How to Make One Thread Wait for Another in .NET?

Detailed explanation of .NET multi-thread waiting mechanism

When dealing with multi-threading in .NET, especially when combined with the main UI thread, managing thread execution order is crucial. This article explores several strategies for making one thread wait for another thread to complete.

This need arises when one thread (for example, the main UI thread) needs to wait for another thread to complete its task before continuing. There are multiple ways to achieve this behavior:

1. Thread.Join:

<code>Thread t1 = new Thread(new ThreadStart(action1));
t1.Start();
t1.Join();</code>

Thread.Join blocks the calling thread (in this case, the main UI thread) until the specified thread (t1 in the example) completes execution. Although this method is simple and straightforward, it may cause the UI to freeze.

2. WaitHandles:

<code>ManualResetEvent resetEvent = new ManualResetEvent(false);
Thread t1 = new Thread(new ThreadStart(() =>
{
    // 执行任务并在完成时发出信号
    resetEvent.Set();
}));
t1.Start();
resetEvent.WaitOne(); // 阻塞直到 resetEvent 发出信号</code>

WaitHandles (such as ManualResetEvent) provide more fine-grained control. Here, resetEvent is used to indicate the completion of thread t1. The main thread blocks on WaitOne() until the event is set, allowing it to continue execution.

3. Events:

<code>ThreadWorker worker = new ThreadWorker();
worker.ThreadDone += HandleThreadDone;

Thread t1 = new Thread(worker.Run);
t1.Start();

void HandleThreadDone(object sender, EventArgs e)
{
    // 线程 `t1` 完成时执行操作
}</code>

Events provide a way for threads to communicate their completion status. The ThreadWorker raises an event after its task is completed, which is then handled in the HandleThreadDone method.

4. Delegates:

<code>ThreadWorker worker = new ThreadWorker();

Thread t1 = new Thread(worker.Run);
t1.Start(HandleThreadDone);

void HandleThreadDone()
{
    // 线程 `t1` 完成时执行操作
}</code>

Delegates can be used to pass methods as parameters to threads. When the ThreadWorker completes its task, the delegate HandleThreadDone is called, allowing the main thread to continue execution.

5. Asynchronous programming:

You can use asynchronous programming techniques, such as using delegates and event handlers with the Task or async/await keywords, to avoid thread blocking and efficiently manage the flow of execution.

Handling thread synchronization:

When using events or delegates, be sure to consider thread synchronization. Event/delegate handler methods may execute on the thread that raised the event/invoked the delegate, not necessarily the main UI thread. To update the user interface, you may need to call a handler method on the main UI thread using Invoke or InvokeRequired.

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