C# MVVM How to update viewmodel string from the model

Question

I am really new to mvvm and wpf in c# and got stuck at some very basic stuff.In this example I am using Fody.PropertyChanged. I have a basic viewmodel that holds a string called Test which is binded to a textblock.

 public class Model : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged = (sender, e) => { };

        public string Test { get; set; }
    }

Then,in a separate file and class called Data,I have a simple function that increments an int and converts it to a string.

public class Data
    {
        public static int i = 0;
        public static string IncTest { get; set; }
        public static void Inc()
        {
            i++;
            IncTest = i.ToString();
        }
    }

How do I update the Test variable inside the viewmodel when calling the Inc() function? For example, when clicking a button

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        this.DataContext = new Model();
        Data.Inc();
    }

    private void Increment_Click(object sender, RoutedEventArgs e)
    {
        Data.Inc();
    }

Answer 1

In MVVM, the model does not update the view model, its actually opposite, The view model updates the model properties.

Here is an example.

MODEL:

public class Model
{
    public string Test
    {
        get;
        set;
    }
}

View Model:

public class ViewModel : INotifyPropertyChanged
{
    private Model _model;

    public string Test
    {
        get
        {
            return _model.Test;
        }
        set
        {
            if(string.Equals(value, _model.Test, StringComparison.CurrentCulture))
            {
                return;
            }
            _model.Test = value;
            OnPropertyChanged();
        }
    }

    public ViewModel(Model model)
    {
        _model = model;
    }

}

Your views will bind to your view models.

UPDATE: In regards to your question

public class SomeClass
{
    public static void Main(string [] args)
    {
        Model model = new Model();
        ViewModel viewModel = new ViewModel(model);

        //Now setting the viewmodel.Test will update the model property
        viewModel.Test = "This is a test";
    }
}