mvvm - WPF binding does not work with a UserControl -

in mainviewmodel have property, set user. when new value received, update information @ usercontrol. problem binding not work. way work following. after selectedday changed, day property in user control code behind changes, invoking calculateday method. however, method never called!

i guess, reason simple , not understand binding mechanism, cannot figure out wrong , need help. have checked number of similar questions, never found appropriate answer.

the code have:

mainviewmodel

private day _selectedday; public day selectedday {     { return _selectedday; }     set     {         _selectedday = value;         raisepropertychanged("selectedday");     } } 

mainwindow xaml

<controls:daycontrol day="{binding selectedday, mode=twoway}"></controls:daycontrol> 

usercotrol code behind

public day day {     { return (day)getvalue(dayproperty); }     set     {         setvalue(dayproperty, value);         calculateday();     } }  public static readonly dependencyproperty dayproperty =     dependencyproperty.register("day", typeof(day), typeof(daycontrol), new propertymetadata(new day())); 

according msdn

in exceptional circumstances, wrapper implementations should perform getvalue , setvalue actions, respectively. reason discussed in topic xaml loading , dependency properties.

you should use propertychangedcallback in definition dependency property instead. following code directly taken msdn.

public static readonly dependencyproperty aquariumgraphicproperty = dependencyproperty.register(   "aquariumgraphic",   typeof(uri),   typeof(aquariumobject),   new frameworkpropertymetadata(null,       frameworkpropertymetadataoptions.affectsrender,        new propertychangedcallback(onurichanged)   ) ); 

define calculateday in place of onurichanged above.