Question

I have MVVM Project and I want to share one object( singleton ) from the model between several viewmodel what is the good practice to do that?
Thank you for the help

Answer 1

If the object is needed and does not provide value without it force the interface within the object via Constructor Injection; do not push a concrete type via injection always make use of an interface.

Since you are not making use of an IoC container such as Unity, you will need to create your singleton instance at the startup of your application and then make sure that the given instance is passed in via the given ViewModels constructor as needed.

A better approach would be pushing the singleton instance to a service which can provide the needed behavior and then disregard pushing the singleton into the Model. This would be a more MVVM purist approach and will separate concerns across your Models/ViewModels.

EDIT:

If you were making use of Unity you would define a Lifetime Manager at the time of registration.

// Register a type to have a singleton lifetime without mapping the type
// Uses the container only to implement singleton behavior
myContainer.RegisterType<MySingletonObject>(new ContainerControlledLifetimeManager());
// Following code will return a singleton instance of MySingletonObject
// Container will take over lifetime management of the object
myContainer.Resolve<MySingletonObject>();

Once you do this any attempt to resolve MySingletonObject via the IUnityContainer would resolve to the same instance providing the singleton behavior you so desire across the application. ViewModels themselves should not need to have the same instance returned. The data it needs should be abstracted away via a service as referenced earlier which could potentially behave like a singleton and provide a stateful implementation if needed but the ViewModel should not need to be a singleton. If you find yourself making either a Model or ViewModel a singleton; take a step back and analyze your design.