Interesting foreach problem

See more:

C#

WPF

I am trying to use a foreach statement to remove the children of my main grid.
When I do this, I get the following runtime exception:

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

Here is my code:

List<DraggableCard> remove=new List<DraggableCard>();
                foreach (UIElement em in cont.Children)//This is where I get my exception
                {
                    if (em is DraggableCard)
                    {
                        var d = em as DraggableCard;
                        if (d.UID == str.Replace("REMOVE_CARD{", "").Replace("}", ""))
                            remove.Add(d);
                    }
                }
                foreach (DraggableCard d in remove)
                {
                    cont.Children.Remove(d);
                }

I don't know why this would happen; any help would be appreciated.