visual studio 2012 - Error in MVVM when building -

i created pcl called tipcalc>core, tutorial witch building one . here tipviewmodel.cs

using cirrious.mvvmcross.viewmodels;  namespace tipcalc.core { public class tipviewmodel : mvxviewmodel {     private readonly icalculation _calculation;     public tipviewmodel(icalculation calculation)     {         _calculation = calculation;     }      public override void start()     {         _subtotal = 100;         _generosity = 10;         recalcuate();         base.start();     }      private double _subtotal;      public double subtotal     {         { return _subtotal; }         set { _subtotal = value; raisepropertychanged(() => subtotal); recalcuate(); }     }      private int _generosity;      public int generosity     {         { return _generosity; }         set { _generosity = value; raisepropertychanged(() => generosity); recalcuate(); }     }      private double _tip;      public double tip     {         { return _tip; }         set { _tip = value; raisepropertychanged(() => tip); }     }      private void recalcuate()     {         tip = _calculation.tipamount(subtotal, generosity);     } }  } 

the problem when cuild pcl, following errors:

error   1   type or namespace name 'icalculation' not found (are missing using directive or assembly reference?) tipcalc.core error   2   type or namespace name 'icalculation' not found (are missing using directive or assembly reference?) 

altough interface , class,are right there in services folder,in project.

calculation.cs

using system; using system.collections.generic; using system.linq; using system.text;  namespace tipcalc.core.services { public class calculation : icalculation {     public double tipamount(double subtotal, int generosity)     {         return subtotal * ((double)generosity) / 100.0;     } }  } 

and icalculation.cs

using system; using system.collections.generic; using system.linq; using system.text;  namespace tipcalc.core.services {  public interface icalculation {     double tipamount(double subtotal, int generosity); } } 

any please?

you need add using in calculation.cs

to use icalculation.cs

using tipcalc.core.services;