如何正确使用Dispatcher.BeginInvoke？

Question

I have searched almost everywhere on the internet, and I have googled so many times and found so many results, but I still can't find the solution to my problem.

I am busy converting an old WinForms application to a new WPF application but I am having trouble with some of the commands. In the Winforms application they use Control.BeginInvoke() and store this in an IAsyncResult object. I have read that the Dispatcher.BeginInvoke() is the WPF equivalent to the Control.BeginInvoke() for WinForms but I get this error when I use

Dispatcher.BeginInvoke(): "Cannot implicitly convert type 'System.Windows.Threading.DispatcherOperation' to 'System.IAsyncResult'. An explicit conversion exists (are you missing a cast?)".

Any help will be appreciated.

Here is the code that I am trying to convert. This is the original WinForms code. I am able to convert everything except the BeginInvoke part.

    private eSkan.api.TeSkanAPI feSkanAPI = null;

    private void MessageFilter_AddRemove_Invoked(bool AddFilter, IMessageFilter Filter)
    {
        if (AddFilter){ Application.AddMessageFilter(Filter); }
        else { Application.RemoveMessageFilter(Filter); }
    }

    private void MessageFilter_AddRemove(bool AddFilter, IMessageFilter Filter)
    {
        {
            IAsyncResult sr = BeginInvoke((ESKAN_ADD_REMOVE_MESSAGEFILTER)MessageFilter_AddRemove_Invoked,
                                          AddFilter, Filter);
            sr.AsyncWaitHandle.WaitOne(2000);
        }
    }

Below is my code that I have converted so far including the BeginInvoke part that I am struggling with.

    private void MessageFilter_AddRemove_Invoked(bool addFilter, System.Windows.Forms.IMessageFilter filter)
    {
        if (addFilter) 
        { 
            System.Windows.Forms.Application.AddMessageFilter(filter); 
        }
        else 
        {
            System.Windows.Forms.Application.RemoveMessageFilter(filter); 
        }
    }

    private void MessageFilter_AddRemove(bool addFilter, System.Windows.Forms.IMessageFilter filter)
    {
        {
            IAsyncResult sr = System.Windows.Threading.Dispatcher.BeginInvoke((ESKAN_ADD_REMOVE_MESSAGEFILTER)MessageFilter_AddRemove_Invoked, addFilter, filter);
            sr.AsyncWaitHandle.WaitOne(2000);
        }
    }

If there are any other mistakes then please let me know.

Thanks

Answer 1

That's because Dispatcher.BeginInvoke , though it may be the equivalent logical operation, doesn't return an IAsyncResult , it returns a DispatcherOperation . Have a look at this blog post and you'll see a good example on how the Dispatcher works. I have copied the relevant code example into here to ensure it exists later.

public Window1()
{
  InitializeComponent();

  CheckBox myCheckBox = new CheckBox();
  myCheckBox.Content = "A Checkbox";

  System.Threading.Thread thread = new System.Threading.Thread(
    new System.Threading.ThreadStart(
      delegate()
      {
        System.Windows.Threading.DispatcherOperation
          dispatcherOp = myCheckBox.Dispatcher.BeginInvoke(
          System.Windows.Threading.DispatcherPriority.Normal,
          new Action(
            delegate()
            {
              myCheckBox.IsChecked = true;
            }
        ));

        dispatcherOp.Completed += new EventHandler(dispatcherOp_Completed);
      }
  ));

  thread.Start();
}

void dispatcherOp_Completed(object sender, EventArgs e)
{
  Console.WriteLine("The checkbox has finished being updated!");
}

Take note to this line:

dispatcherOp.Completed += new EventHandler(dispatcherOp_Completed);

that's how you're going to know when it's completed - it's going to call back to you via that event.