Support for TypeDescriptionProvider when resolving the TypeConverter used while parsing xaml files

[nalka0]

Issue to fix

Considering the following class :

public class Wrapper<T>
{
    public T Wrapped { get; set; }

    public override string ToString()
    {
        return $"{Wrapped.GetType()} : {Wrapped}";
    }
}

If I have an object with a property of type Wrapper<T> and want to use it in XAML like MyProperty="SomeValue" a TypeConverter is required.
Here, the TypeConverter implementation would look like :

public class WrapperConverter : TypeConverter
{
    private readonly Type wrappedType;

    public WrapperConverter(Type wrappedType)
    {
        this.wrappedType = wrappedType;
    }

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

    public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
    {
        Type retType = typeof(Wrapper<>).MakeGenericType(wrappedType);
        var conversionResult = TypeDescriptor.GetConverter(wrappedType).ConvertFrom(context, culture, value);
        var ret = Activator.CreateInstance(retType); // var ret = new Wrapper<T>();
        retType.GetProperty(nameof(Wrapper<object>.Wrapped)).SetValue(ret, conversionResult); // ret.Wrapped = conversionResult;
        return ret;
    }
}

The issue is, if I apply[TypeConverter(typeof(WrapperConverter))] to my Wrapper<T> class, the XAML parser doesn't find the constructor for WrapperConverter (since it has no default constructor) and hence throws a XamlParseException.

Most likely solution to me

Now if I implement a TypeDescriptionProvider and a CustomTypeDescriptor :

public class WrapperTypeDescriptor : CustomTypeDescriptor
{
    private readonly Type objectType;

    public WrapperTypeDescriptor(Type objectType)
    {
        this.objectType = objectType;
    }

    public override TypeConverter GetConverter()
    {
        Type wrappedType = objectType.GenericTypeArguments[0];
        return new WrapperConverter(wrappedType);
    }
}

public class WrapperTypeDescriptionProvider : TypeDescriptionProvider
{
    public override ICustomTypeDescriptor GetTypeDescriptor(Type objectType, object instance)
    {
        return new WrapperTypeDescriptor(objectType);
    }
}

I can control the WrapperConverter's instantiation and use my WrapperConverter's parameterized constructor and convert strings to Wrapper<T> instances by applying [TypeDescriptionProvider(typeof(WrapperTypeDescriptionProvider))] to my Wrapper<T> class :

Console.WriteLine(TypeDescriptor.GetConverter(typeof(Wrapper<int>)).ConvertFromString("18"));

will output :

System.Int32 : 18

The only problem with this solution is that WPF's xaml parser doesn't seem to look for a TypeDescriptionProviderAttribute but only a TypeConverterAttribute.

[ryalanms]

Thanks, @nalka0. Please don't assign bugs directly to WPF developers. They are assigned based on priority.

[nalka0]

Thanks, @nalka0. Please don't assign bugs directly to WPF developers. They are assigned based on priority.

@ryalanms SamBent is the default assignee when doing "new issue" => "feature request", and the only way I can change this is removing the assignees=SamBent part of the URL, I think there's a setting that you'd like to change for the feature request button?