c# - Remove event from ViewModel when the User Control is unloading -

in code, attaching event @ vm method @ user control during @ loaded event, want remove event when user control unloaded, how can it?

at vm

public delegate bool checkcondition(); public class vmclass {      event checkcondition check; } 

at user control

public partial class acontrol:usercontrol {    public acontrol()    {        initializecomponent();     loaded += (s, e) =>            vm.check +=() => checkingcode();    }    public vmclass vm => (vmclass)datacontext; } 

i thinking using unloaded event, problem @ unloaded event, datacontext null, can't unsubscribe event, ie:

   unloaded +=(s, e) =>                vm.check -=() => checkingcode();  

doesn't work because vm null @ unloaded .

how can remove event attached vm.check when user control unloaded?

you can use datacontextchanged event.

datacontextchanged += (object sender, dependencypropertychangedeventargs e) => {     var oldvm = e.oldvalue vmclass;     var newvm = e.newvalue vmclass;      if (oldvm != null)     {         oldvm.check -= checkingcode;     }      if (newvm != null)     {         newvm.check += checkingcode;     } }