Canvas UserControl DataContext update issue - wpf

Question

I have developed a usercontrol, inside it has a canvas. I draw some lines and arcs in the canvas. Everytime when the datacontext changes, I need to force usercontrol's datacontext to null before updating new datacontext. Unless the new drawing is not getting updated. why is that? How to update Usercontrol's content (Actually canvas's content) without resetting the datacontext to null prior to update the new datacontext.
I use the following code. FlashControl is the usercontrol which contains canvas inside.
(FlashControl.Content as FrameworkElement).DataContext = null;
(FlashControl.Content as FrameworkElement).DataContext = this.DataContext;
FlashControl.DataContext = this.DataContext;
Update
::::::: This is my usercontrol
<UserControl x:Class="PricingRoutine.FlashGeneratorUserControl"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:Converters="clr-namespace:PricingRoutine.Converters"
xmlns:local ="clr-namespace:CustomFlashGenerator"
mc:Ignorable="d" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" HorizontalContentAlignment="Stretch" VerticalContentAlignment="Stretch">
<UserControl.Resources>
<Converters:TextPositionConverter x:Key="TextPositionConverter" />
<Converters:LinetPositionConverter x:Key="LinePositionConverter" />
<Converters:PointPositionConverter x:Key="PointPositionConverter" />
</UserControl.Resources>
<Grid x:Name="MajorGrid" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
<ItemsControl ItemsSource="{Binding Path = CompositeObjectList}">
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<!--<ViewBox Stretch="Uniform">-->
<Canvas x:Name="MainCanvas" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" ClipToBounds="False">
<Canvas.RenderTransformOrigin>
<Point X="0.5" Y="0.5" />
</Canvas.RenderTransformOrigin>
<Canvas.RenderTransform>
<TransformGroup>
<TranslateTransform X="0" Y="0" /> <!--Handle this one through binding-->
<ScaleTransform ScaleX="1" ScaleY="1" CenterX=".5" CenterY=".5" /> <!--Handle this one through binding-->
</TransformGroup>
</Canvas.RenderTransform>
</Canvas>
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<ItemsControl.Resources>
<DataTemplate DataType="{x:Type local:Line}">
<Line Stroke="Black" StrokeThickness="1">
<Line.X1>
<MultiBinding Converter="{StaticResource LinePositionConverter}" ConverterParameter="X1">
<Binding ElementName="MainCanvas" Path="ActualWidth" />
<Binding ElementName="MainCanvas" Path="ActualHeight" />
<Binding Path = "StartPoint.X" />
</MultiBinding>
</Line.X1>
<Line.Y1>
<MultiBinding Converter="{StaticResource LinePositionConverter}" ConverterParameter="Y1">
<Binding ElementName="MainCanvas" Path="ActualWidth" />
<Binding ElementName="MainCanvas" Path="ActualHeight" />
<Binding Path = "StartPoint.Y" />
</MultiBinding>
</Line.Y1>
<Line.X2>
<MultiBinding Converter="{StaticResource LinePositionConverter}" ConverterParameter="X2">
<Binding ElementName="MainCanvas" Path="ActualWidth" />
<Binding ElementName="MainCanvas" Path="ActualHeight" />
<Binding Path = "EndPoint.X" />
</MultiBinding>
</Line.X2>
<Line.Y2>
<MultiBinding Converter="{StaticResource LinePositionConverter}" ConverterParameter="Y2">
<Binding ElementName="MainCanvas" Path="ActualWidth" />
<Binding ElementName="MainCanvas" Path="ActualHeight" />
<Binding Path = "EndPoint.Y" />
</MultiBinding>
</Line.Y2>
</Line>
</DataTemplate>
</ItemsControl.Resources>
</ItemsControl>
</Grid>
</UserControl>