Drawing two polylines from one PointCollection throws exception (Silverlight 5)

Question

I'm using Silverlight 5 with MVVM.

I have a ViewModel that is a Singleton. The ViewModel exposes a PointCollection that I am using to draw a Polyline in one of my views.

If I try to draw the same Polyline in a second view, by databinding to the PointCollection again, I get a "value does not fall within the expected range" exception.

As far as I could find out (with my limited Silverlight knowledge), this is caused by the fact that PointCollections are not shareable.

Is there a workaround? How can I get a second Polyline drawn that is identical to the first? I want to databind two Polylines to one PointCollection at the same time.

Edit: I haven't found a solution, but someone with the same problem here. According to Microsoft:

This MSDN page mentions that some objects are not shareable and will genereate a "value out of range" exception. http://msdn.microsoft.com/en-us/library/system.windows.resourcedictionary(VS.95).aspx

The PointCollection page also mentions that it is not shareable. http://msdn.microsoft.com/en-us/library/system.windows.media.pointcollection(VS.95).aspx

Currently, this is by design behavior. However, we are evaluating this to see whether we can either change the behavior or at least the exception text.

Answer 1

Have a look at this question: Why doesn't this data binding work?

And at this one too: 2nd time binding to PointCollection not being rendered

As you gave little details I am not quite sure what is going on but these posts might help out. If not, please post your code.

I did some testing and the best solution I can think of is this:

using System.ComponentModel;
using System.Windows;
using System.Windows.Media;

namespace SilverlightApplication6
{
    public class DemoVM : INotifyPropertyChanged
    {
        #region PointsClone Property
        private PointCollection _pointsClone;
        public PointCollection PointsClone
        {
            get
            {
                return _pointsClone;
            }
            set
            {
                if (_pointsClone != value)
                {
                    _pointsClone = value;
                    OnPropertyChanged("PointsClone");
                }
            }
        }
        #endregion

        #region Points Property
        private PointCollection _points;
        public PointCollection Points
        {
            get
            {
                return _points;
            }
            set
            {
                if (_points != value)
                {
                    _points = value;
                    PointsClone.Clear();
                    foreach (var point in _points)
                    {
                        PointsClone.Add(point);
                    }
                    OnPropertyChanged("Points");
                }
            }
        }
        #endregion

        public DemoVM()
        {
            PointsClone = new PointCollection();
            Points = new PointCollection();
        }

        public void AddPoint(Point point)
        {
            Points.Add(point);
            PointsClone.Add(point);
        }

        public void ClearPoints()
        {
            Points.Clear();
            PointsClone.Clear();
        }


        public event PropertyChangedEventHandler PropertyChanged;

        protected void OnPropertyChanged(string propertyName)
        {
            var p = PropertyChanged;
            if (p != null)
            {
                p(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }
}

Bind one PolyLine.Points to Points and the other PolyLine.Points to PointsClone.

It is a bit ugly because it will break when you use vm.Points.Add(point) instead of vm.AddPoint(point). By applying proper encapsulation you might be able to solve that.

Answer 2

I found a solution here: duplicate the PointCollection in the getter.

    private PointCollection sourcePoints;
    public PointCollection SourcePoints
    {
        get
        {
            // create a new instance of PointCollection for binding
            PointCollection newPoints = new PointCollection();
            foreach (Point p in sourcePoints)
            {
                newPoints.Add(p);
            }
            return newPoints;
        }

Answer 3

Maybe your PointCollection is Freezed and that's making the trouble.

MSDN:

Freezable Features: Because it inherits from the Freezable class, the PointCollection class provides several special features: PointCollection objects can be declared as resources, shared among multiple objects, made read-only to improve performance, cloned, and made thread-safe. For more information about the different features provided by Freezable objects, see the Freezable Objects Overview.