C# - Dynamic properties and RaisePropertyChanged

Question

I have following class which I use for radio button binding

public class RadioButtonSwitch : ViewModelBase
    {

        IDictionary<string, bool> _options;
        public RadioButtonSwitch(IDictionary<string, bool> options)
        {
            this._options = options;
        }

        public bool this[string a]
        {
            get
            {
                return _options[a];
            }
            set
            {
                if (value)
                {
                    var other = _options.Where(p => p.Key != a).Select(p => p.Key).ToArray();
                    foreach (string key in other)
                        _options[key] = false;
                    _options[a] = true;

                    RaisePropertyChanged("XXXX");
                else
                    _options[a] = false;
            }
        }
    }

XAML

<RadioButton Content="Day" IsChecked="{Binding RadioSwitch[radio1], Mode=TwoWay}" GroupName="Monthly" HorizontalAlignment="Left" VerticalAlignment="Center" />

ViewModel

RadioSwitch = new RadioButtonSwitch(
                new Dictionary<string, bool> {{"radio1", true},{"radio2", false}}
                );

I'm having problem with RaisePropertyChanged() in my Class. I'm not sure what value I should be putting in order to raise the change.

I tried putting:

Item[]
a
[a]

I keep getting following error:

Error

This is so in case of any change I can handle it in my view accordingly. Please do not give me solutions for List for radio buttons etc.

`ViewModelBase` extends `GalaSoft.MvvMLight` `ViewModelBase`. It implements `INotifyPropertyChanged`. `RaisePropertyChanged` pretty much just calls `if (PropertyChanged != null) PropertyChanged(this, new PropertyChangedEventArgs(property));` — Luke, May 21 '11 at 02:44
Hmm, then where is the error *coming* from? Can you post the full stack trace? — Kirk Woll, May 21 '11 at 02:46
@KirkWoll added screenshot of the error. I'm assuming error is happening because I'm not calling RaisePropertyChanged() with correct parameter. `RadioButton` is binded to `[radio1]`, I tried using that as property name but it didnt work either. — Luke, May 21 '11 at 02:55

Rick Sladkey · Accepted Answer · 2011-05-21T07:27:46.447

The problem is that you are implementing an indexer, not an ordinary property. Although the binding subsystem supports indexers, MVVMLight and INotifyPropertyChanged do not.

If you want to use an indexer you need to:

Use a collection base class such as ObservableCollection<T>
Implement INotifiyCollectionChanged and raise that event instead

The first option isn't realistic because you are already deriving from ViewModelBase and have to continue to do that. Since implementing INotifiyCollectionChanged is a little bit of work, the easiest approach is to:

add a property to RadioButtonSwitch that is an observable collection of boolean values (ObservableCollection<bool>)

Then change you binding to add one more path element and you are done.

Edit:

Based on your comment and rereading your question, I think implementing INotifyCollectionChanged is the easiest. Here is the rewrite of your RadioButtonSwitch class which actually no longer needs to derive from the MVVMLight base class, although you still could if you wanted to.

The careful reader will notice that we use a sledgehammer and "reset" the whole collection when any element of the collection is modified. This is not just laziness; it is because the indexer uses a string index instead of an integer index and INotifyCollectionChanged doesn't support that. As a result, when anything changes we just throw up our hands and say the whole collection has changed.

public class RadioButtonSwitch : INotifyCollectionChanged
{
    public event NotifyCollectionChangedEventHandler CollectionChanged;

    protected void RaiseCollectionChanged()
    {
        if (CollectionChanged != null)
            CollectionChanged(this, new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
    }

    IDictionary<string, bool> _options;
    public RadioButtonSwitch(IDictionary<string, bool> options)
    {
        this._options = options;
    }

    public bool this[string a]
    {
        get
        {
            return _options[a];
        }
        set
        {
            if (value)
            {
                var other = _options.Where(p => p.Key != a).Select(p => p.Key).ToArray();
                foreach (string key in other)
                    _options[key] = false;
                _options[a] = true;

                RaiseCollectionChanged();
            }
            else
                _options[a] = false;
        }
    }
}

Second option sounds good. However I'm don't know how to bind to a specific row of ObservableCollection. Do you have any examples? — Luke, May 21 '11 at 06:17

score 2 · Answer 2 · answered May 21 '11 at 03:40

GalaSoft.MvvmLight has the following code to check property name before raising PropertyChanged event.

public void VerifyPropertyName(string propertyName)
{
    if (GetType().GetProperty(propertyName) == null)
        throw new ArgumentException("Property not found", propertyName);
}

GetType().GetProperty("Item[]") obviously returns null.
This is why it is failing.

I think, the quickest workaround for you would be not to use ViewModelBase from this library, but implement your own version, that doesn't do this check:

public class ViewModelBase : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

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

If you implement this class you will be able to run RaisePropertyChanged("Item[]").

I wouldn't want to implement everything from scratch nor change existing methods used everywhere else. — Luke, May 21 '11 at 06:25

C# - Dynamic properties and RaisePropertyChanged

2 Answers2