I've got errors: The type or namespace name 'KinectSensorManager' could not be found (are you missing an assembly or an assembly reference?)

I have these referenced:

using Microsoft.Kinect;
using Microsoft.Samples.Kinect.WpfViewers;
using Microsoft.Kinect.Toolkit;

My piece of code:

public KinectSensorManager KinectSensorManager
get { return (KinectSensorManager)GetValue(KinectSensorManagerProperty); }
set { SetValue(KinectSensorManagerProperty, value); }

The very first KinectSensorManager is the error and has a blue underline. But others KinectSensorManager are not errors.

Does anyone know what is wrong?