The calling thread cannot access this object because a different thread owns it

0
votes

There seems to be a lot of these errors found on the forum, but I can't apply most of them on my situation..

My problem:

I have a page: PosterHome.xaml, with a uniform grid on it. In my code-behind, I have a drawthread: 

drawThread = new Thread(new ThreadStart(drawPosters));
                drawThread.SetApartmentState(ApartmentState.STA);
                drawThread.Start();

This threadmethod (drawPosters) is occasionally woken up by another class, using a autoresetevent. I get the error in this method the moment i'm changing the uniform grid rows:

while (true)
{
  waitEvent.WaitOne();
  //do some calculations
  // change uniform grid rows & cols
  posterUniformGrid.Rows = calculatedRows; //**-> error is first thrown here**
  posterUniformGird.Columns = calculatedCols;             
}

How should I handle this? Thanks in advance.

Greets Daan

2
c#wpfmultithreading
Which forum posts did you look at and why don't they apply to your situation? Did you look for posts about Dispatcher? - mbeckish
If you had googled exactly the title of your question, the first hit had given you a profound explanation of what's going on. - Clemens
I did try using a dispatcher, but apparently i did something wrong because it didn't work for me..but I used the example aaron gave and it worked! - Daan

2 Answers

7
votes

You're attempting to access posterUniformGrid which was created on the UI thread from your background thread.

To avoid this, leverage the Dispatcher.

Dispatcher.Invoke(DispatcherPriority.Normal, 
                    new Action<object[]>(SetGrid), 
                    new object[] { calculatedRows, calculatedColumns });
0
votes

You could try this : 

 while (true)
 {
   waitEvent.WaitOne();
   this.InvokeEx(t => t.posterUniformGrid.Rows = calculatedRows); 
   this.InvokeEx(t => t.posterUniformGird.Columns = calculatedCols);             
 }

 public static class ISynchronizeInvokeExtensions
    {
        public static void InvokeEx<T>(this T @this, Action<T> action) where T : ISynchronizeInvoke
        {
            if (@this.InvokeRequired)
            {
                try
                {
                    @this.Invoke(action, new object[] { @this });
                }
                catch { }
            }
            else
            {
                action(@this);
            }
        }
    }