C# 多线程返回值

ThreadStart 委托既没有参数也没有返回值。这意味着不可以使用需要参数的方法启动线程，或从方法中获得返回值。

为向线程传递数据，需要创建一个用来保持数据和线程方法的对象，如下面的两个代码示例所示。
为检索线程方法的结果，您可以使用回调方法，如第二个代码示例中所示。
using System;
using System.Threading;

// The ThreadWithState class contains the information needed for
// a task, and the method that executes the task.
//
public class ThreadWithState {
    // State information used in the task.
    private string boilerplate;
    private int value;

    // The constructor obtains the state information.
    public ThreadWithState(string text, int number) {
        boilerplate = text;
        value = number;
    }
         
    // The thread procedure performs the task, such as formatting
    // and printing a document.
    public void ThreadProc() {
        Console.WriteLine(boilerplate, value);
    }
}

// Entry point for the example.
//
public class Example {
    public static void Main() {
        // Supply the state information required by the task.
        ThreadWithState tws =
            new ThreadWithState("This report displays the number {0}.", 42);
        // Create a thread to execute the task, and then
        // start the thread.
        Thread t = new Thread(new ThreadStart(tws.ThreadProc));
        t.Start();
        Console.WriteLine("Main thread does some work, then waits.");
        t.Join();
        Console.WriteLine("Independent task has completed; main thread ends."); 
    }
}
用回调方法检索数据
下面的示例演示了一个从线程中检索数据的回调方法。包含数据和线程方法的类的构造函数也接受代表回调方法的委托；在线程方法结束前，它调用该回调委托。
using System;
using System.Threading;

// The ThreadWithState class contains the information needed for
// a task, the method that executes the task, and a delegate
// to call when the task is complete.
//
public class ThreadWithState {
    // State information used in the task.
    private string boilerplate;
    private int value;
    // Delegate used to execute the callback method when the
    // task is complete.
    private ExampleCallback callback;

    // The constructor obtains the state information and the
    // callback delegate.
    public ThreadWithState(string text, int number,
                  ExampleCallback callbackDelegate)
    {
        boilerplate = text;
        value = number;
        callback = callbackDelegate;
    }
   
    // The thread procedure performs the task, such as
    // formatting and printing a document, and then invokes
    // the callback delegate with the number of lines printed.
    public void ThreadProc() {
        Console.WriteLine(boilerplate, value);
        if (callback != null)
            callback(1);
    }
}

// Delegate that defines the signature for the callback method.
//
public delegate void ExampleCallback(int lineCount);

// Entry point for the example.
//
public class Example {
    public static void Main() {
        // Supply the state information required by the task.
        ThreadWithState tws = new ThreadWithState(
            "This report displays the number {0}.",
            42,
            new ExampleCallback(ResultCallback)
        );

        Thread t = new Thread(new ThreadStart(tws.ThreadProc));
        t.Start();
        Console.WriteLine("Main thread does some work, then waits.");
        t.Join();
        Console.WriteLine("Independent task has completed; main thread ends.");
    }

    // The callback method must match the signature of the
    // callback delegate.
    //
    public static void ResultCallback(int lineCount) {
        Console.WriteLine("Independent task printed {0} lines.", lineCount); 
    }
}