Typeconverter not working for single properties

Question

I have a simple TypeConverter to convert a comma-separated string into an IEnumerable<T> to shorten the url when calling my API-endpoints.

Therefore I have an request-object which is set on client and passed to the server. So, on server it's the same object.

This is what the type-converter looks like:

public class EnumerableTypeConverter : TypeConverter
{
    private readonly Type _innerType;
    private readonly MethodInfo _enumerableCastMethodInfo = typeof(Enumerable).GetMethod(nameof(Enumerable.Cast));

    public IEnumerableTypeConverter(Type type)
    {
        // check if the type is somewhat ienumerable-like
        if (type.BaseType != null && type.BaseType.IsGenericType && typeof(IEnumerable<>).MakeGenericType(type.BaseType.GetGenericArguments()[0]).IsAssignableFrom(type.BaseType) && type.BaseType.GetGenericArguments().Length == 1)
        {
            _innerType = type.BaseType.GetGenericArguments()[0];
        }
        else
        {
            throw new ArgumentException("Incompatible type", nameof(type));
        }
    }

    public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType) => sourceType == typeof(string) || base.CanConvertFrom(context, sourceType);
    public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType) => (destinationType.BaseType != null && typeof(IEnumerable<>).MakeGenericType(destinationType.BaseType.GetGenericArguments()[0]).IsAssignableFrom(destinationType.BaseType)) || base.CanConvertFrom(context, destinationType);

    public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
    {
        var source = value as string;
        if (source == null)
            return base.ConvertFrom(context, culture, value);

        var temp = source.Split(new[] { ',' }, StringSplitOptions.RemoveEmptyEntries).Select(s => TypeDescriptor.GetConverter(_innerType).ConvertFromInvariantString(s));
        var castMethod = _enumerableCastEmthMethodInfo.MakeGenericMethod(_innerType);
        var casted = castMethod.Invoke(null, new object[] {temp});
        return casted;
    }

    public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType)
    {
        var s = value as IEnumerable<string>;
        return s != null ? string.Join(",", s) : base.ConvertFrom(context, culture, value);
    }
}

A sample request could look something like

public class MyRequest 
{
    [TypeConverter(typeof(EnumerableTypeConverter))]
    public IEnumerable<string> Names {get;set;}

    [TypeConverter(typeof(EnumerableTypeConverter))]
    public IEnumerable<Guid> Ids {get;set;}
}

I've decorated the properties with [TypeConverter(typeof(EnumerableTypeConverter))]-attribute. However, the ConvertFrom-method is never called. When I add the attribute on a class instead of a property it is working.

Answer 1

the TypeConverterAttribute (used in [TypeConverter]) is different from the base class TypeConverter.

For the EnumerableTypeConverter to be invoked, it needs to be instantiated, and it can only be automagically instantiated if a parameterless constructor exists.

In your case, you might want to have 2 converters, a EnumerableStringConverter and a EnumerableGuidConverter, or use a generic one:

public class EnumerableTypeConverter<T> : TypeConverter
{
}

and you can decorate your properties like this:

public class MyRequest 
{
    [TypeConverter(typeof(EnumerableTypeConverter<string>))]
    public IEnumerable<string> Names {get;set;}

    [TypeConverter(typeof(EnumerableTypeConverter<Guid>))]
    public IEnumerable<Guid> Ids {get;set;}
}