dependency injection - How does IoC container know which named instance to inject? -

when there multiple named implementations given interface, how container (i using unity in prism application) know 1 inject unless call container.resolve registered name? here simple example:

public interface idependencyclass {     void dosomething(); }  public class dependencyclassa : idependencyclass {     void dosomething() { } }  public class dependencyclassb : idependencyclass {     void dosomething() { } }  public interface iconsumer {     void takeuserspecificaction(); }  public class consumer : iconsumer {     idependencyclass dependencyinstance;     public consumer(idependencyclass _dependencyinstance)     {         dependencyinstance = _dependencyinstance;     }      public void takeuserspecificaction()     {         dependencyinstance.dosomething();     } }  public class mybootstrapper : unitybootstrapper {      protected override void configurecontainer()     {         base.configurecontainer();         container.registertype<idependencyclass, dependencyclassa>( "instancea" );         container.registertype<idependencyclass, dependencyclassb>( "instanceb" );         container.registertype<iconsumer, consumer>();     } } 

and here mainviewmodel application. "raisesomecommand" command not enabled until user has logged in. when enabled, can execute reaiseconsumercommandrequest, in turn calls consumer. here viewmodel.

public class mainwindowviewmodel     {         private readonly iregionmanager regionmanager;         private readonly iloginservice loginservice;         private readonly iconsumer consumer;         public icommand raisesomecommand { get; set; }         public mainwindowviewmodel( iregionmanager regmgr, iloginservice _loginservice, iconsumer _consumer )         {             regionmanager = regmgr;             loginservice = _loginservice;             consumer = _consumer;             navigatecommand = new delegatecommand<string>( navigate );             loginviewrequest = new interactionrequest<inotification>();             raisesomecommand = new delegatecommand( raiseconsumercommandrequest );         }  private void raiseconsumercommandrequest()             {                 consumer.takeuserspecificaction();             }          } 

so, when execute

consumer.takeuserspecificaction(); 

which dependencyclass instance using? dependencyclassa or dependencyclassb. also, if want use dependencyclassb, need make happen. don't want call

container.reslove<idependencyclass>("instanceb")  

in viewmodel because using container service locator. passing container reference around.

i have seen in code examples constructor parameter consumer class decorated dependency attribute below.

public class consumer {     idependencyclass dependencyinstance;     public consumer([dependency("instanceb")]idependencyclass _dependencyinstance)     {         dependencyinstance = _dependencyinstance;     } } 

but then, putting hard constraint on consumer use "instanceb" implementation. secondly, creating dependency unity. thirdly, have clone consumer class use "instancea" implementation. goes against dry principle.

i have heard these conditions application decisions , not ioc related logic. can agree argument. then, , how in application resolve right implementation without violating 1 rule or another?

i can't see how can inject right concrete instance unless choose use 1 of above 2 options. container.resolve or dependency attribute. can please?