C＃：连续两次调用方法

Question

This is my code.

BackgroundWorker exportWorker = new BackgroundWorker();

private void btnOK_Click(object sender, RoutedEventArgs e)
{
    exportWorker.DoWork += new DoWorkEventHandler(ExportWorkerDoWork);
    exportWorker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(ExportWorkerRunWorkerCompleted);
    exportWorker.RunWorkerAsync();
}


void ExportWorkerDoWork(object sender, DoWorkEventArgs e)
{
    MethodToPerformInThisThread();
    **Dispatcher.Invoke(new Action(() => {MethodofAnotherThreadThatChangesUIStuff();}**
}

void ExportWorkerRunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
}

• When I click on the OK Button ( btnOK_Click ) the first time, it goes ahead and runs everything just fine.
• Now, if I click on the OK Button the second time after the code has finished executing, the code in the bold runs twice, meaning the method MethodofAnotherThreadThatChangesUIStuff(); gets called twice in a row.
• Again, If I click the OK Button the third time, MethodofAnotherThreadThatChangesUIStuff(); gets called three times in a row.
• and so on.

I want the MethodofAnotherThreadThatChangesUIStuff(); to be called just once, no matter which click it is. I mean, generally that's how things should work.

What am I missing here?

Any help will be really appreciated.

Answer 1

Since you are declaring the background worked outside of the Click handler each time you press the button you add the event handlers again. This is why the second time your method is called twice and the third time it will be called three times...

To solve this, add the event handlers outside the click method (in the constructor for example) and only leave RunAsync() call in the click method.

Answer 2

Each time you click OK, you are creating new event handlers which will all call your DoWork methods.

Place this code in the initialization of your application:

exportWorker.DoWork += new DoWorkEventHandler(ExportWorkerDoWork);
exportWorker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(ExportWorkerRunWorkerCompleted);

Only include the following in your btnOK_Click method:

exportWorker.RunWorkerAsync();

Answer 3

This is being caused because this line:

exportWorker.DoWork += new DoWorkEventHandler(ExportWorkerDoWork);

is being run inside the the btnOK_Click handler. If you setup the events outside the handler like this:

BackgroundWorker exportWorker = new BackgroundWorker();
exportWorker.DoWork += new DoWorkEventHandler(ExportWorkerDoWork);
exportWorker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(ExportWorkerRunWorkerCompleted);

private void btnOK_Click(object sender, RoutedEventArgs e)
{
exportWorker.RunWorkerAsync();
}

then everything should work like you expect.

Another option would actually be to create a new BackgroundWorker inside the btnOK_Click handler. Essentially, you would be creating a new worker for each click, setting it up with handlers, and then running it. You would then discard it and create a new one for each click.