wpf MVVM in WPF The Model

Example

The model is the first "M" in MVVM. The model is usually a class containing the data that you want to expose via some kind of user interface.

Here is a very simple model class exposing a couple of properties:-

public class Customer : INotifyPropertyChanged
{
    private string _forename;
    private string _surname;
    private bool _isValid;

    public event PropertyChangedEventHandler PropertyChanged;

    /// <summary>
    /// Customer forename.
    /// </summary>
    public string Forename
    {
        get
        {
            return _forename;
        }
        set
        {
            if (_forename != value)
            {
                _forename = value;
                OnPropertyChanged();
                SetIsValid();
            }
        }
    }

    /// <summary>
    /// Customer surname.
    /// </summary>
    public string Surname
    {
        get
        {
            return _surname;
        }
        set
        {
            if (_surname != value)
            {
                _surname = value;
                OnPropertyChanged();
                SetIsValid();
            }
        }
    }

    /// <summary>
    /// Indicates whether the model is in a valid state or not.
    /// </summary>
    public bool IsValid
    {
        get
        {
            return _isValid;
        }
        set
        {
            if (_isValid != value)
            {
                _isValid = value;
                OnPropertyChanged();
            }
        }
    }

    /// <summary>
    /// Sets the value of the IsValid property.
    /// </summary>
    private void SetIsValid()
    {
        IsValid = !string.IsNullOrEmpty(Forename) && !string.IsNullOrEmpty(Surname);
    }

    /// <summary>
    /// Raises the PropertyChanged event.
    /// </summary>
    /// <param name="propertyName">Name of the property.</param>
    private void OnPropertyChanged([CallerMemberName] string propertyName = "")
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

This class implements the INotifyPropertyChanged interface which exposes a PropertyChanged event. This event should be raised whenever one of the property values changes - you can see this in action in the above code. The PropertyChanged event is a key piece in the WPF data-binding mechanisms, as without it, the user interface would not be able to reflect the changes made to a property's value.

The model also contains a very simple validation routine that gets called from the property setters. It sets a public property indicating whether or not the model is in a valid state. I've included this functionality to demonstrate a "special" feature of WPF commands, which you'll see shortly. The WPF framework provides a number of more sophisticated approaches to validation, but these are outside the scope of this article.