Doesn't seem to work with generic extension methods

[MuTsunTsai]

In my project I have a generic interface implemented by a few entity classes, and a generic extension method for that interface. When I tried using Expressionify on that method, the generator does come up with a static expression, but the type of it contains the method's generic parameter which is undefined in this context and causes compiling error.

Is there a way to fix this? Thanks!

[MuTsunTsai]

I think in order to support generic methods, the generator will also have to output a generic static method (that returns the lambda expression) instead of a static property. For a given set of generic parameter assignment, the expression visitor will invoke the method for the first time, and the returned lambda expression can then be cached in a dictionary mapping sets of generic parameter assignments to the returned expressions.

[jeffward01]

Thats a very creative way of solving identifying the source expression types. What about if you simply generate the constraint types in the extension methods?

I will test this with Expressionfy, but I think that this might be the outcome: koenbeuk/EntityFrameworkCore.Projectables#54 (comment)

[MuTsunTsai]

Replacing with the contraint type might work in some cases, but I don't think it will work in general. For example, if the returned type of the expression also involves the generic parameter, such a replacement could make the result too general for whoever is using the expression.

[mariusGundersen]

Can you show an example of what you mean? How would it be too general?

[MuTsunTsai]

@mariusGundersen Consider something as simple as

public interface IProp<T> {
    T prop { get; set; }
}

public static T GetProp<T>(this IProp<T> object) => object.prop;

In this case replaceing T with its constraint (none in this case, so it will simply be object) will result in the extension method returning just an object instead of a T, and thus runtime error will occur if other part of the expression tries to access anything specific about T on the returned result.

[mariusGundersen]

I am working on a version that supports generics. Can you try with pre-release version 6.7.0-generics.41?

[MuTsunTsai]

@mariusGundersen Still not working. It still shows an error message saying that "cannot find the type or namespace of [name of the generic parameter]" (paraphrased). It seems that the generated .g.cs file is immediately deleted afterwards so I have no idea what code was being generated.