In which class should I load my data when using MVVM

Question

I'm currently learning C#, and recently I've learned about the MVVM design pattern for WPF. I am writing a simple program as a way to practice this, but I'm not sure where I should write the method that loads the data.

I have a SalesSheet class, as shown below. This holds the data that I load from a .xls file.

class SalesSheet
{
    public List<Row> Rows { get; set; }

    public SalesSheet()
    {
        Rows = new List<Row>();
    }

    public class Row
    {
        public string CallType { get; set; }
        public string HistoryText { get; set; }
    }

}

My question is, where should I write the method that loads the data? Is it bad practice to write a method like:

private void LoadData(string filePath)

in the model, and call it the constructor?

Should I load it from the ViewModel?

Answer 1

In general, a small WPF project should have the following approximate folder structure:

• ProjectName
  • Converters
  • DataAccess
  • DataTypes
  • Images
  • ViewModels
  • Views

DataAccess is the folder where you should store your data access classes. It is good practice to separate the various aspects of the application; the views, the view models and the data access classes. This is known as Separation of Concerns and is good practice because (among other things) it enables you to switch out layers... this means that you could later add a web interface (or change your database) while still keeping the majority of your code the same, and it also makes testing your code easier.

You might only have one class in this folder, let's call it DataProvider. In this DataProvider class, you put all of your data access methods. You now have one point of entry to your data access and you can add a reference to it in a base view model:

protected DataProvider DataProvider
{
    get { return new DataProvider(); }
}

Now your view models all have access to the project data source and then you can do something like this:

SomeObject someObject = DataProvider.LoadData(filePath);

Of course, there are many different ways of implementing this pattern, but hopefully now, you get the idea.

Answer 2

In my understanding of MVVM your LoadData method belongs in the model. The viewmodels can then access the loaded data through a property or method from the model. The important point here is, that the viewmodels don't know about the concrete file access logic, it is abstracted by the model.