<ItemsControl Name="DialogItemsControl" ItemsSource="{Binding Messages, Mode=OneWay}" Background="Transparent"
                          BorderBrush="Transparent" TargetUpdated="DialogItemsControl_TargetUpdated">
                <ItemsControl.ItemTemplate><!-- For ever message -->
                    <DataTemplate>
                        <Grid Margin="0,0,0,20">
                            <ItemsControl Name="SubDialogItemsControl"
                                  Foreground="{DynamicResource ButtonTextBrush}"
                                  ItemsSource="{Binding Lines,NotifyOnTargetUpdated=True}"
                                  Margin="0,0,0,12"
                                  Grid.Column="0">
                                <ItemsControl.ItemTemplate><!-- For every line -->
                                    <DataTemplate>
                                        <TextBlock Name="DialogMessageText"
                                                   Text="{Binding NotifyOnTargetUpdated=True}"
                                            VerticalAlignment="Top"
                                            Margin="0,2,0,2"
                                            TextTrimming="WordEllipsis"/>
                                    </DataTemplate>
                                </ItemsControl.ItemTemplate>
                            </ItemsControl>
                        </Grid>
                    </DataTemplate>
                </ItemsControl.ItemTemplate>
            </ItemsControl>

private void DialogItemsControl_TargetUpdated(object sender, System.Windows.Data.DataTransferEventArgs e)
        {
          ItemsControl itemControl = sender as ItemsControl;

            ContentPresenter dp =   itemControl.ItemContainerGenerator.ContainerFromItem(itemControl.Items.CurrentItem) as ContentPresenter;

            // Finding textBlock from the DataTemplate that is set on that ContentPresenter
            DataTemplate myDataTemplate = dp.ContentTemplate;
            ItemsControl itc = (ItemsControl)myDataTemplate.FindName("SubDialogItemsControl", dp);
            if (itc != null && itc.ItemContainerGenerator.Status == System.Windows.Controls.Primitives.GeneratorStatus.ContainersGenerated)
            {
                ContentPresenter cp = itc.ItemContainerGenerator.ContainerFromIndex(0) as ContentPresenter;
                DataTemplate dt = cp.ContentTemplate;
                TextBlock tb = dt.LoadContent() as TextBlock;

                tb.TargetUpdated += new EventHandler<System.Windows.Data.DataTransferEventArgs>(myTextBlock_TargetUpdated);
            }
        }

 void myTextBlock_TargetUpdated(object sender, System.Windows.Data.DataTransferEventArgs e)

       {

            TextBlock tb = sender as TextBlock;
           //When i access the text property of tb, its showing null, how to get the text
        }

public class CustomTextBlock : TextBlock
    {
        static CustomTextBlock()
        {
            TextProperty.OverrideMetadata(typeof(CustomTextBlock), new FrameworkPropertyMetadata(null,
                new PropertyChangedCallback(
                    (dpo, dpce) =>
                    {
                        //Flash the background to yellow for 2 seconds
                        var myTxtblk = dpo as CustomTextBlock;
                        if (myTxtblk != null)
                        {
                            myTxtblk.Background = Brushes.Yellow;
                            Task.Factory.StartNew(
                                () =>
                                {
                                    Thread.Sleep(2000);
                                    Application.Current.Dispatcher.Invoke(
                                        new Action(() =>
                                        {
                                            myTxtblk.Background = Brushes.Transparent;
                                        }));
                                });
                        }
                    })));
        }
    }

<local:CustomTextBlock Text="{Binding MyDynamicText}"/>