Question

I’m so frustrated trying to solve this. Why is creating threads and updating UI after completing them so complicated?! I have tried this, this and this and I still can’t get my head around the whole multi-threading thing clearly at all. It was so easy in Java…

Anyway here’s a simplified version of my code as the result of third link:

private delegate void SimpleDelegate2(BitmapImage bi);
private delegate void SimpleDelegate(string st1, string st2);

private void Process(string st1)
{
    try
    {
        string st2 = "test st2";
        SimpleDelegate del = new SimpleDelegate(LongRunningProcess);
        del.BeginInvoke(st1, st2, null, null);
    }
    catch
    {
        //do some failsafe thing
    }
}

private void LongRunningProcess(string st1, string st2)
{
    //do some long processing with the strings
    BitmapImage bi = new BitmapImage();//there will be an actual bitmap here

    SimpleDelegate2 del1 = delegate(BitmapImage bimg)
    {
        ImageControlOnWPFform.Source = bimg; //null;
    };
    this.Dispatcher.BeginInvoke(DispatcherPriority.Send, del1, bi);
}

The problem here is that I can’t set the Source value of the Image control to bimg but I can set it to null! Whenever I try to set it to bimg I get the exception that says the calling thread cannot access this object because a different thread owns it. I have also tried setting it directly to bi which also gives the same exception.

But I can set the source to null without any problem, which means I can modify the Source value of the Image control. But how do I access bi or bimg? Am I doing something wrong?

Also: I notice that the given arguments of the last BeginInvoke do not match any overload of the method but they are still accepted as valid and works properly. When I point at BeginInvoke I am shown a completely different set of method overloads than the ones that appear when I type a ( following the method name. Why?

Answer 1

Answer for main question

Execute bi.Freeze() before using the BitmapImage in the UI thread. The Freeze operation will make the BitmapImage read-only and thread-safe.

    ...
    //do some long processing with the strings
    BitmapImage bi = new BitmapImage();//there will be an actual bitmap here
    bi.Freeze();

    SimpleDelegate2 del1 = delegate(BitmapImage bimg)
    ...

Answer for Also

The class System.Windows.Threading.Dispatcher has five signatures for the BeginInvoke method:

public DispatcherOperation BeginInvoke(Delegate method, params object[] args);
public DispatcherOperation BeginInvoke(DispatcherPriority priority, Delegate method);
public DispatcherOperation BeginInvoke(Delegate method, DispatcherPriority priority, params object[] args);
public DispatcherOperation BeginInvoke(DispatcherPriority priority, Delegate method, object arg);
public DispatcherOperation BeginInvoke(DispatcherPriority priority, Delegate method, object arg, params object[] args);

However, three of the signatures (those with DispatcherPriority priority as their first argument) have the following tags on them:

[Browsable(false)]
[EditorBrowsable(EditorBrowsableState.Never)]

This is why they do not show up when typing a ( after the method name.