c# - Type universe cannot resolve assembly: System.Xaml -

i'm trying create custom renderer progressbar can change color. classes below:

public class colorprogressbar : progressbar {     public static bindableproperty barcolorproperty = bindableproperty.create( "barcolor", typeof( color ), typeof( progressbar ), constants.orange   );      public color barcolor     {         { return (color)getvalue( barcolorproperty ); }         set { setvalue( barcolorproperty, value ); }     } } 

then, in myapp.uwp project, have:

[assembly: exportrenderer (typeof(colorprogressbar), typeof(progressbarrenderer))] namespace myapp.uwp {     public class colorprogressbarrenderer : progressbarrenderer     {         public colorprogressbarrenderer() {}          protected override void onelementchanged( elementchangedeventargs<progressbar> e )         {             base.onelementchanged( e );              if ( e.newelement == null )                 return;              if ( control != null )                 updatebarcolor();         }      protected override void onelementpropertychanged( object sender, propertychangedeventargs e )     {         base.onelementpropertychanged( sender, e );          if ( e.propertyname == colorprogressbar.barcolorproperty.propertyname )         {             updatebarcolor();         }     }      private void updatebarcolor()     {         var element = element colorprogressbar;         var color = windows.ui.colorhelper.fromargb( (byte)255, (byte)247, (byte)137, (byte)30 );         var brush = new windows.ui.xaml.media.solidcolorbrush( color );         control.foreground = brush;     } } } 

however, error in subject line along cannot resolve assembly or windows metadata file 'system.xaml.dll' when building. suspect has setting color , brush, i'm not sure how correct class otherwise.

i wanted change color :/

edit: added system.xaml.dll assembly, , error in app.xaml when building: object reference not set instance of object. there conflict between xamarin , system.xaml.dll?