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)