MultiBinding in Xamarin Forms: element is null

Question

I'm trying to use MultiBinding with Xamarin.Forms 4.8, and I can't get it to work. I have this very simple converter:

public class QuantityLabelConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        return "Hello World";
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

And I'm trying to use it in an XAML label, like this:

<Label>
    <Label.Text>
        <MultiBinding Converter="{StaticResource QuantityLabelConverter}">
            <Binding Path="SelectedQuantity" />
            <Binding Path="OutstandingQuantity" />
        </MultiBinding>
    </Label.Text>
</Label>

Now, when I debug, I can see that the converter is called three times: one with two null values, then another one when the SelectedQuantity value is set, and a last time when OutstandingQuantity value is assigned. So the link between the bindings and the converter seems to be working fine.

However, the page crashes and I get a "Value cannot be null.\nParameter name: element" System.ArgumentNullException.

If I replace the previous XAML with a simple

<Label Text="Hello World">

The page shows without problems, so there is something wrong with how the MultiBinding is set, but I don't know what...

Answer 1

You need to return BindableProperty.UnsetValue to use the binding FallbackValue .

public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        foreach (var value in values)
        {
            if (!(value is string b))
            {
                return "xxx";
                // set a default value when unset
            }

        }

        string str1 = values[0].ToString();
        string str2 = values[1].ToString();

        return str1+str2;
    }

##Update

I had tested the sample and found out the cause

If you want to binding the property of ViewModel . You need to implement the interface INotifyPropertyChanged of the property instead of BindableObject .

public class MainViewModel : INotifyPropertyChanged
{



    string _name="";
    string _surname="";

    public string Name
    {
        get { return _name; }
        set
        {
            if(_name!=value)
            {
                _name = value;
                OnPropertyChanged("Name");
            }
        }
    }

    public string Surname
    {
        get { return _surname; }
        set
        {
            if(_surname!=value)
            {
                _surname = value;
                OnPropertyChanged("Surname");
            }
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

public class NameConverter : IMultiValueConverter
    {
        public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
        {
            foreach (var value in values)
            {
                if (!(value is string b))
                {
                    return " ";
                    // set a default value when unset
                }

            }

            string str1 = values[0].ToString();
            string str2 = values[1].ToString();

            return str1 + str2;
        }

        public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }