数据绑定/ PropertyChanged为空

Question

Ok, I'm lost. I'm searching since yesterday why my databinding doesn't work as expected. As I'm new to WPF and also to MVVM it would be understandable if I would not use the same approach on the same project with 2 other views.

The essential configuration is that I have 3 views, for all of them I set the DataContext in the MainWindows-Constructor to the responsible ViewModel.

The view model itself does not implement IPropertyChanged but the model has a few nested objects which do.

I bind with the following XAML:

<TextBox Grid.Column="1" Grid.Row="0" Margin="1">
                <TextBox.Text>
                    <Binding Path="Model.InsertLine.Destination.Value" UpdateSourceTrigger="PropertyChanged">
                        <Binding.ValidationRules>
                            <local:MinMaxLengthValidatonRule Min="7" Max="7"/>
                        </Binding.ValidationRules>
                    </Binding>
                </TextBox.Text>
            </TextBox>

InsertLine has PropertyChanged:

    public TLTMoveULLine InsertLine
    {
        get { return _insertline; }
        internal set
        {
            _insertline = value;
            if (PropertyChanged != null) PropertyChanged(this, new PropertyChangedEventArgs("InsertLine"));
        }
    }

The general binding to Destination and Unit works. As does the validation.

However I do at some point in time set a new insertline, after it has been inserted into an ObeservableCollection (_document.Lines). What I expect to happen (and what does happen in the 2 other views) is that the textbox get's cleared.

    public void AddLine()
    {
        _document.Lines.Add(InsertLine);
        TLTMoveULLine newline = new TLTMoveULLine();

        newline.Destination.Value = InsertLine.Destination.Value;
        newline.Unit.Value = "";

        InsertLine = newline;
    }

The property on InsertLine get's called, however the PropertyChanged delegate is null.

Does anybody have an idea why it's null? The same approach works with 2 separate views, but not with this one...

Or do I have a wrong idea and there is a better way to do it?

Answer 1

Ok, I found it. And it was an absolutely stupid oversight.

For anybody else who has this problem: Make sure that you not only have the PropertyChanged event and you are calling it, you should also inherit from the interface. I simply forgot to put INotifyPropertyChanged after the model's definition.

Which brings me to a question out of curiosity: I always thought that an interface was nothing more than a contract, essentially it should not make any difference, but I assume that WPF uses reflection/is-operator to find out if an object can be bound to. Am I roughly on the right track there?

Answer 2

Just in case: I had a similar problem but my mistake was that class which implemented INotifyPropertyChanged was private. Making it public resolved my case.