C#•2y ago

✅ Invalid cast between interface types

Hello. I'm trying to learn functional programming in C#, so I'm ending up with some unusual code. A part of that is having an array of options marked with their length, like so

IMarked<Arr<Option<Unit>>, CountOfField>

IMarked<Arr<Option<Unit>>, CountOfField>

Since this is so long, I've defined this

public interface FieldUnits : IMarked<Arr<Option<Unit>>, CountOfField> {}

public interface FieldUnits : IMarked<Arr<Option<Unit>>, CountOfField> {}

This inevitably means casting to FieldUnits at some point, and doing so is giving me this error

System.InvalidCastException: 'Unable to cast object of type 'Marked`2[LanguageExt.Arr`1[LanguageExt.Option`1[Unit]],CountOfField]' to type 'FieldUnits'.'

System.InvalidCastException: 'Unable to cast object of type 'Marked`2[LanguageExt.Arr`1[LanguageExt.Option`1[Unit]],CountOfField]' to type 'FieldUnits'.'

I'm not sure why though, since they seem to be the same. I also swear this was working the other day. Any ideas what's wrong here? Here's a sample line that has this problem

var units = (FieldUnits)Enumerable.Repeat(Option<Unit>.None, CountOfField.Count).ToArr().mark(default(CountOfField));

var units = (FieldUnits)Enumerable.Repeat(Option<Unit>.None, CountOfField.Count).ToArr().mark(default(CountOfField));

18 Replies

Aaron•2y ago

they might seem the same, but they are not FieldUnits is not the same interface anything that implements FieldUnits is just guaranteed to implement the long one

LukeJOP•2y ago

Right. I figured that meant I could cast them like this. Looking at it I realised I'm not actually casting to an object at all. VS not complaining about it has me a little confused though.

Aaron•2y ago

the inverse isn't true though not everything that implements the long one is guaranteed to implement FieldUnits

LukeJOP•2y ago

Up v down casting, right.

Aaron•2y ago

VS doesn't complain because it's possible that the return value implements it it doesn't know

LukeJOP•2y ago

It doesn't know it's casting to an interface and not an object?

Aaron•2y ago

casting to an interface is a valid thing to do

MODiX•2y ago

Windows10CE#8553

REPL Result: Success

interface A {}
interface B {}
class C : A, B {}

A GetA() => new C();
B casted = (B)GetA();

interface A {}
interface B {}
class C : A, B {}

A GetA() => new C();
B casted = (B)GetA();

Compile: 474.880ms | Execution: 33.290ms | React with ❌ to remove this embed.

Aaron•2y ago

because C implements both interfaces

Anton•2y ago

you could define aliases with global usings, which only works for .net6+ I think

LukeJOP•2y ago

Ah, that would be perfect. It's what I was trying to do with this.

Anton•2y ago

you won't be able to export them as symbols tho, they would ve purely compile time things and local to the project they are defined in

LukeJOP•2y ago

Ah. That's a shame. Fine in this context, but probably shouldn't get into the habit of it. Is there a way I can get this to cast nicely?

Anton•2y ago

the language is not designed to handle this much generic complexity nicely, so you either have to make your design more dynamic, which is preferable by the design of the language, or have to deal with the noise and unmaintainability of repeating those types source generators is also a way

LukeJOP•2y ago

I think I crossed that bridge when I started using Language.Ext 😅

Anton•2y ago

in some cases

LukeJOP•2y ago

C# is not happy with me, nor is my use case.

Accord•2y ago

Closed!

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✅ Invalid cast between interface types