调用RunWorkerAsync一次时,会调用BackgroundWorker的DoWork两次？

Question

我在它工作的类中创建了一个后台工作者,但是如果我调用并等到最后一次运行,那么第二次调用它会两次执行相同的过程

我认为bw.DoWork + =有问题

private void button1_Click(object sender, EventArgs e)
{
    nptest.test.start("null", "null");    
}


namespace nptest
{
    class test
    {
        public static void start(string str, string strb)
        {
            if (bw.IsBusy != true)
            {
                bw.WorkerSupportsCancellation = true;
                bw.DoWork += (obj, e) => bw_DoWork(str, strb);
                bw.RunWorkerCompleted += new RunWorkerCompletedEventHandler(bw_RunWorkerCompleted);
                bw.RunWorkerAsync();
            }
        }
        private static BackgroundWorker bw = new BackgroundWorker();
        private static void bw_DoWork(string str, string strb)
        {
            System.Windows.Forms.MessageBox.Show("initializing BackgroundWorker");
        }
        private static void bw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            if ((e.Cancelled == true))
            {
                Console.WriteLine("Canceled");
            }
            else if (!(e.Error == null))
            {
                Console.WriteLine("Error: " + e.Error.Message);
            }
            bw.Dispose();

        }
    }
}

问题解决了

  class test
    {
        private static List<object> arguments = new List<object>();

        // initializing with program startup
        public static void bwinitializing()
        {
            bw.WorkerSupportsCancellation = true;
            bw.DoWork += new DoWorkEventHandler(bw_DoWork);
            bw.RunWorkerCompleted += new RunWorkerCompletedEventHandler(bw_RunWorkerCompleted);
        }

        public static void start(string str, string strb)
        {
            if (bw.IsBusy != true)
            {
                arguments.Clear();
                arguments.Add(str);
                arguments.Add(strb);
                bw.RunWorkerAsync(arguments);
            }
        }
        private static BackgroundWorker bw = new BackgroundWorker();
        private static void bw_DoWork(object sender, DoWorkEventArgs e)
        {
            List<object> genericlist = e.Argument as List<object>;
            System.Windows.Forms.MessageBox.Show("BackgroundWorker " + genericlist[0]);

        }

Answer 1

我怀疑无意中添加了多个 DoWork事件.

也就是说,每次start调用该方法时,它都会注册一个新的 DoWork事件处理程序.这会添加并不会替换现有的处理DoWork程序处理程序.那么将有多个DoWork处理程序称为后续时间.. 1,2,3等.

// creates a NEW delegate and adds a NEW handler
bw.DoWork += (obj, e) => bw_DoWork(str, strb);

我建议不要在这里使用闭包,而只是使用一个方法组(隐式转换为委托),然后将数据传递给RunWorkerAsync调用(有一个接受数据参数的表单).

该RunWorkerCompleted +=行没有此问题,因为它从方法组传递委托(保证始终评估为同一委托对象¹).因此+=,对该行的重复调用将替换处理程序.

例:

class MyData {
   public string StrA { get; set; }
}

// These only need to be setup once (and should be for clarity).
// However it will be "ok" now if they are called multiple times
// as, since the delegates are the same, the += will
// act as a replacement (as it replaces the previous delegate with itself).
bw.WorkerSupportsCancellation = true;
bw.DoWork += bw_DoWork;
bw.RunWorkerCompleted += bw_RunWorkerCompleted;

// Pass data via argument
bw.RunWorkerAsync(new MyData {
    StrA = str,
});

void bw_DoWork (object sender, DoWorkEventArgs e) {
    var data = (MyData)e.Argument;
    var str = data.StrA;
    // stuff
}

¹,我不知道这是否是保证引用等于平等,但使用这种方法允许稳定的调用+=,并-=从方法组,即使所获得的代表new DelegateType(MethodGroup).

WRT.我在主帖中的评论:如果从未创建UI元素的线程访问UI元素,那么将会有趣的"跨线程操作异常".我相信消息框的这种用法是"好的"(当不是与另一个线程的所有者一起创建时),但是在BackgroundWorker的DoWork中访问UI的做法通常是可疑的.

另外,不要bw.Dispose()在这里打电话; 将其与拥有的容器或上下文一起处理.在这种情况下它似乎很好而且温和,但只有在BGW实例永远不会再次使用时才会这样做.从事件处理程序调用它也是可疑的,因为BGW仍然"活跃".

Answer 2

我遇到了与上述评论者"Power-Mosfet"相同的问题

最后,添加一个new BackgroundWorker()然后分配给全局bw值将解决我的问题.

代码是,改为:

private BackgroundWorker gBgwDownload;

private void yourFunction_bw(xxx)
{
    // Create a background thread
    gBgwDownload.DoWork += bgwDownload_DoWork;
    gBgwDownload.RunWorkerCompleted += bgwDownload_RunWorkerCompleted;
    //omited some code
    gBgwDownload.RunWorkerAsync(paraObj);
}

至:

private BackgroundWorker gBgwDownload;

private void yourFunction_bw(xxx)
{
    // Create a background thread
    gBgwDownload = new BackgroundWorker(); /* added this line will fix problem */
    gBgwDownload.DoWork += bgwDownload_DoWork;
    gBgwDownload.RunWorkerCompleted += bgwDownload_RunWorkerCompleted;
    //omited some code
    gBgwDownload.RunWorkerAsync(paraObj);

}

Answer 3

还有另一个原因.DoWorkEventHandler在其生成的代码中查找InitializeComponent()如果您通过compnent UI属性生成它并自己注册它.

因为如果你再次注册它,它将不会覆盖前一个,但会添加另一个事件,并将调用两次.