set
    End Property
    Private m_Text As String
    ' Data can be anything or another class
    Public Property Data() As Object
        Get
            Return m_Data
        End Get
        Set(value As Object)
            m_Data = value
        End Set
    End Property
    Private m_Data As Object
End Class

Answer 14

可以使用此代码：

 private Object MyThread(Object Data)
      {
        Object response = null;
        Thread newThread = new Thread(() =>
        {
            response = MyFunction(Data);
            //MyFunction Is Function that you Define
        });
        newThread.Start();
        newThread.Join();
        return response;
      }

Answer 15

class Program
{
    static void Main(string[] args)
    {
        string returnValue = null;
       new Thread(
          () =>
          {
              returnValue =test() ; 
          }).Start();
        Console.WriteLine(returnValue);
        Console.ReadKey();
    }

    public static string test()
    {
        return "Returning From Thread called method";
    }
}

Answer 16

一个简单的解决方案是通过 ref 将参数传递给正在线程中运行的函数，并在线程中更改其值。

       // create a list of threads
        List<Thread> threads = new List<Thread>();


        //declare the ref params
        bool is1 = false;
        bool is2 = false;

        threads.Add(new Thread(() => myFunction(someVar, ref is1)));
        threads.Add(new Thread(() => myFunction(someVar, ref is2)));

        threads.ForEach(x => x.Start());

        // wait for threads to finish
        threads.ForEach(x => x.Join());

        //check the ref params
        if (!is1)
        {
          //do something
        }

        if (!is2)
        {
           //do somethign else
        }

如果你不能改变正在运行的函数，你可以把它换成另一个函数：

 bool theirFunction(var someVar){
   return false;
}


 void myFunction(var someVar ref bool result){
  result = theirFunction(myVar);
 }

Answer 17

class Program
{
    public static void ActionResultPrint(string i)
    {
        Console.WriteLine(i);
    }

    static void Main(string[] args)
    {
        var tl = new List<Thread>();

        tl.Add(new Thread(() => Run(10, ActionResultPrint)));
        tl.Add(new Thread(() => Run(20, ActionResultPrint)));
        tl.ForEach(x => x.Start());
        tl.ForEach(x => x.Join());
    }

    public static void Run(int j, Action<string> action)
    {
        string rvalue = string.Empty;

        for (int i = 0; i <= j; i++)
        {
            Thread.Sleep(100);
            rvalue = i.ToString();
            Console.WriteLine(rvalue);
        }
        action($@"output {j}");
    }
}

Answer 18

使用线程时，可以通过以下方式传递和返回值：

int value = -1;
  Thread t1 = new Thread(() => { value = compute(a); });
   t1.Start();
 if(value!=-1)
 {...}

   public int compute(int[] a1)
    {
    //...code logic
        return -1;
    }

Answer 19

我不是线程方面的专家，这就是我这样做的原因：

我创建了一个设置文件并

在新线程内：

Setting.Default.ValueToBeSaved;
Setting.Default.Save();

然后我会在需要时获取该值。