Namespace Xamarin.Forms.Core not found

Hi,

I'm just starting with Xamarin.Forms. I now ran into a problem when trying to write a XAML markup extension as described in Petzolds book to access embeded images.

using System;

using Xamarin.Forms;
using Xamarin.Forms.Core;

namespace EmbeddedResources
{

    [ContentProperty("Source")]
    public class ImageResourceExtension : IMarkupExtension
    {
        public string Source { get; set; }
        public object ProvideValue(IServiceProvider serviceProvider)
        {
            if (Source == null)
                return null;
            return ImageSource.FromResource(Source);
        }
    }
}

But I get the message Type or Namespace Core is not included in Xamatin.Forms. But As shown in the project explorer it's there as you can see on the screenshot.

All the best
Thomas