Using SetMemberAccessor to override object method

[luttje]

Hi y'all, loving Jint so far! Though I'm stuck atm.

I'm trying to make it so when I expose objects to JS, I can specify which members can be accessed from JS by adding a "[ScriptAccessible]" attribute. I can make this work for fields and properties, but am unsure how to approach returning a Method as a JsValue.

My setup:

var engine = new Engine((_, opts) =>
{
    opts.SetMemberAccessor(GetObjectMember);
});

// [...]

// Hides anything that isn't tagged with [ScriptAccessible]
// Ensures lowerCamelCase is converted to UpperCamelCase
private static JsValue GetObjectMember(Engine engine, object target, string memberName)
{
    var builder = new StringBuilder();
    builder.Append(char.ToUpper(memberName[0]));

    if (memberName.Length > 1)
        builder.Append(memberName.Substring(1));

    var upperCamelCaseMemberName = builder.ToString();
    var type = target.GetType();
    var members = type.GetMember(upperCamelCaseMemberName);

    MemberInfo targetMember = null;

    for (var i = 0; i < members.Length; i++)
    {
        if (members[i].GetCustomAttribute<ScriptAccessibleAttribute>() != null)
            if (targetMember == null)
                targetMember = members[i];
            else
                // TODO: Implement checking overloaded method somehow
                throw new NotImplementedException($"Members name ambigous. Multiple members with name `{upperCamelCaseMemberName}` are marked [ScriptAccessible].");
    }

    // If no members exist marked [ScriptAccessible], do not allow accessing them
    if (targetMember == null)
        return JsValue.Undefined;

    switch (targetMember.MemberType)
    {
        case MemberTypes.Field:
            return JsValue.FromObject(engine, ((FieldInfo)targetMember).GetValue(target));
        case MemberTypes.Property:
            return JsValue.FromObject(engine, ((PropertyInfo)targetMember).GetValue(target));
        case MemberTypes.Method:
            return ???; // Return the method somehow?
        default:
            throw new NotImplementedException("Can't access this type of member from JS yet!");
    }
}

How can I return a method from C# such that it can be called in JavaScript?

I feel like the classes and methods I could use are marked internal or private so it seems I have to rewrite/copy a lot.

Anyone got an idea or pointers?

[lahma]

Things are usually internal and hidden to allow some room for change. In your case I think you're after a filter/predicate that could be called to verify whether member should be resolved?

I guess there's no such thing at the moment, but there's been an ask to hide GetType() for example which currently allows caller to use reflection and thus bypass some security. So a filter like that would probably fit the bill for both use cases.

[luttje]

Yeah that sounds like what I'm looking for. The SetMemberAccessor is really a bit overkill for what I need. Indeed I'd just like to specify per member if it should be accessible from JS

[luttje]

I adjusted my question a bit, because I now think a filter or predicate is not really what I'm looking for. This test already demonstrates how filtering could be done.

No I want to adjust which value or method is returned based on the request. In my use-case I want to maintain UpperCamelCase/PascalCase method and property names in C#, but be able to call those as lowerCamelCase from JavaScript

[luttje]

I'll keep trying. Perhaps providing a custom IReferenceResolver is the way to go.

I also just noticed that in js someObject.test by default resolves to the Test field on that object in C#. I wasn't aware of this feature.

[luttje]

I've found a way to return the method using ClrFunctionInstance. It was easier than it seemed though I'm not sure if there's a better way:

return new ClrFunctionInstance(engine, memberName, (thisObject, arguments) =>
{
    return CallMethod(engine, method, methodParameters, (ObjectWrapper)thisObject, arguments);
}, methodParameters.Length, PropertyFlag.Configurable); // Why PropertyFlag.Configurable?? No idea.

I uploaded the full code to this gist for future reference. Maybe it can help someone.

[lahma]

Great that you got that sorted and thanks for the provided sample. I think I'd like there to be an easier configuration story. So far what I've understood this is both filter (has attribute) and property name resolution (UpperCamelCase/IgnoreCase etc) configuration. So in this case implementing the member accessor is quite heavy-weight and also causes code duplication (which might also change in Jint when improvements/fixes are being made).

[luttje]

Thanks for your time. That sounds great. Curious to see which solution you will go for. Good luck

[lahma]

I think this should handle the filtering part #951 .