C# – WPF Data Binding and IValueConverter

cconverterdata-bindingwpf

Why is it that when I use a converter in my binding expression in WPF, the value is not updated when the data is updated.

I have a simple Person data model:

class Person : INotifyPropertyChanged
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

My binding expression looks like this:

<TextBlock Text="{Binding Converter={StaticResource personNameConverter}" />

My converter looks like this:

class PersonNameConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        Person p = value as Person;
        return p.FirstName + " " + p.LastName;
    }

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

If I bind the data without a converter it works great:

<TextBlock Text="{Binding Path=FirstName}" />
<TextBlock Text="{Binding Path=LastName}" />

What am I missing?

EDIT:
Just to clarify a few things, both Joel and Alan are correct regarding the INotifyPropertyChanged interface that needs to be implemented. In reality I do actually implement it but it still doesn't work.

I can't use multiple TextBlock elements because I'm trying to bind the Window Title to the full name, and the Window Title does not take a template.

Finally, it is an option to add a compound property "FullName" and bind to it, but I'm still wondering why updating does not happen when the binding uses a converter. Even when I put a break point in the converter code, the debugger just doesn't get there when an update is done to the underlying data 🙁

Thanks,
Uri

Best Answer

You can also use a MultiBinding.. Bind to the Person object, the FirstName and LastName. That way, the value gets updated as soon as FirstName or LastName throws the property changed event.

<MultiBinding Converter="{IMultiValueConverter goes here..}">
    <Binding />
    <Binding Path="FirstName" />
    <Binding Path="LastName" />
</MultiBinding>

Or if you only use the FirstName and LastName, strip the Person object from the binding to something like this:

<MultiBinding Converter="{IMultiValueConverter goes here..}">
    <Binding Path="FirstName" />
    <Binding Path="LastName" />
</MultiBinding>

And the MultiValueConverter looks like this:

class PersonNameConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
            return values[0].ToString() + " " + values[1].ToString();
    }

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

But of course, the selected answer works as well, but a MultiBinding works more elegantly...

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

R – Silverlight Databinding – Binding a ValueConverter to a property on a view-model

Since you're using a ViewModel to sit between your actual model and your view, I wonder if it's easier just to implement the IValueConverter logic directly in there. Sort of like:

public class ViewModel
{
    public ObservableCollection Root { get; set: }

    public ObservableCollection Children
    {
        get { /* return children items */ }
    }
}

Then you can simply bind directly to your second property:

<control:HierarchicalDataTemplate ItemsSource="{Binding Children}">

I think the main purpose of a ViewModel object is to limit the number of "tricks" (such as IValueConverters) you need to pull to get the data you need from the original model. Since you have one, you might as well make use of it.

Edit 1

... and of course now that I re-read your post I see that there's more to it. You're getting the children for each item in your "Root" collection.

How about implementing the IValueConverter as a static instance in your ViewModel itself?

public class ViewModel : IValueConverter
{ 
    public static readonly IValueConverter ChildrenConverter
        = new LoadChildrenValueConverter();
}

Now you should be able to say:

<control:HierarchicalDataTemplate 
    ItemsSource="{Binding Converter={x:Static local:ViewModel.ChildrenConverter}}">

Edit 2

Ok, you're using Silverlight so {x:Static} isn't available to you.

The only other option I can think of that will let you reuse the one static resource instead of having to declare two is to implement the IValueConverter directly in your ViewModel. This is no good if you will need to do more than one type of conversion, but if your ViewModel is very narrow-focused then it could do the job. So:

public class ViewModel : IValueConverter
{
    // move your Convert and ConvertBack methods into here
}

Now you can do this:

<control:HierarchicalDataTemplate
    ItemsSource="{Binding Converter={StaticResource ViewModel}}">

Best Answer

Related Solutions

C# – the difference between String and string in C#

This is the style that Microsoft tends to use in their examples.

R – Silverlight Databinding – Binding a ValueConverter to a property on a view-model

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