Regarding INotifyPropertyChanged

Question

i never work wuth INotifyPropertyChanged. i was reading article and from there i found code related with INotifyPropertyChanged.

public class ContactModel : INotifyPropertyChanged
{
    private string _firstName;

    public string FirstName
    {
        get { return _firstName; }
        set
        {
            _firstName = value;
            RaisePropertyChanged("FirstName");
            RaisePropertyChanged("FullName");
        }
    }

    private string _lastName;

    public string LastName
    {
        get { return _lastName; }
        set
        {
            _lastName = value;
            RaisePropertyChanged("LastName");
            RaisePropertyChanged("FullName");
        }
    }

    public string FullName
    {
        get { return string.Format("{0} {1}", FirstName, LastName); }
    }



    protected void RaisePropertyChanged(string propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    public override bool Equals(object obj)
    {
        return obj is ContactModel && ((ContactModel) obj).FullName.Equals(FullName);
    }

    public override int GetHashCode()
    {
        return FullName.GetHashCode();
    }
}

from the above code i found RaisePropertyChanged("FirstName"); is always called from property setter. why RaisePropertyChanged is required to call. i search google for good explaination but i did not found any good link. so please if someone explain why RaisePropertyChanged need to call from setter.

thanks

Answer 1

Because WPF registers to the PropertyChanged event and this will make the binding work, ie you change the property and textbox text changes.

Also the event raising pattern is a standard one: Event is raised through a protected method so that the subclasses can turn it off/or change the behaviour.

I actually have a bit more on my view models that verify the name - originally by Josh Smith:

    public event PropertyChangedEventHandler PropertyChanged;

    protected void OnPropertyChanged(string propertyName)
    {

        VerifyPropertyName(propertyName);

        PropertyChangedEventHandler handler = this.PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    protected void OnPropertyChanged(object sender, string propertyName)
    {
        PropertyChanged(sender, new PropertyChangedEventArgs(propertyName));
    }

    public void VerifyPropertyName(string propertyName)
    {

        if (TypeDescriptor.GetProperties(this)[propertyName] == null)
        {
            throw new Exception("Invalid property name: " + propertyName);
        }
    }

Answer 2

You should check inside the property setter if the property actually changes, then by rasing the event you can notify that your property has changed :

        set
        {
            if (lastname != value)
            {
               _lastName = value;
               RaisePropertyChanged("LastName");
               RaisePropertyChanged("FullName");
            }
        }

Any class that has subscribed to the PropertyChanged event, will then be notified that the property has changed, so they can update their bindings.

Answer 3

This is so all subscribing recipients get a message stating that the property has changed, therefore they should update their values accordingly. For example, a grid will change text.

The reason they've updated the FullName property as well, is because the First/Last name properties directly affect the FullName of the person, therefore bindings should reflect these values.