Question

I’m trying to understand WPF binding. As simple as it gets:

I have a ClassWithProperty that has a public uint Prop1.

The main window has a public ClassWithProp object and uses it for data context. This is set in the main Windows’s constructor:

this.ClassWithProp = new ClassWithProp();
this.DataContext = this.ClassWithProp;

ClassWithProp’s default constructor sets Porp1 value to 1.

The main windows contains a label:

<Label Content="{Binding Prop1}" ...  />

It also contains a button that, when click, sets the ClassWithProp.Prop1 to 2.

When the window first appears, the label correctly shows 1. When the button is clicked the property’s value is changed to 2, but the lable does not refresh.

Sorry – probably obvious but I’m a novice in WPF:

Why doesn’t the bound label update when the undelying property changes?

Answer 1

Your ClassWithProperty needs to implement the INotifyPropertyChanged interface (which has just the one event on it, PropertyChanged), this way the WPF binding subsystem can listen for property changes and update the value. When you have changed the value of a property, you raise the event.

Here is an example:

pulic class ClassWithProperty : INotifyPropertyChanged
{
    public uint Prop1
    {
        get { return _prop1; }
        set 
        {
            _prop1 = value;
            OnPropertyChanged("Prop1");
        }
    }

    protected void OnPropertyChanged(string propertyName)
    {
        if (this.PropertyChanged != null)
            this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }

    public event PropertyChangedEventHandler PropertyChanged;


    private uint _prop1;
}