Question

I’ve been playing about with data binding and using the INotifiedProperty interface (including the new .Net 4.5 CallerMemberName attribute).

All is working well but I can’t understand why updating an object’s property refreshes the label it’s bound to but refreshing the object itself doesn’t re-fresh the label.

For example, if I have the following Window:

<Grid Name="TestGrid">
    <!-- Grid definitions here -->

    <Label Grid.Column="0" Grid.Row="0">The value is :</Label>
    <Label Grid.Column="1" Grid.Row="0" Content="{Binding TestVal1}"/>

    <Button Grid.Column="0" Grid.Row="1" Click="Button_Click_1">Refresh</Button>
    <Button Grid.Column="1" Grid.Row="1" Click="Button_Click_2">New class instance</Button>
</Grid>

With the following code behind it:

public MainWindow()
    {
        InitializeComponent();

        TestGrid.DataContext = TestClass1;
    }

    public TestClass TestClass1 = new TestClass();

    private void Button_Click_1(object sender, RoutedEventArgs e)
    {
        TestClass1.ChangeTestVal1();
    }

    private void Button_Click_2(object sender, RoutedEventArgs e)
    {
        TestClass1 = new TestClass();
    }

Which is bound the following class:

public class TestClass : INotifyPropertyChanged
{
    public TestClass()
    {
        ChangeTestVal1();
    }


    public event PropertyChangedEventHandler PropertyChanged;

    internal void OnPropertyChanged([CallerMemberName] String caller = null)
    {
        var handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(caller));
        }
    }

    private string _TestVal1;
    public string TestVal1
    {
        get { return _TestVal1; }
        set
        {
            if (value != _TestVal1)
            {
                _TestVal1 = value;
                OnPropertyChanged();
            }
        }
    }

    public void ChangeTestVal1()
    {
        TestVal1 = "TestVal1 = " + DateTime.Now.ToString("dd-MMM-yyyy HH:mm:ss");
    }

The result of is that clicking the “Refresh” button works and clicking the “New class instance” doesn’t.

My question is, I know I can add “TestGrid.DataContext = TestClass1” to the code for the second button to get it to work but surely it should detect the instance of the TestClass changing when it’s refreshed? Am I setting the binding up incorrectly?

Answer 1

You’re assigning TestGrid.DataContext = TestClass1; Changing the object reference of the variable to a different object does not change the object reference in the DataContext property of the TestGrid. Take a look at the basics of OOP for more details

Edit: I mean, doing TestClass1 = new TestClass(); does not change the fact that the Datacontext of your grid is still the same object instance it was before.