void runtime_SkeletonFrameReady(object sender, SkeletonFrameReadyEventArgs e)
        {
            SkeletonFrame skeletonSet = e.OpenSkeletonFrame();

            Skeleton data = (from s in skeletonSet // <- Here is the error
                             where s.TrackingState == SkeletonTrackingState.Tracked
                             select s).FirstOrDefault();
            if (data != null)
            {
                SetEllipsePosition(Head, data.Joints[JointType.Head]);
                SetEllipsePosition(leftHand, data.Joints[JointType.HandLeft]);
                SetEllipsePosition(rightHand, data.Joints[JointType.HandRight]);
                ProcessGesture(data.Joints[JointType.Head], data.Joints[JointType.HandLeft], data.Joints[JointType.HandRight]);
            }

        }

private Skeleton[] mySkeletons;

private void OnSkeletonFrameReady(object sender, SkeletonFrameReadyEventArgs e)
    {
        bool haveSkeletons = false
        using (SkeletonFrame skeletonFrame = e.OpenSkeletonFrame())
        {
            if (null != skeletonFrame)
            {
                if ((null == mySkeletons) || (mySkeletons.Length != skeletonFrame.SkeletonArrayLength))
                {
                    mySkeletons = new Skeleton[skeletonFrame.SkeletonArrayLength];
                }
                skeletonFrame.CopySkeletonDataTo(mySkeletons);
                haveSkeletons = true;
            }
        }

if (haveSkeletons)
       {
          Skeleton data = (from s in mySkeletons
             where s.TrackingState == SkeletonTrackingState.Tracked
             select s).FirstOrDefault();
       }
   }