PropertyChanged event is null after changing values inside property

Question

I am having problems with updating the UI after updating my ObservableCollection property.

My constructor:

public ObservableCollection<Positions> Positions { get; set; }

        public BindableBase BindableBase { get; set; }

        public MainWindow()
        {
            InitializeComponent();

            Positions = new ObservableCollection<Positions>();
            BindableBase = new BindableBase();

            Positions.Add(new Positions
            {
                Position1 = "Red",
                Position2 = "Red",
                Position3 = "Red",
                Position4 = "Gray",
                Position5 = "Green",
                Position6 = "Green",
                Position7 = "Green",
            });

            this.DataContext = this;
        }

This is the if statement that changes the values inside of the ObservableCollection and then I call the RaisePropertyChanged event with the name of my property. Because I did not include all the code, imagine that it goes inside the ifstatement.

if (Positions[0].Position4 == "Gray")
            {
                Positions[0].Position1 = "Red";
                Positions[0].Position2 = "Red";
                Positions[0].Position3 = "Gray";
                Positions[0].Position4 = "Red";
                Positions[0].Position5 = "Green";
                Positions[0].Position6 = "Green";
                Positions[0].Position7 = "Green";

                BindableBase.RaisePropertyChanged("Positions");
            }

This is my code to RaisePropertyChanged:

public class BindableBase : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        public void RaisePropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged.Invoke(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }

The problem is that, it does not update the UI if I call the RaisePropertyChanged event. After some debugging I found out that the PropertyChanged has the value NULL, so it won't be updated even tho I made a change. Does anyone how to fix this problem?

Clemens Clemens · Accepted Answer · 2018-10-22T18:39:57

PropertyChanged is always null because there is no Binding that actually uses the BindableBase property value as source object (the source object is the MainWindow, by DataContext = this).

Fortunately you don't need the BindableBase here. Positions is an ObservableCollection, and doesn't need any additional property change notification. If it would need one, Positions had to be a property of BindableBase, otherwise firing the PropertyChanged event is pointless.

Replace

Positions[0].Position1 = "Red";
Positions[0].Position2 = "Red";
Positions[0].Position3 = "Gray";
Positions[0].Position4 = "Red";
Positions[0].Position5 = "Green";
Positions[0].Position6 = "Green";
Positions[0].Position7 = "Green";

by

Positions[0] = new Positions
{
    Position1 = "Red",
    Position2 = "Red",
    Position3 = "Gray",
    Position4 = "Red",
    Position5 = "Green",
    Position6 = "Green",
    Position7 = "Green",
};

Besides that, replace the combination of null check and PropertyChanged.Invoke by this:

public void RaisePropertyChanged(string propertyName)
{
    PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}

PropertyChanged event is null after changing values inside property

1 Answers