gistfile1.cs

public class FormulaSample : ObservableBase
{
    private decimal a;
    public decimal A
    {
       get { return a; }
       set
       {
          this.a = value;
          RaisePropertyChanged("A");
       }
   }

   private decimal b;
   public decimal B
   {
       get { return b; }
       set
       {
          this.b = value;
          RaisePropertyChanged("B");
       }
    }

    public Formula<decimal> C
    {
        get
        {
           return Formula(() => this.A + this.B, "A", "B");
        }
    }
}