IValueConverter from string

Question

I have an Enum that needs to be shown in ComboBox. I have managed to get enum values to combobox using ItemsSource and I'm trying to localize them. I thought that that could be done using value converter but as my enum values are already strings compiler throws error that IValueConverter can't take string as input. I'm not aware of any other way to convert them to other string value. Is there some other way to do that (not the localization but conversion)?

I'm using this marku extension to get enum values

[MarkupExtensionReturnType(typeof (IEnumerable))]
public class EnumValuesExtension : MarkupExtension {
    public EnumValuesExtension() {}

    public EnumValuesExtension(Type enumType) {
        this.EnumType = enumType;
    }

    [ConstructorArgument("enumType")]
    public Type EnumType { get; set; }
    public override object ProvideValue(IServiceProvider serviceProvider) {
        if (this.EnumType == null)
            throw new ArgumentException("The enum type is not set");
        return Enum.GetValues(this.EnumType);
    }
}

and in Window.xaml

<Converters:UserTypesToStringConverter x:Key="userTypeToStringConverter" />
....
<ComboBox ItemsSource="{Helpers:EnumValuesExtension Data:UserTypes}" 
            Margin="2" Grid.Row="0" Grid.Column="1" SelectedIndex="0" TabIndex="1" IsTabStop="False">
    <ComboBox.ItemTemplate>
        <DataTemplate DataType="{x:Type Data:UserTypes}">
            <Label Content="{Binding Converter=userTypeToStringConverter}" />
        </DataTemplate>
    </ComboBox.ItemTemplate>
</ComboBox>

And here is converter class, it's just a test class, no localization yet.

public class UserTypesToStringConverter : IValueConverter {
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture) {
        return (int) ((Data.UserTypes) value) == 0 ? "Fizička osoba" : "Pravna osoba";
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture) {
         return default(Data.UserTypes);
     }
}

-- EDIT --

Enum is generated by ADO.NET Diagram and can't be changed.

Answer 1

Yes, by the time you pass the value into the converter it will be a string as the default type converter for Enum (EnumConverter) for GetStandardValues (i.e. Enum.GetValues()) returns an enumerable of the fields as strings.

The best way to solve this to write a custom type converter to decorate your Enums with. Fortunately you are not the first person that has needed to this, see below for code sample.

public class EnumTypeConverter : EnumConverter
{
    public EnumTypeConverter()
        : base(typeof(Enum))
    {
    }

    public EnumTypeConverter(Type type)
        : base(type)
    {
    }

    public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
    {
        return sourceType == typeof(string) || TypeDescriptor.GetConverter(typeof(Enum)).CanConvertFrom(context, sourceType);
    }

    public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
    {
        if (value is string)
            return GetEnumValue(EnumType, (string)value);

        if (value is Enum)
            return GetEnumDescription((Enum)value);

        return base.ConvertFrom(context, culture, value);
    }

    public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType)
    {
        if (value is Enum && destinationType == typeof(string))
            return GetEnumDescription((Enum)value);

        if (value is string && destinationType == typeof(string))
            return GetEnumDescription(EnumType, (string)value);

        return base.ConvertTo(context, culture, value, destinationType);
    }

    public static bool GetIsEnumBrowsable(Enum value)
    {
        var fieldInfo = value.GetType().GetField(value.ToString());
        var attributes = (BrowsableAttribute[])fieldInfo.GetCustomAttributes(typeof(BrowsableAttribute), false);

        return !(attributes.Length > 0) || attributes[0].Browsable;
    }

    public static string GetEnumDescription(Enum value)
    {
        var fieldInfo = value.GetType().GetField(value.ToString());
        var attributes = (DescriptionAttribute[])fieldInfo.GetCustomAttributes(typeof(DescriptionAttribute), false);

        return (attributes.Length > 0) ? attributes[0].Description : value.ToString();
    }

    public static string GetEnumDescription(Type value, string name)
    {
        var fieldInfo = value.GetField(name);
        var attributes = (DescriptionAttribute[])fieldInfo.GetCustomAttributes(typeof(DescriptionAttribute), false);
        return (attributes.Length > 0) ? attributes[0].Description : name;
    }

    public static object GetEnumValue(Type value, string description)
    {
        var fields = value.GetFields();
        foreach (var fieldInfo in fields)
        {
            var attributes = (DescriptionAttribute[])fieldInfo.GetCustomAttributes(typeof(DescriptionAttribute), false);

            if (attributes.Length > 0 && attributes[0].Description == description)
                return fieldInfo.GetValue(fieldInfo.Name);

            if (fieldInfo.Name == description)
                return fieldInfo.GetValue(fieldInfo.Name);
        }

        return description;
    }

    public override bool GetPropertiesSupported(ITypeDescriptorContext context)
    {
        return true;
    }

    public override StandardValuesCollection GetStandardValues(ITypeDescriptorContext context)
    {
        return base.GetStandardValues(context);
    }

}

Usage

[TypeConverter(typeof(EnumTypeConverter))]
public enum UserTypes : int
{
    [Browsable(false)]
    Unkown,    
    [Description("Local")]
    LocalUser,
    [Description("Network")]
    NetworkUser,
    [Description("Restricted")]
    RestrictedUser
} 

As you can see, the above enum we have used the Description attribute to decorate each field with a user friend description and have overridden the type converter to first look for this attribute.

Not 100% but to get this to work with your code, you will also need to change your MarkupExtension to be the following (Note: I have not tested this, so some work on your part is required).

[MarkupExtensionReturnType(typeof (IEnumerable))]
public class EnumValuesExtension : MarkupExtension {

    public EnumValuesExtension() {}

    public EnumValuesExtension(Type enumType) 
    {
        this.EnumType = enumType;
    }

    [ConstructorArgument("enumType")]
    public Type EnumType { get; set; }

    public override object ProvideValue(IServiceProvider serviceProvider) 
    {
        if (this.EnumType == null)
            throw new ArgumentException("The enum type is not set");

        var converter = TypeDescriptor.GetConverter(this.EnumType);
        if (converter != null && converter.GetStandardValuesSupported(this.EnumType))        
            return converter.GetStandardValues(this.EnumType);

        return Enum.GetValues(this.EnumType);
    }
}

Also, I have only done limited localisation for an application however I believe this is the best and most maintainable approach as will be able to leverage the existing .NET localisation tools (e.g. satellite assemblies)

Answer 2

Edit:

You could simply add a property on your view model that returns MyEnumType[] and simply return MyEnumType.GetValues(). Then at the point of data binding you would have enum values to reference instead of string values.

The helper class you specified seems clean upon first look, but it really isn't - there is a lot of cruft involved, and you're making decisions in the view that might be better left to the view model. Depending on how you look at the problem, it makes sense to expose the enum values in the view model. The allowed values (even if it is all of them) can be seen as an aspect of the business logic and not an aspect of the view.

The rest of my answer might help you still if this isn't quite enough to solve the problem.

---

Before edit:

You could solve this problem by simply using string resources, except that resource keys are typically hard-coded in the view.

So I looked into whether there is a way to dynamically bind string resource keys, and found these other answers:

• How can I specify resources in an MVVM view model?
• Binding a datacontext string property to a StaticResource key

The second of these seems like a clean and simple option.

While I was looking, I also found this blog post:

• http://blog.functionalfun.net/2009/12/specifying-resource-keys-using-data_31.html

Here's the code from their sample.

View:

<DataTemplate>
  <Button Command="{Binding}" Padding="2" Margin="2" Width="100" Height="100">
    <StackPanel>
      <Image HorizontalAlignment="Center"
             Width="60"
             app:ResourceKeyBindings.SourceResourceKeyBinding="{Binding Converter={StaticResource ResourceKeyConverter}, ConverterParameter=Image.{0}}"/>
      <TextBlock Text="{ext:ResourceKeyBinding Path=Name, StringFormat=Caption.{0} }" HorizontalAlignment="Center" FontWeight="Bold" Margin="0,2,0,0"/>
    </StackPanel>
  </Button>
</DataTemplate>

Resources:

<Application.Resources>
  <BitmapImage x:Key="Image.AngryCommand" UriSource="Angry.png"/>
  <BitmapImage x:Key="Image.CoolCommand" UriSource="Cool.png"/>
  <BitmapImage x:Key="Image.HappyCommand" UriSource="Happy.png"/>

  <sys:String x:Key="Caption.Angry">Angry. Rrrr!</sys:String>
  <sys:String x:Key="Caption.Happy">Happy. Ha ha!</sys:String>
  <sys:String x:Key="Caption.Cool">Chilled out</sys:String>
</Application.Resources>

Markup extension to enable string resource key binding:

public class ResourceKeyBindingExtension : MarkupExtension
{
    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        var resourceKeyBinding = new Binding()
        {
            BindsDirectlyToSource = BindsDirectlyToSource,
            Mode = BindingMode.OneWay,
            Path = Path,
            XPath = XPath,
        };

        //Binding throws an InvalidOperationException if we try setting all three
        // of the following properties simultaneously: thus make sure we only set one
        if (ElementName != null)
        {
            resourceKeyBinding.ElementName = ElementName;
        }
        else if (RelativeSource != null)
        {
            resourceKeyBinding.RelativeSource = RelativeSource;
        }
        else if (Source != null)
        {
            resourceKeyBinding.Source = Source;
        }

        var targetElementBinding = new Binding();
        targetElementBinding.RelativeSource = new RelativeSource()
        {
            Mode = RelativeSourceMode.Self
        };

        var multiBinding = new MultiBinding();
        multiBinding.Bindings.Add(targetElementBinding);
        multiBinding.Bindings.Add(resourceKeyBinding);

        // If we set the Converter on resourceKeyBinding then, for some reason,
        // MultiBinding wants it to produce a value matching the Target Type of the MultiBinding
        // When it doesn't, it throws a wobbly and passes DependencyProperty.UnsetValue through
        // to our MultiBinding ValueConverter. To circumvent this, we do the value conversion ourselves.
        // See http://social.msdn.microsoft.com/forums/en-US/wpf/thread/af4a19b4-6617-4a25-9a61-ee47f4b67e3b
        multiBinding.Converter = new ResourceKeyToResourceConverter()
        {
            ResourceKeyConverter = Converter,
            ConverterParameter = ConverterParameter,
            StringFormat = StringFormat,
        };

        return multiBinding.ProvideValue(serviceProvider);
    }

    [DefaultValue("")]
    public PropertyPath Path { get; set; }

    // [snipped rather uninteresting declarations for all the other properties]
}

Answer 3

I use a generic resource converter to do this. You just need to specify the resource manager to use, and pass a prefix as the converter parameter:

class ResourceConverter : IValueConverter
{
    public ResourceManager ResourceManager { get; set; }

    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (ResourceManager == null)
            throw new InvalidOperationException("The resource manager is not set");

        if (value == null)
            return string.Empty;
        string prefix = parameter as string ?? string.Empty;
        string resourceKey = prefix + value;
        if (string.IsNullOrEmpty(resourceKey))
            return string.Empty;

        return ResourceManager.GetString(resourceKey);
    }

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

Assuming you have an enum like this:

class MyEnum
{
    Foo,
    Bar,
    Baz
}

And resources named MyEnum_Foo, MyEnum_Bar and MyEnum_Baz, you can use it like this:

<Window.Resources>
    <my:ResourceConverter x:Key="resourceConverter" ResourceManager="{x:Static prop:Resources.ResourceManager}" />
</Window.Resources>

...


<Label Content="{Binding Converter=resourceConverter, ConverterParameter=MyEnum_}" />

Answer 4

You might be interested in the LocalizedList Class which is described in the following blog post: http://wpfglue.wordpress.com/2010/01/14/localized-value-formatting-in-wpf/

It can be used to define a localized translation for enum values, at the same time defining their order. Also, it provides an ItemsSource for ComboBoxes which allows selecting localized string items while setting typed enum values.

It's written in VB.net, but you should easily be able to translate it to C#, or you could use the library which contains it in binary form.