use*_*787 7 c# events multithreading
我阅读了很多事件和线程讨论,但是如果我不知道某个事件并尝试稍后调用它,那么所有这些都集中在"会发生什么".我的问题是不同的...如果我在线程A中有一个进程以毫秒1触发事件"我完成",并且在线程B中有一个进程以毫秒2激活事件"我完成",将会发生什么.
两个进程都被用于监听和处理事件的相同方法.因此,C#必须执行处理事件2次的方法:1次用于在线程A中触发的事件,1次用于从线程B触发的事件.
会发生什么？？当"来自线程A的第一个事件"开始执行处理事件的方法时,C#是否锁定该方法,并在完成执行时解锁该方法,从而允许其他等待"事件"执行方法内容？
或者从线程A触发的事件将开始执行处理事件的方法,1毫秒后,从线程B触发的事件也将在相同的方法上开始执行,而不会注意到当前该方法正由其他人执行"处理"？？？
我问这个,因为我想在捕获事件的方法中写一些文件,但如果方法可以同时执行(取决于事件被触发的时间),我想我不能在这里做,因为关于该文件将是两个进程同时写入同一文件的混合(不是文件的有效信息).
我的代码看起来像这样(有点长,对不起).请注意这不会编译,只是一个示例,以显示我在做什么:
public partial class MainForm : Form
{
FTPClientManager client = null;
public MainForm()
{
InitializeComponent();
}
private void btnConnect_Click(object sender, EventArgs e)
{
Connect(this.tbFTPServer.Text.Trim());
this.lstLog.Items.Add("Connected"); //This lstLog is a list box that will have a list of downloaded files from all threads.
}
void Connect(string urlStr)
{
try {
client = new FTPClientManager();
//subscribe to the event
client.FileDownloadCompleted += new EventHandler<FileDownloadCompletedEventArgs>(client_FileDownloadCompleted);
}
catch (Exception ex)
{
MessageBox.Show(ex.Message);
}
}
void client_FileDownloadCompleted(object sender, FileDownloadCompletedEventArgs e)
{
this.Invoke(new EventHandler<FileDownloadCompletedEventArgs>(
client_FileDownloadCompletedHandler), sender, e);
}
void client_FileDownloadCompletedHandler(object sender, FileDownloadCompletedEventArgs e)
{
string log = string.Format("{0} Instance {5} Download from {1} to {2} is completed. Length: {3} Time: {4}. ",
DateTime.Now, e.ServerPath, e.LocalFile.FullName, e.LocalFile.Length, e.DownloadTime, e.ftpInstance);
this.lstLog.Items.Add(log);
}
private void btnDownload_Click(object sender, EventArgs e)
{
client.DownloadFiles();
}
}
public class FTPClientManager {
FTPDownloadClient[] arrayDownloadClient = new FTPDownloadClient[2];
public event EventHandler<FileDownloadCompletedEventArgs> FileDownloadCompleted;
public void DownloadFiles()
{
for (int i = 0; i < 2; i++)
{
arrayDownloadClient[i] = new FTPDownloadClient();
//subscribe to the event. each instance of FTPDownloadClient will suscribe to the same event.
arrayDownloadClient[i].FileDownloadCompleted += new EventHandler<FileDownloadCompletedEventArgs>(downloadClient_FileDownloadCompleted);
}
//download one set of files in thread A
arrayDownloadClient[0].DownloadFiles(list_of_files_to_download);
//download another set of files in thread B
arrayDownloadClient[1].DownloadFiles(another_list_of_files_to_download);
}
//In theory, the method downloadClient_FileDownloadCompleted will be executed by any instance of FTPDownloadClient
//running in either thread A or thread B, whichever finish first downloading a file.
//My question comes in the execution of this method.
//Lets say the process in thread A finish downloading and fires the event.
//Lets say the process in thread B finish downloading 1 millisecond after thread A finish, so it also fires the event.
//how C# manage the execution of the downloadClient_FileDownloadCompleted??
//does the event coming from thread A will lock the method downloadClient_FileDownloadCompleted, execute it, and when finish execution unlock the method
//and allows the event coming from thread B start locking, processing, unlock ??
//Or the method will be executed "at the same time" (1 millisecond difference) by each event fired from thread A and thread B??
void downloadClient_FileDownloadCompleted(object sender, FileDownloadCompletedEventArgs e)
{
this.OnFileDownloadCompleted(e);
}
protected virtual void OnFileDownloadCompleted(FileDownloadCompletedEventArgs e)
{
if (FileDownloadCompleted != null)
{
//this will fire the event, so the main form will catch it
//again, this fire can be triggered from process in thread A or from process in thread B
FileDownloadCompleted(this, e);
}
}
}
public class FTPDownloadClient {
public event EventHandler<FileDownloadCompletedEventArgs>
FileDownloadCompleted;
public void DownloadFiles(string [] files_to_download)
{
ParameterizedThreadStart threadStart =
new ParameterizedThreadStart(StartDownloadFiles);
Thread downloadThread = new Thread(threadStart);
downloadThread.IsBackground = true;
downloadThread.Start(new object[] { files_to_donwload });
}
//This metod will download all the files in the list passed as parameter.
//Every file downloaded will raise the event FileDownloadComplete, so a message can be added to the lstlog on the main form
void StartDownloadFiles(object state)
{
var paras = state as object[];
string [] files = paras[0] as string [];
foreach (var file in files)
{
DownloadOneFile(file);
}
}
void DownloadFile(string onefile)
{
//Donwload file done here
var fileDownloadCompletedEventArgs = new FileDownloadCompletedEventArgs
{
LocalFile = new FileInfo(destPath),
ServerPath = onefile,
DownloadTime = fileDownloadTime.ElapsedMilliseconds.ToString()
};
this.OnFileDownloadCompleted(fileDownloadCompletedEventArgs);
}
protected virtual void OnFileDownloadCompleted(FileDownloadCompletedEventArgs e)
{
if (FileDownloadCompleted != null)
{
//the event is fired when the file being downloaded by this thread is finish.
//so, thread A will fire this event from its current thread
//and also thread B will fire the same event from its own thread.
FileDownloadCompleted(this, e);
}
}
}
Mat*_*son 12
C#不会为你做任何锁定.如果事件可以由多个线程同时引发,则必须编写代码来处理(如果需要).
您可以使用lock语句来阻止多个线程执行它:
private void MyEventHandler(object sender, EventArgs e)
{
lock (lockingObject)
{
// Handle event here.
// Only one thread at a time can reach this code.
}
}
凡lockingObject被宣告你的类中的字段,如:
private readonly object lockingObject = new object();
您还必须小心在引发事件的方法中进行线程化.
假设您的班级中有一个名为的活动MyEvent.你不应该这样做:
private void RaiseMyEvent()
{
if (MyEvent != null) // {1}
MyEvent(this, new EventArgs()); // {2}
}
如果另一个线程可以脱离MyEvent,那么它可能会在行{1}和行{2}之间分离.如果发生这种情况,第{2}行将抛出一个空引用异常,因为它MyEvent会突然变为null!
正确的方法是:
private void RaiseMyEvent()
{
var handler = MyEvent;
if (handler != null)
handler (this, new EventArgs());
}
现在不能发生null引用异常.
但是,请注意,当使用多个线程时,可以在线程分离之后调用事件处理程序!