Question

I have read several articles and tutorials on the MVVM pattern but there is one thing I couldn’t find distinct information about. Let’s take the following example:

The model contains two fields and properties:

private string username;
public string Username {
  get; set;
}

private string password;
public string Password {
  get; set;
}

Now, what should the view model contain? Should it only contain an instance of the model or should it also “re-expose” the model’s properties?

private Model _model;
public Model Model {
  get; set;
}

Or…

private Model _model;
public Model Model {
  get; set;
}

public Username {
  get { return _model.Username; }
  set { _model.Username = value; }
}

I have seen both variants in several articles and now am unsure which way is correct.

Answer 1

There is no general right or wrong answer to this question. It depends on the context and on personal preference.

I personally tend to not expose the model to the view directly. I like to have a ViewModel that is specifically tailored for the view. I don’t want to implement anything into model just because the view needs it (and I might be tempted to do so when the the model is exposed to the view).

In my ViewModel, I like to have have as little dependencies on the model as possible. If I can get away with it, I like to have a ViewModel that does not have a direct dependency to the Model at all (and have some external entity/service fill do the mapping).