Silverlight PropertyChanged event not firing in two-way textbox binding -

if class property two-way bound text property of textbox, when change text in text box, "propertychanged" event of class supposed fire? doesn't in case - message box never pops when change text box content. binding works though, if properties change textbox changes, , vice versa.

edit: realize fires, when textbox loses focus. guess there must reason behind behavior.

code:

    public class baselineareas : inotifypropertychanged     {         public event propertychangedeventhandler propertychanged;         private double _impervious = 0;         public double impervious         {             { return _impervious; }             set             {                 if (value != this._impervious)                 {                     _impervious = value;                     notifypropertychanged("impervious");                 }             }         }         private double _pervious = 0;         public double pervious         {             { return _pervious; }             set             {                 if (value != this._pervious)                 {                     _pervious = value;                     notifypropertychanged("pervious");                 }             }         }         public void notifypropertychanged(string propertyname)         {             if (propertychanged != null)             {                 propertychanged(this, new propertychangedeventargs(propertyname));             }         }     }      private baselineareas baselineareas = new baselineareas();      public mainpage()     {         initializecomponent();          baselineareas.impervious = 100;         baselineareas.pervious = 500;         textboxtotalimperviousarea.datacontext = baselineareas;         textboxtotalperviousarea.datacontext = baselineareas;         baselineareas.propertychanged += new propertychangedeventhandler(baselineareas_propertychanged);     }      void baselineareas_propertychanged(object sender, propertychangedeventargs e)     {         messagebox.show(baselineareas.impervious.tostring());     } 

xaml:

                <textbox height="23" horizontalalignment="left" margin="138,16,0,0" name="textboxtotalimperviousarea" verticalalignment="top" width="62"                           text="{binding impervious, mode=twoway}"/>                 <textbox height="23" horizontalalignment="right" margin="0,45,226,0" name="textboxtotalperviousarea" verticalalignment="top" width="62"                           text="{binding pervious, mode=twoway}"/>  

it's possible give textbox behavior in xaml make update on every character change:

http://dotneteers.net/blogs/vbandi/archive/2009/12/24/make-a-silverlight-textbox-update-its-binding-on-every-character.aspx