Listbox Selected item

file1.xaml

<ListBox x:Name="mylistbox" Margin="0,0,0,101" SelectionChanged="Selectionchanged_Eventhandler_of_Listbox">
                <ListBox.ItemTemplate>
                    <DataTemplate>
                        <Grid Height="80" VerticalAlignment="Top">
                            <TextBlock x:Name="nameblock" Text="{Binding name}" HorizontalAlignment="Left" TextWrapping="Wrap" VerticalAlignment="Top" Height="57" Width="236" FontSize="36" Margin="10,10,0,0" />
                            <Button Content="Marks" HorizontalAlignment="Left" Margin="275,0,0,0" VerticalAlignment="Top" Height="80" Width="181" Click="Marks_button_click"/>
                        </Grid>
                    </DataTemplate>
                </ListBox.ItemTemplate>
            </ListBox>
            <TextBlock x:Name="Studentnameblock" HorizontalAlignment="Left" Margin="40,543,0,0" TextWrapping="Wrap" Text="Student name" VerticalAlignment="Top" Height="64" Width="193" FontSize="30"/>
            <TextBlock x:Name="marksblock" HorizontalAlignment="Left" Margin="322,543,0,0" TextWrapping="Wrap" Text="Marks" VerticalAlignment="Top" Height="64" Width="134" FontSize="30"/>