如何将变量传递给C＃中的事件？

Question

I am using FileSystemWatcher On the Changed Event, I want to pass an integer variable.

eg

int count = someValue;
 FileSystemWatcher watcher = new FileSystemWatcher();
watcher.Path = "C:\temp";
watcher.Changed += new FileSystemEventHandler(fileSystemWatcher_Changed);

on the fileSystemWatcher_Changed, I want to take the count value and then do some work. But how do I get that value.

if I make count a global variable, it wouldn't be valid because count changes with each file changed event and it is passed from user.

Answer 1

The simplest approach is to use a lambda expression:

watcher.Changed += (sender, args) => HandleWatcherChanged(count);

It sounds like you may want to pass count by reference though, if the method wants to update the value.

Answer 2

为什么不将FileSystemWatcher子类化并将计数传递给子类的构造函数？

Answer 3

You could maintain a global dictionary mapping each file (path) to its count:

readonly Dictionary<string, int> filesChangeCount= 
    new Dictionary<string, int>();

Then, in your event handler, just increment the appropriate count in the dictionary:

void fileSystemWatcher_Changed(object sender, FileSystemEventArgs e)
{
    lock (filesChangeCount)
    {
        int count;
        filesChangeCount.TryGetValue(e.FullPath, out count);
        filesChangeCount[e.FullPath] = count++;
    }
}

Answer 4

If you want to know how often fileSystemWatcher_Changed was called globally, you could use a static variable as well. If you want to know the number of calls in one specific instance of the class, drop the static keyword.

private static int _count;

private void fileSystemWatcher_Changed(object sender, EventArgs e)
{
    Console.WriteLine("fileSystemWatcher_Changed was called {0} times",
                      ++_count);
}

Answer 5

Just be aware that if you are using a global variable in your changed handler you should surround any use of your variable with a lock since your changed event will be called by more than one thread.

private static int _count;
private object lockerObject = new object();
private void fileSystemWatcher_Changed(object sender, EventArgs e)
{
    lock(lockerObject)
    {
        Console.WriteLine("fileSystemWatcher_Changed was called {0} times",
                  ++_count);
    }
}