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How to inject dependencies into view models in Caliburn.Micro?

I really hope this is not a stupid question, but I'm somehow unable to recognize a straight-forward way to inject dependencies into view models using Caliburn.Micro.
I'm having a main shell (conductor) like so:
public class ShellViewModel : Conductor<IScreen>.Collection.OneActive, IShell
{
public ShellViewModel(IEventAggregator eventAggregator) {
ActivateItem(new DashboardViewModel());
}
}
Now I'd like to inject a service into the DashboardViewModel but since the ActivateItem method requires me to pass an instance (rather than e.g. a type), I'm forced to provide the service myself. And since the ShellViewModel isn't aware of an underlying IoC container, I have to have the service injected into the shell.. to me, it looks like Caliburn is trying to enforce a complete graph of all view models and dependencies within an application.
I am aware that I could use a static accessor for my inversion of control container, but I really don't like this approach because I'd like to have a single composition root for my application (the bootstrapper) without having the other parts knowing about dependency injection and such.

couple of ways to do it dependent completely on your choice of container.
MEF [ImportMany] used in the constructor on the parameter that will do the actual import reference the Hello Screens example
The baked in IoC static class you could use IoC.Get<IDashBoard>() or IoC.GetAll<IDashBoard>(), this assumes you have register your types into the container you use. Caution with this one it can be over used and result in anti-pattern behavior. I have done this in one of my apps that does a dashboard, anything that is tagged with IDashBoard in my Container instance, in association with the actual implementation class will get pulled into the collection IoC.GetAll<IDashboard>() or the first item in the collection based on the IoC.Get<IDashBoard>().
You could also make your dashboard inherit Conductor<IDashBoard>.Collection.AllActive, there by allowing you access to the Items property (as part of the Collection) and populate it with the CTOR of your DashBoardViewMode, using IoC.GetAll<IDashboard>() in that one place get all the items you need on your dashboard. From there, I query the Items collection property in OnActivate and match the other viewmodels to the properties that I needed and placed named ContentControls on the DashBoardView accordingly.
this does pull from the container that you have chosen, keeping that in mind you might just want to you the containers' methods to grab the necessary items through its intended design.
I actually moved away from MEF since version used in CM doesn't work with Open Generics and debugging missing Export() attributes was starting to wear me out.

Change current implementation of basic MVVM to adhere to SOLID pattern

I have been writing all my MVVM application with basic design pattern generally mentioned in MVVM examples available online. The pattern that I am following is described below:
Model
This section include DTO classes with their properties and Interfaces IDataService and like:
public class Employee
{
public string EmployeeName { get; set; }
public string EmployeeDesignation { get; set; }
public string EmployeeID { get; set; }
}
public interface IDataService
{
public Task<Employee> GetEmployeeLst();
}
Proxy
This layer contains Dataservice calls which implement IDataservice like:
public class DataService : IDataService
{
public async Task<Employee> GetEmployeeLst()
{
// Logic to get employee data from HTTPClient call
}
}
ViewModel
This layer contains ViewModel and reference to Model and Proxy layer from which all data is received:
public class BaseViewModel
{
public BaseViewModel(INavigationService nav, IDataService data, IAESEnDecrypt encrypt, IGeoLocationService geoLocation, IMessageBus msgBus, ISmartDispatcher smtDispatcher)
{
}
// This also include common methods and static properties that are shared among most of the ViewModels
}
All the ViewModel inherits BaseViewModel. Each viewModel also contains Delegatecommand which is executed when UI triggers an event. Which then fetches the data from the server by calling DataService in proxy layer and perform business logic and populates the properties in ViewModel which is binded to the view. For each View there is a VM which is binded to the Datacontext of the View.
ViewModel is also responsible for starting an animation I have used trigger to start storyboard which is binded to my enums in VM for state change of these trigger as in example in: http://www.markermetro.com/2011/05/technical/mvvm-friendly-visual-state-management-with-windows-phone-7/
View
In this layer I have all my Views, Usercontrols and business logic with implementation of certain dependencies like GeoLocation Service, AES encryption, NavigationService between Views etc.
Every View has .xaml and .xaml.cs file. In .xaml.cs file I have binded the data context of the view with VM like this:
this.DataContext = App.IOConatiner.GetInstance<DashboardViewModel>();
and from here on all binding happens.
My problem is that recently I had the knowledge that this pattern is not following a SOLID design pattern which I got know in this answer of my question:
Simple Injector inject multiple dependency in BaseClass
I am trying very hard to change my design as per the suggestion given in my previous question's answer. But I am not able to get some of the things like:
Currently View Datacontext is binded to ViewModel hence all the controls are controlled by a property in VM. How would I change this to your above mentioned pattern with Processor/Service or DialogHandler?
I am using Delegatecommands which are binded to command property of UI element. Execution of these command certain action happens like animation, usercontrol is displayed. How to do it in command pattern?
How can I start changing my current implementation to accommodate all those changes with best possible approach?

First of all an answer to your question 3
How can I start changing my current implementation to accommodate all those changes with best possible approach?
This is the very first step you need to take. It is not a case of some smart refactoring of your current code. You will need to take a step back and design the application. I once read some nice blog about (re)design.
Before starting to write any code, define how many different basic types of views you will want to show to the user? E.g.:
Just show (any type of) data
Edit data
Alert user
Ask user for input
...
When you defined your different requirements, you can translate this to specific interfaces that are tailor made for the job they serve. For example a view that lets the user edit data will typically have an interface that will look something like:
public interface IEditViewModel<TEntity>
{
public EditResult<TEntity> EditEntity(TEntity entityToEdit)();
}
Once you every detail of this design in place, you must decide how you will show your views to the user. I used another interface for this to handle this task for me. But you could also decide to let a navigation service handle this kind of task.
With this framework in place, you can start coding your implementations.
Currently View Datacontext is binded to ViewModel hence all the controls are controlled by a property in VM. How would I change this to your above mentioned pattern with Processor/Service or DialogHandler?
This will not change in this design. You will still bind your view to your viewmodel and set the datacontext to the viewmodel. With a lot of views the use of an MVVM framework like Caliburn Micro will come in handy. This will do a lot of the MVVM stuff for you, based on Convention over Configuration. To start with this model, would make the learning curve even higher, so my advice to start of by hand. You will learn this way what happens under the covers of such an MVVM tool.
I am using Delegatecommands which are binded to command property of UI element. Execution of these command certain action happens like animation, usercontrol is displayed. How to do it in command pattern?
I'm not sure if the command pattern you mention here is the command pattern I advised you in the previous answer. If so, I think you need to reread this blog, because this is totally unrelated to the commands I think you mean in this question.
Animation and that sort of stuff is the responsibility of the view, not the viewmodel. So the view should handle all this stuff. XAML has a lot of ways to handle this. More than I can explain here. Some ideas: Triggers, Dependency Properties
Another option: Code behind! If the logic is purely view related IMO it is not a mortal sin to place this code in the code behind of your view. Just don't be temped to do some gray area stuff!
For commands that just perform a method call in your viewmodel, ICommand is still possible and MVVM tools like Caliburn will do this automagically...
And still: Loose the base class....

Why are you injecting all these services in your viewmodel base class if the viewmodel base class does not make use of these services himself ?
Just inject the services you need in the derived viewmodels that do need those services.

Simple Injector inject multiple dependency in BaseClass

I have a BaseViewModel which is inherited by multiple ViewModel classes. In my BaseViewModel I have a couple of dependencies which get injected from ViewModel. Now if I need to add a new dependency in my BaseViewModel I need to change all the VM which inherit BaseViewModel. Please let me know how can it be handled in Simple Injector. Following is my code structure:
How can I make my base class injection independent so that I don't need to make changes in all my inherited class?
Code:
public class BaseViewModel
{
protected readonly IAESEnDecrypt AESEnDecrypt;
protected readonly IDataService DataService;
protected readonly INavigationService NavigateToPage;
public BaseViewModel(INavigationService nav, IDataService data, IAESEnDecrypt encrypt)
{
AESEnDecrypt= encrypt;
NavigateToPage = nav;
DataService = data;
}
}
public class ViewModel
{
public ViewModel(INavigationService nav, IDataService data, IAESEnDecrypt encrypt) : base (nav, data, encrypt)
{
}
}
My BaseViewModel Contains some of the following Interfaces whose implementation is injected through constructor:
- NavigationService
- DataService
- GeoLocationService
- SmartDispatcher
- MessageBus which implement Message Aggregator
It also Contains some common properties as static variables whose data is used throughout the application like UserDetails. And also contains CancellationToken, IsBusy to display progressbar.
BaseViewModel also contain HandleException method which handle all the incoming exceptions from all ViewModel.
Also Contains some common Commands which are used in all the Views like Si
gnoutCommand, NavigationBar Commands.
Actually it has started to contain all kinds of common methods used among various ViewModel.
Please suggest how can i refactor this code?

Your last sentence:
Actually it has started to contain all kinds of common methods used among various ViewModel
Precisely describes your problem! As Steven already described, that you're building almost the complete application through a single base class. Thereby infringing the Open-Closed principle which you are heavinly experiencing now.
The trick is design your application around very small SOLID ViewModels of which you compose the application at runtime. By splitting the ViewModels and using a UserControl as your views you can compose big complicated views for the user, while you still get all the benefits from using a SOLID design. So let’s take a look at some of your different interfaces that you implement and some of the functions you ‘handle’ in the base class:
NavigationService
This sounds like a service which controls the flow in your application. This sounds to me like your mainview(model). You could create a single MainViewModel which as a single property, let’s say CurrentView.Assuming you’re using WPF you typically would bind this property to a ContentControl. The content of this control can be everything from a single TextBlock to a complete UserControl. The UserControls can still be very complicated as they could be composed of multiple child usercontrol and so on. Using a MVVM framework (like e.g. Caliburn Micro or MVVM Light) for this is optionally but will come in handy.
It could also be an application global service with some of kind of callback or delegate function to perform navigation to a certain View(Model). It is in any case an infrastructural part of your application that deserves it own class and shouldn't be put away in a base class.
DataService
A single dataservice was the way I worked for over 10 years. Every time I hit my head against the wall. There comes a point in time that you need something special which is not included in your dataservice and you will probably go through your complete code base to make the right adjustments. Speaking of the Open-Closed principle…
Than I learned about the Command/Handler and Query/Handler pattern. You can read about this here and here. Using this pattern everywhere you need data you just inject the correct IQueryHandler<,> and use it right there. Not every view(model) needs data and certainly not the same data. So why use a global DataService? This is will also improve your Lifetime management of your DBContext object.
HandleException
Why is your base class responsible for handling the exceptions of your viewmodel? What does the base class know about this exceptions? What does the base class do? Log the exception, show a message to the user (what kind of message?) and silently continue? Letting the application break down 3 minutes later and leaving a user ignorant of what happened?
I.M.O. exception should not be catched if you didn’t expect them to be thrown in the first place. Than log the exception at an application level (e.g. in your Main), show an ‘Excuse me’ message to the user and close the application. If you expect an exception, handle it right there and then and handle according.
UserDetails
Ask yourself the question how many of your 40 ViewModels actually need this information? If all 40 are in need of this information, there is something else wrong with your design. If not, only inject this details (or even better an IUserContext) in the ViewModels that actually use them.
If you use it for some kind of authentication consider using a decorator wrapping the task they need permission for performing it.
IsBusyIndicator
Again: do you need this in every ViewModel? I think not. I think furthermore, showing the user a busy indicator is a responsibility of the View, not the ViewModel and the as the length of the task determines if you need to show this, make it a responsibility of the task (assuming you’re looking at your tasks also in a SOLID manner by using e.g. the already mentioned Command/Handler pattern).
With WPF you could define a Dependency Property that you can bind to the view, thereby showing some kind of busy indicator. Now just inject a ShowBusyIndicatorService in the task that needs to show it. Or wrap all your (lengthy) tasks in a ShowBusyIndicatorDecorator.
Design
Now let’s look at some simple interfaces you could define to build up your View(Model)s. Let’s say we decide to make every ViewModel responsible for a single task and we define the following (typical LoB) tasks:
Show (any kind of) data
Select or choose data
Edit data
A single task can be stripped down to ‘Show data of single datatype (entity)’. Now we can define the following interfaces:
IView<TEntity>
ISelect<TEntity>
IEdit<TEntity>
For each interface type you would create a Processor/Service or DialogHandler depending on your semantic preferences which would do the typical MVVM stuff like finding a corresponding view and binding this to viewmodel and show this in some way (a modal window, inject it as usercontrol in some contentcontrol etc.).
By injecting this single Processor/Service or DialogHandler in the your ‘Parent’ ViewModel where you need to navigate or show a different view you can show any type of entity by a single line of code and transfer the responsibility to the next ViewModel.
I’m using these 3 interfaces now in a project and I really can do everything I could do in the past, but now in SOLID fashion. My EditProcessor, interface and viewmodel look like this, stripped down from all not so interesting stuff. I’m using Caliburn Micro for the ViewModel-View Binding.
public class EditEntityProcessor : IEditEntityProcessor
{
private readonly Container container;
private readonly IWindowManager windowManager;
public EditEntityProcessor(Container container, IWindowManager windowManager)
{
this.container = container;
this.windowManager = windowManager;
}
public void EditEntity<TEntity>(TEntity entity) where TEntity : class
{
// Compose type
var editEntityViewModelType =
typeof(IEntityEditorViewModel<>).MakeGenericType(entity.GetType());
// Ask S.I. for the corresponding ViewModel,
// which is responsible for editing this type of entity
var editEntityViewModel = (IEntityEditorViewModel<TEntity>)
this.container.GetInstance(editEntityViewModelType);
// give the viewmodel the entity to be edited
editEntityViewModel.EditThisEntity(entity);
// Let caliburn find the view and show it to the user
this.windowManager.ShowDialog(editEntityViewModel);
}
}
public interface IEntityEditorViewModel<TEntity> where TEntity : class
{
void EditThisEntity(TEntity entity);
}
public class EditUserViewModel : IEntityEditorViewModel<User>
{
public EditUserViewModel(
ICommandHandler<SaveUserCommand> saveUserCommandHandler,
IQueryHandler<GetUserByIdQuery, User> loadUserQueryHandler)
{
this.saveUserCommandHandler = saveUserCommandHandler;
this.loadUserQueryHandler = loadUserQueryHandler;
}
public void EditThisEntity(User entity)
{
// load a fresh copy from the database
this.User = this.loadUserQueryHandler.Handle(new GetUserByIdQuery(entity.Id));
}
// Bind a button to this method
public void EndEdit()
{
// Save the edited user to the database
this.saveUserCommandHandler.Handle(new SaveUserCommand(this.User));
}
//Bind different controls (TextBoxes or something) to the properties of the user
public User User { get; set; }
}
From you IView<User> you can now edit the current selected User with this line of code:
// Assuming this property is present in IView<User>
public User CurrentSelectedUser { get; set; }
public void EditUser()
{
this.editService.EditEntity(this.CurrentSelectedUser);
}
Note that by using this design you can wrap your ViewModels in a decorator to do crosscutting concerns, like logging, authentication and so on.
So this was the long answer, the short one would be: loose the base class, it is biting you and it will bite you more and harder!

Prevent having this base class in the first place. This base class is a big code smell and the result is your current pain. Such a base class will violate the Single Responsibility Principle (SRP) and will just act as a big helper class for all derived view models, or it even seems that you are putting cross-cutting concerns in there. The base class might even hide the fact that your view models violate the SRP. They probably do too much; have too many responsibilities.
Instead, try to do the following:
Move cross-cutting concerns out of the base class into decorators or find another way to apply cross-cutting concerns.
Group related dependencies together into a aggregate service and inject such aggregate service into your view model.
In a well designed application, there is hardly ever a need for having such base class that takes dependencies.
If you aren't able to change your design (but please do take a look it this; you will be in a much better place without that base class), you can revert to explicit property injection. Simple Injector does not do this out-of-the-box, but the documentation describes how to do this.
Basically, it comes down to writing a custom IPropertySelectionBehavior, moving the constructor dependencies of the BaseViewModel to public properties and marking them with a custom attribute.
But again, only use property injection as a last resort. Property injection will only hide the design problem; it will not solve it.

You can use the ServiceLocator (anti)pattern to make the injection independent, HOWEVER you should not do this as it violates the principles of SOLID. Mark Seemann - Service Locator violates SOLID
You should rather stick to adding the dependencies in the constructor as this is in line with SOLID OO design principles.

Having my Viewmodel appear in namespace dropdown

I'm trying to expose the ViewModel as a static resource on the page so that it can be easily accessible by the binding.
TestViewModel.cs
namespace Test.WPFUI.Home
{
public class TestViewModel....
HelloWorldView.Xaml
xmlns:local="clr-namespace:Test.WPFUI.Home"
<UserControl.Resources>
<local:TestViewModel x:Key="mainPageViewModel" />
</UserControl.Resources>
TestViewModel Can't be found. May I ask for some tips or suggestions Please.
Getting help from http://www.telerik.com/help/silverlight/gridview-troubleshooting-blank-cells.html
public class LoanViewModel : ScreenViewModelBase<LoanViewModel>, IRecord, INotifyPropertyChanged
{
public LoanViewModel(IEventAggregator events) .............

It sounds like your initial problem was not having the full xmlns definition. You usually need both the namespace and assembly.
The easiest way to get it right, in my experience, is to let intellisense do it for you. Just start typing the namespace you want, and as long as its in a referenced project, there will be an autocomplete option.
Your second problem is due to not having a default constructor. You wrote this:
<local:TestViewModel x:Key="mainPageViewModel" />
Which will invoke the default constructor. However, you define a constructor here:
public LoanViewModel(IEventAggregator events) .............
Which removes the provided (paramaterless) default constructor. I'm going to take a wild guess and say that creating the correct IEventAggregator is not simple or desired from XAML, so I see two choices:
You didn't really need that parameter in the constructor. Simply add a default constructor to your view model and you are good to go!
You really need that parameter, so instantiating from XAML just isn't a good idea. Pass in your view model from somewhere else on the view's constructor.
If you feel like you can instantiate the correct object from XAML, use this post to invoke the paramaterized constructor: Calling a parameterized constructor from XAML
In my opinion, putting truly regular classes into XAML is not a good pattern to follow, so I wouldn't. By regular, I mean not related at all to the view.

WPF MVVM - Accessing properties of other ViewModels using delegates

I have a MainViewModel, which features PersonViewModel and a HouseViewModel as properties. HouseViewModel has the property GetRooms. What is the best way to access this property from the PersonViewModel?
My solution at the minute is to pass through an instance of MainViewModel to PersonViewModel, then I can call MainViewModel.HouseViewModel.GetRooms. However, this seems a little wasteful.
I am happy to pass a function as a delegate, but I can't seem to do this with a Property. I have searched for an example of this and only come up with overly complicated techniques. I'm assuming there must be a simple way of doing this, as it seems like a common problem. Can anyone point out a strong example?
Or is there another, alternative method that I haven't considered?

If a method has to be shared across two viewmodel, it should be defined in base viewmodel or a service. The best way is a common Service class should hold all common methods like GetRooms, CheckIn, CheckOut, etc. And this service should be provided to every viewmodel using Dependency Injection.
public class HomeViewModel
{
public HomeViewModel(IRoomService roomservice)
{
}
}
public class PersonViewModel
{
public PersonViewModel(IRoomService roomservice)
{
}
}