wpf - how to refresh multi binding -

in listbox have items checkboxes , in textblock want update counter showing how many of them checked. simple scenario, have problems refreshing binding counter.

my xaml:

<window x:class="listboxwithcounting.mainwindow"         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"         xmlns:vm="clr-namespace:listboxwithcounting"         title="mainwindow" height="350" width="525">     <window.datacontext>         <vm:myviewmodel />     </window.datacontext>     <window.resources>         <datatemplate x:key="checkboxlistitem">             <border x:name="checkboxitemelement"                      background="transparent"                      padding="0,2,0,2">                 <checkbox content="{binding name}"                            ischecked="{binding path=ischecked,mode=twoway,                     updatesourcetrigger=propertychanged}"/>             </border>         </datatemplate>     </window.resources>     <grid>         <grid.columndefinitions>             <columndefinition width="*" />             <columndefinition width="*"/>         </grid.columndefinitions>         <grid.rowdefinitions>             <rowdefinition height="*" />         </grid.rowdefinitions>          <textblock grid.column="1">             <textblock.text>                 <multibinding stringformat="items checked: {0} of {1}">                     <binding path="columnschecked" />                     <binding path="columnscount" />                 </multibinding>             </textblock.text>         </textblock>          <listbox itemtemplate="{staticresource checkboxlistitem}"                           x:name="columns"                           itemssource="{binding columns}" />     </grid> </window> 

my view model , single item:

public class item : inotifypropertychanged     {         public string name { set; get; }          private bool ischecked;         public bool ischecked         {                         {                 return ischecked;             }             set             {                 ischecked = value;                 propertychanged(this, new propertychangedeventargs("ischecked"));             }         }          public event propertychangedeventhandler propertychanged = delegate { };     }      public class myviewmodel     {         public int columnscount         {                         {                 return columns.count;             }         }          public int columnschecked         {                         {                 return columns.where(x => x.ischecked).count();             }         }          public list<item> columns         {                         {                 var data = new list<item>()                 {                     new item(){ name = "item1", ischecked=true },                     new item(){ name = "item2" },                     new item(){ name = "item3" }                 };                  data.foreach(x => x.propertychanged += (s, e) => { });                  return data;             }         }     } 

how can trigger multibinding when each , every checkbox on list state changed?

[update]

thanks slugster, working code:

public class item : inotifypropertychanged     {         public string name { set; get; }          private bool ischecked;         public bool ischecked         {                         {                 return ischecked;             }             set             {                 if (ischecked != value)                 {                     ischecked = value;                     propertychanged(this, new propertychangedeventargs("ischecked"));                 }             }         }          public event propertychangedeventhandler propertychanged = delegate { };     }      public class myviewmodel : inotifypropertychanged     {         public int columnscount         {                         {                 return columns.count;             }         }          public int columnschecked         {                         {                 return columns.where(x => x.ischecked).count();             }         }          private list<item> columns;         public list<item> columns         {                         {                 if (columns == null)                 {                     columns = new list<item>()                     {                         new item(){ name = "item1", ischecked=true },                         new item(){ name = "item2" },                         new item(){ name = "item3" }                     };                 }                  columns.foreach(x =>                 {                     x.propertychanged -= x_propertychanged;                     x.propertychanged += x_propertychanged;                 });                  return columns;             }         }          void x_propertychanged(object sender, propertychangedeventargs e)         {             propertychanged(this, new propertychangedeventargs("columnschecked"));         }          public event propertychangedeventhandler propertychanged = delegate { };     } 

your viewmodel needs implement inotifypropertychanged binding can made aware of property change on list items.

to honest can't tell off top of head whether notifying on 1 property cause whole multibinding evaluated (in case can find out), want notify on both properties in case of other bindings.

public class myviewmodel : inotifypropertychanged {      [...snip...]      public list<item> columns     {                 {             var data = new list<item>()             {                 new item(){ name = "item1", ischecked=true },                 new item(){ name = "item2" },                 new item(){ name = "item3" }             };              data.foreach(x => x.propertychanged += (s, e) => {                                                                        onpropertychanged("columnscount");                                                                 onpropertychanged("columnschecked");                                                              });              return data;         }     }      private void onpropertychanged(string propertyname)     {         var handler = propertychanged;         if (handler != null)             handler(this, new propertychangedeventargs(propertyname));     }      public event propertychangedeventhandler propertychanged;  }