Should I reuse view models in different views?

Question

I noticed that I have views that need the same information like others. But sometimes you need 5 properties of the view model and sometimes only 2.

Do you share such view model over many views or do you create a separate view model for each view or maybe do you prefere an inheritance or composition strategy?

For me there are some disadvantages for sharing view models:

1. Principle of Least Surprise: It is strange to fill only 2 properties of 5 of a view model and get null reference exception, because you don't want to query additional data of the database. When the view model has 5 properties I expect that all are filled. The exceptions prove the rule.
2. Separation of Concerns/Single Responsibility Principle: The view model cluttered up on complex sites, because you have to suit different needs for each view. If logic is involved its getting more complex, too.

What do you think? How do you handle such circumstances?

Answer 1

People tend to have different philosophies of ViewModels based on their perspective of their use. ViewModels are the glue between a view and a model and people will typically base their answer on which of the two ends they like to hold more rigid.

• If you like your model/data objects to be more rigid, then you'll tend to tie the ViewModel closer to the model/data—i.e. you'll have a single ViewModel that is used in multiple views and let the ViewModel determine which properties to retrieve based on how you want to handle data loading (and defer things like images or other long-load properties, etc.).
• If you like your Views to be more rigid, then you'll tie the ViewModel closer to the View—i.e. have a separate ViewModel for each view and let the model/data objects handle things like syncronization as you move from view to view.

Personally, I prefer the first as my data tends to be more rigid because it's less likely to change than the views are (in my projects—I don't think that's a universal property of data and views). Since change notifications are a natural feature of ViewModels, I don't have to make my model objects communicate changes if a user happens to have two views up that show the same/similar data.

Answer 2

In the project I am working on, each view has its own ViewModel, however we also have CollectionViewModels, which are shared/referenced by multiple view models.

Think - a list of Suppliers, that needs to be displayed in multiple screens in your application - and is bound to a variety of controls - a list box, grid view, whatever you need. Having just one ViewModel makes for simpler update/refresh logic of the list of Suppliers.

TLDR: I would only reuse view models, if all usage cases use the ViewModel in the same way. I.e. they all use the same properties etc.