Question

I’ve been playing around with WPF for a while and I like how it is extremely versatile. I have read how to bind to a method with a single parameter (using a converter). However, purely hypothetically, is there a way to bind to a function that has access to local variables in the class?

For example, say I have a ListBox of TextBoxes to be bound to a List of Strings, defined and manipulated in the code behind, through a DataTemplate. Instead of displaying the entire string, however, each TextBox only displays the first N words of its corresponding string, which is easily done by a method that accepts the string and N as parameters. N, however, is free for the user to configure, and thus must be a local variable. For this to be a flexible control, N cannot be a static variable as we want it to be possible to have many windows with such ListBoxes with different Ns; besides, that would be extremely inelegant.

Now we can’t use a converter as converters only accept a single variable, while we need to access an N. We can’t cheat and use a static variable either.

We also cannot bind the TextBoxes to a local property that has access to local variables as we do to normal objects, as we are binding to a whole List of objects.

From what I see, ObjectDataProvider only allows you to call a method of the class of a given object, and not a method defined in the window. It is, I believe, a waste to create a new class and copy N to every instance of the class just for the sake of achieving such simple functionality.

Of course in a real scenario it’s possible to sidetrack this problem through such measures as creating an object including the string and N as in the previous paragraph, or including both the string and another string comprising the first two words of the string precomputed. I’m just asking whether such a functionality exists for WPF because it fits well into the concept WPF is based on and I believe that such a functionality would be useful some way or another.

I’m sorry if the solution is immediately obvious; I’ve been searching for hours.

If there is a solution, is there a way to do so completely in XAML, with only the method defined in the code behind?

EDIT: Thanks a lot for the suggestion, Clemens… N can’t be passed as ConvertParameter as it is not possible to bind to ConvertParameter. As for the first suggestion, I’m trying it out, but I’m getting an error Specified cast is not valid. at the Convert method of the IMultiValueConverter. Rather than N, values[1] is DependencyProperty.UnsetValue. Below is the code:

XAML

<MultiBinding>
    <MultiBinding.Converter>
        <my:StringShorternerConverter />
    </MultiBinding.Converter>
    <Binding Path="Title" />
    <Binding ElementName="me" Path="N" />
</MultiBinding>

Inside StringShorternerConverter

public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
    return ShortenText((String)values[0], (Int32)values[1]);
}

EDIT 2: Looks like that error was due to some irrelevant mistake. Yeah, it works. Thanks a lot.

Answer 1

You can easily bind to multiple properties by using a MultiBinding in conjunction with a converter that implements IMultiValueConverter.

You could also pass your variable N to the parameter argument of the Convert method of a normal converter by means of the Binding.ConverterParameter property.