AppointmentItemContentTemplate not working properly

[Mohammed]

Presently I'm working on RadScheduleView control (Silverlight 4 version)  
to display two different type of appointments.  Following is scale downversion of what I'm trying to do,

public abstract class AppointmentTypeBase : AppointmentBase
   {

   }

   public class AppointmentTypeOne : AppointmentTypeBase
   {
       public override IAppointment Copy()
       {
           var appt = new AppointmentTypeOne();

           appt.CopyFrom(this);

           return appt;
       }
   }

   public class AppointmentTypeTwo : AppointmentTypeBase
   {
       public override IAppointment Copy()
       {
           var appt = new AppointmentTypeTwo();

           appt.CopyFrom(this);

           return appt;
       }
   }

Then my actual container class which represents the appointment in the AppointmentSource is as follows,

public class AppointmentContainer : AppointmentBase
    {
        public override IAppointment Copy()
        {
            return this;
        }

        public AppointmentTypeBase AppointmentBase { get; set; }

        public override DateTime Start
        {
            get
            {
                if (this.AppointmentBase != null)
                    return this.AppointmentBase.Start;

                return base.Start;
            }
            set
            {
                if (this.AppointmentBase != null)
                    this.AppointmentBase.Start = value;

                base.Start = value;
            }
        }

        public override DateTime End
        {
            get
            {
                if(this.AppointmentBase!=null)
                    return this.AppointmentBase.End;

                return base.End;
            }
            set
            {
                if (this.AppointmentBase != null)
                    this.AppointmentBase.End = value;

                base.End = value;
            }
        }

        public override string Subject
        {
            get
            {
                if (this.AppointmentBase != null)
                    return this.AppointmentBase.Subject;

                return base.Subject;
            }
            set
            {
                if (this.AppointmentBase != null)
                    this.AppointmentBase.Subject = value;

                base.Subject = value;
            }
        }
    }

Then my AppointmentItemContentTemplate is as follows

<DataTemplate x:Key="AppointmentViewTemplate">
           <Grid DataContext="{Binding Appointment.AppointmentBase}">
               <ContentPresenter Content="{Binding}">
                   <ContentPresenter.ContentTemplate>
                       <Binding>
                           <Binding.Converter>
                               <local:TypeToTemplateConverter>
                                   <local:TypeToTemplateConverter.Resources>
                                       <ResourceDictionary>
                                           <DataTemplate x:Key="appointmentTypeOne">
                                               <local:AppointmentTypeOneCtrl />
                                           </DataTemplate>
                                           <DataTemplate x:Key="appointmentTypeTwo">
                                               <local:AppointmentTypeTwoCtrl />
                                           </DataTemplate>
                                       </ResourceDictionary>
                                   </local:TypeToTemplateConverter.Resources>
                               </local:TypeToTemplateConverter>
                           </Binding.Converter>
                       </Binding>
                   </ContentPresenter.ContentTemplate>
               </ContentPresenter>
           </Grid>
       </DataTemplate>

AppointmentTypeOneCtrl is 

<UserControl x:Class="ScheduleViewError.AppointmentTypeOneCtrl"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    d:DesignHeight="300" d:DesignWidth="400">

    <Grid x:Name="LayoutRoot" Background="White">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="*" />
        </Grid.RowDefinitions>
        <Rectangle Grid.Row="0"
                   Grid.RowSpan="3"
                   Stroke="{x:Null}">
            <Rectangle.Fill>
                <LinearGradientBrush EndPoint="0.5,1"
                                     StartPoint="0.5,0">
                    <GradientStop Color="#FF0036FF"
                                  Offset="0" />
                    <GradientStop Color="#FF6B86E2"
                                  Offset="1" />
                </LinearGradientBrush>
            </Rectangle.Fill>
        </Rectangle>
        <TextBlock Grid.Row="1"
                   Margin="2"
                   Foreground="White"
                   Text="{Binding Subject}"
                   TextWrapping="Wrap" />
    </Grid>
</UserControl>

and AppointmentTypeTwoCtrl is 

<UserControl x:Class="ScheduleViewError.AppointmentTypeTwoCtrl"

    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"

    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"

    mc:Ignorable="d"

    d:DesignHeight="300" d:DesignWidth="400">

 

    <Grid x:Name="LayoutRoot"

          Background="White">

        <Grid.RowDefinitions>

            <RowDefinition Height="Auto" />

            <RowDefinition Height="Auto" />

            <RowDefinition Height="*" />

        </Grid.RowDefinitions>

        <Rectangle Grid.Row="0"

                   Grid.RowSpan="3"

                   Stroke="{x:Null}">

            <Rectangle.Fill>

                <LinearGradientBrush EndPoint="0.5,1"

                                     StartPoint="0.5,0">

                    <GradientStop Color="#FF4B0348"

                                  Offset="0" />

                    <GradientStop Color="#FFBC5CB9"

                                  Offset="1" />

                </LinearGradientBrush>

            </Rectangle.Fill>

        </Rectangle>

            <TextBlock Grid.Row="1"

                   Margin="2"

                   Foreground="White"

                   Text="{Binding Subject}"

                   TextWrapping="Wrap" />

    </Grid>

</UserControl>


I generate two of each kind in the viewmodel for ScheduleView as follows

public class CalendarViewModel : ViewModelBase
   {
       public CalendarViewModel()
       {
             if(!IsDesignTime)
             {
                 VisibleRangeChangeCmd = new DelegateCommand(OnVisibleRangeExecuted, OnVisibleRangeCanExecute);
             }
       }

       private ObservableCollection<AppointmentContainer> _collection;
       public ObservableCollection<AppointmentContainer> Collection
       {
           get { return _collection ?? (_collection = new ObservableCollection<AppointmentContainer>()); }
       }

       public ICommand VisibleRangeChangeCmd { get; private set; }

       private static bool? _isInDesignMode;
       public static bool IsDesignTime
       {
           get
           {
               if (!_isInDesignMode.HasValue)
                   _isInDesignMode = DesignerProperties.IsInDesignTool;

               return _isInDesignMode.Value;
           }
       }

       private void OnVisibleRangeExecuted(object param)
       {
           var dateSpan = param as Telerik.Windows.Controls.ScheduleView.IDateSpan;

           if (dateSpan != null)
               GenerateAppointmentsSourceForTimeRange(dateSpan.Start, dateSpan.End);
       }

       private bool OnVisibleRangeCanExecute(object param)
       {
           return param != null;
       }

       /// <summary>
       /// Generates the appointments source for time range.
       /// </summary>
       /// <param name="startTime">The start time.</param>
       /// <param name="endTime">The end time.</param>
       protected void GenerateAppointmentsSourceForTimeRange(DateTime startTime, DateTime endTime)
       {
           Collection.Clear();

           GenerateTypeOne(startTime, endTime);
           GenerateTypeTwo(startTime, endTime);
       }

       private void GenerateTypeTwo(DateTime startTime, DateTime endTime)
       {
           string subjectStr = string.Format("Subject - {0}", typeof (AppointmentTypeOne).Name);

           Collection.Add(new AppointmentContainer
                              {
                                  AppointmentBase =
                                      new AppointmentTypeOne
                                          {Start = startTime.AddHours(3), End = startTime.AddHours(4), Subject = subjectStr}
                              });

           Collection.Add(new AppointmentContainer
           {
               AppointmentBase =
                   new AppointmentTypeOne { Start = startTime.AddHours(8), End = startTime.AddHours(9), Subject = subjectStr }
           });
       }

       private void GenerateTypeOne(DateTime startTime, DateTime endTime)
       {
           string subjectStr = string.Format("Subject - {0}", typeof(AppointmentTypeTwo).Name);

           Collection.Add(new AppointmentContainer
           {
               AppointmentBase =
                   new AppointmentTypeTwo { Start = startTime.AddHours(6), End = startTime.AddHours(7), Subject = subjectStr }
           });

           Collection.Add(new AppointmentContainer
           {
               AppointmentBase =
                   new AppointmentTypeTwo { Start = startTime.AddHours(11), End = startTime.AddHours(12), Subject = subjectStr }
           });
       }
   }

Things show fine as expected when the application is lauched. But when I scroll up/down in the Daily view the appointment views which were initially visible just go blank. I was not able figure why this is happening intermittently. This problem can well understood if it is seen live. I would like to post repro project that I created.

     Please help me in this problem

Thanks
-Obaid