Updating Source on PropertyChanged on Silverlight

<UserControl x:Class="UpdateSourceTrigger.MainPage"
    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"
    xmlns:local="clr-namespace:UpdateSourceTrigger"
    mc:Ignorable="d"
    >

    <UserControl.DataContext>
        <local:PersonViewModel/>
    </UserControl.DataContext>
    
    <Grid x:Name="LayoutRoot" Background="White" Width="400" Height="300" HorizontalAlignment="Center" VerticalAlignment="Center">
        <Grid.RowDefinitions>
            <RowDefinition Height="*"/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <Grid>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="Auto"/>
                <ColumnDefinition Width="*"/>
            </Grid.ColumnDefinitions>
            <Grid.RowDefinitions>
                <RowDefinition Height="Auto"/>
                <RowDefinition Height="Auto"/>
                <RowDefinition Height="Auto"/>
                <RowDefinition Height="Auto"/>
            </Grid.RowDefinitions>
            <TextBlock VerticalAlignment="Center" HorizontalAlignment="Right" Text="First Name :" Grid.Column="0" Grid.Row="0"/>
            <TextBlock VerticalAlignment="Center" HorizontalAlignment="Right" Text="Middle Name :" Grid.Column="0" Grid.Row="1"/>
            <TextBlock VerticalAlignment="Center" HorizontalAlignment="Right" Text="Last Name :" Grid.Column="0" Grid.Row="2"/>
            <TextBlock VerticalAlignment="Center" HorizontalAlignment="Right" Text="Monthly Salary :" Grid.Column="0" Grid.Row="3"/>
            <TextBox Text="{Binding Context.FirstName, Mode=TwoWay}" Grid.Column="1" Grid.Row="0" Margin="5" local:UpdateSourceManager.UpdateOnPropertyChanged="True"/>
            <TextBox Text="{Binding Context.MiddleName, Mode=TwoWay}" Grid.Column="1" Grid.Row="1" Margin="5" local:UpdateSourceManager.UpdateOnPropertyChanged="True"/>
            <TextBox Text="{Binding Context.LastName, Mode=TwoWay}" Grid.Column="1" Grid.Row="2" Margin="5" local:UpdateSourceManager.UpdateOnPropertyChanged="True"/>
            <TextBox Text="{Binding Context.Salary, Mode=TwoWay}" Grid.Column="1" Grid.Row="3" Margin="5" local:UpdateSourceManager.UpdateOnPropertyChanged="True"/>
        </Grid>
        <Button Content="Save Context..." Margin="5" Grid.Row="1" HorizontalAlignment="Right" Width="125" Command="{Binding SaveContext}"/>
    </Grid>
</UserControl>