Windows phone 8 Camera is not initialize

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C#

hello greetings

I am trying to initialize camera with the following code is not getting initialize.

protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
        {
            // Initialize the camera, when available.
            camera   = new PhotoCamera();

            // Event is fired when the PhotoCamera object has been initialized.
            camera.Initialized += new EventHandler<CameraOperationCompletedEventArgs>(cam_Initialized);


            //Set the VideoBrush source to the camera.
            viewfinderBrush.SetSource(camera);
        }

        void cam_Initialized(object sender, CameraOperationCompletedEventArgs e)
        {
            if (e.Succeeded)
            {
                this.Dispatcher.BeginInvoke(delegate()
                {
                    MessageBox.Show("Camera initialized.");
                });
            }
        }

        protected override void OnOrientationChanged(OrientationChangedEventArgs e)
        {
            if (camera != null)
            {
                // LandscapeRight rotation when camera is on back of phone.
                int landscapeRightRotation = 180;

                // Change LandscapeRight rotation for front-facing camera.
                if (camera.CameraType == preferences.CameraDevice.Value) landscapeRightRotation = -180;

                // Rotate video brush from camera.
                if (e.Orientation == PageOrientation.LandscapeRight)
                {
                    // Rotate for LandscapeRight orientation.
                    viewfinderBrush.RelativeTransform =
                        new CompositeTransform() { CenterX = 0.5, CenterY = 0.5, Rotation = landscapeRightRotation };
                }
                else
                {
                    // Rotate for standard landscape orientation.
                    viewfinderBrush.RelativeTransform =
                        new CompositeTransform() { CenterX = 0.5, CenterY = 0.5, Rotation = 0 };
                }
            }

            base.OnOrientationChanged(e);
        }

Thank you in advance