Object must implement IConvertible

[martinbu]

From a technical point of view, I do not understand why exchanging an object directly works but not via a Func. Is this an error in Jint or are there are some technical reasons for that?

var engine = new Jint.Engine();
engine.SetValue("Example", new Example());

engine.Execute("""
    const obj = {
        value: 42
    };

    const res = Example.ExchangeObject(obj); // works
    const res2 = Example.ExchangeObjectViaFunc(() => 3); // works, only to ensure that not Func is the issue
    const res3 = Example.ExchangeObjectViaFunc(() => obj); // does not work, throws 'Object must implement IConvertible'

    """);


class Example()
{
    public T ExchangeObject<T>(T obj)
    {
        return obj;
    }

    public T ExchangeObjectViaFunc<T>(Func<T> objViaFunc)
    {
        return objViaFunc();
    }
}

[lahma]

This is quite exotic use case due to the generic and probably hasn't been tested/implemented. As JS doesn't have generics the type system isn't prepared for such..

[martinbu]

Hmm, do you mean the generic T in the Example class, or that Func is based on generics.
If you mean the T used in the Example class, it does not make any difference if I change to code to object or JsValue:

var engine = new Jint.Engine();
engine.SetValue("Example", new Example());

engine.Execute("""
    const obj = {
        value: 42
    };

    const res = Example.ExchangeObject(obj); // works

    const res2 = Example.ExchangeObjectViaFunc(() => 3); // works, only to ensure that not Func is the issue
    const res3 = Example.ExchangeObjectViaFunc(() => obj); // does not work, throws 'Object must implement IConvertible'

    """);


class Example()
{
    public object ExchangeObject(object obj)
    {
        return obj;
    }

    public object ExchangeObjectViaFunc(Func<object> objViaFunc)
    {
        return objViaFunc();
    }
}

What I do not understand is why it makes a difference if an object is passed directly or is returned by a Func. I both cases the object is passed from js to c#.

[lahma]

OK so generics aren't in play here, Jint probably just gets confused by something.

[martinbu]

I have started to analyze the issue. Everything works as expected until the point where ToObject is called and the result is returned.

1. Why is ToObject called at all, couldn' find a reason fo it when debugging? It doesn't really matter here, but I am interessted in... Maybe you could give me a hint.
2. When ToObject returns, System.Convert gets called, I don't know from whom, but there the exception occures because value has the type ExpandoObject and conversionType is Object

After that observation, I have tried to change the method signature to

public object ExchangeObjectViaFunc(Func<ExpandoObject> objViaFunc)
{
    ExpandoObject res = objViaFunc();
    return res;
}

and this is working fine. What I am really asking myself now is why System.Convert get called after ToObject or in other words by/after method.Method.Invoke(thisObj, parameters) in MethodInfoFunction:Call(...)

[lahma]

I think that Jint doesn't correctly read target type from Func''s generic definition and goes to wrong path and tries to convert - like it would not a catch all object that should basically accept anything.

[martinbu]

I have found the code part that is responsible for the issue. It is the DefaultTypeConverter, there is the "Setup" for the calls to ToObject() and System.Convert. The problem could be solved if a check is added for IConvertible, else just try to "cast" with Expression.Convert.

With this change these Func definitions work as expected

public object ExchangeObjectViaFunc(Func<object> objViaFunc)
{
    object res = objViaFunc();
    return res;
}

public T ExchangeObjectViaFunc<T>(Func<T> objViaFunc)
{
    T res = objViaFunc();
    return res;
}

public int ExchangeBasicValueViaFunc<int>(Func<int> objViaFunc)
{
    int res = objViaFunc();
    return res;
}

What do you think about it? Would you accept a PR with this kind of code change or in this direction assuming that all other tests still succeeds?

[lahma]

Generally if you can make a new failing test case pass and all the old ones still pass as before, help in form of a PR is always welcome.

[martinbu]

Ok, then I will create a PR with test cases. Then we will see if all other test-cases still pass.

[martinbu]

Looks good to me. Let me know if you have any concerns or further suggestions. #2053