Exemplo de como criar um barcode scanner com Xamarin.Forms http://julianocustodio.com/scanner

Scanner.xaml.cs

        private void OpenScanner(object sender, EventArgs e)
        {
            Scanner();
        }

        public async void Scanner()
        {

            var ScannerPage = new ZXingScannerPage();

            ScannerPage.OnScanResult += (result) => {
                // Parar de escanear
                ScannerPage.IsScanning = false;

                // Alert com o código escaneado
                Device.BeginInvokeOnMainThread(() => {
                    Navigation.PopAsync();
                    DisplayAlert("Código escaneado", result.Text, "OK");
                });
            };

            
            await Navigation.PushAsync(ScannerPage);

        }