Code XAML :
<Grid>
<TextBox x:Name="txtPrixHT" HorizontalAlignment="Left" Height="35" Margin="33,137,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="123" />
<TextBox x:Name="txtPrixTTC" HorizontalAlignment="Left" Height="35" Margin="327,137,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="94" />
<Button x:Name="btnVersTTC" Content="-->" HorizontalAlignment="Left" Height="47" Margin="187,103,0,0" VerticalAlignment="Top" Width="111" Click="btnVersTTC_Click"/>
<Button x:Name="btnVersHT_" Content="<--" HorizontalAlignment="Left" Height="59" Margin="187,172,0,0" VerticalAlignment="Top" Width="111" Click="btnVersHT__Click"/>
<Label x:Name="label" Content="Prix HT:
" HorizontalAlignment="Left" Height="42" Margin="33,108,0,0" VerticalAlignment="Top" Width="113"/>
<Label x:Name="label_Copy" Content="Prix TTC:

" HorizontalAlignment="Left" Height="42" Margin="327,108,0,0" VerticalAlignment="Top" Width="113"/>
<Label x:Name="label1" Content="Taux:" HorizontalAlignment="Left" Height="23" Margin="45,231,0,0" VerticalAlignment="Top" Width="90"/>
<TextBox x:Name="txtTaux" HorizontalAlignment="Left" Height="23" Margin="45,254,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="120"/>
</Grid>