C#•17mo ago

❔ Attempting to create a Result<T, E> type, running into issues with subclasses

I want to create a Result<T, E> and I'm running into trouble with the following: let's say I have a class Foo and a subclass Bar : Foo. If I create an instance of Result<Bar, _> I had expected it to be assignable to Result<Foo, _> but that doesn't seem to be the case. For a fuller example see: https://dotnetfiddle.net/UEgC3P Is there a way that allows such an assignment? Else how can I ergonomically work with the compiler? I could of course create a new Result instance (like the below), but it's very verbose.

return result.IsSuccess
    ? Result<Foo, int>.AsOk(result.Value)
    : Result<Foo, int>.AsError(result.Error);

return result.IsSuccess
    ? Result<Foo, int>.AsOk(result.Value)
    : Result<Foo, int>.AsError(result.Error);

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7 Replies

Tarcisio•17mo ago

Contravariance and covariance are supported only on interfaces and delegates, not on concrete classes. Your Result<T, E> class is a concrete class, so contravariance and covariance don't apply. Read this to learn how you'd implement it

Tarcisio•17mo ago

https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/covariance-contravariance/creating-variant-generic-interfaces

Creating Variant Generic Interfaces (C#)

Learn how to create variant generic interfaces with covariant or contravariant generic type parameters.

Tarcisio•17mo ago

Also you might also want to learn about implicit operators if you're making your own result type

Tarcisio•17mo ago

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/user-defined-conversion-operators

User-defined explicit and implicit conversion operators - provide c...

Learn how to define custom implicit and explicit type conversions in C#. The operators provide the functionality for casting an object to a new type.

akaJBOP•17mo ago

In my defense it's late, but I'm having a hard time understanding how to implement a result type via contravariant/covariance. Would you have a real world example of these concepts in practice? Fwiw I plan on looking at it in the morning with fresh eyes

Omnissiah•17mo ago

your types should inherit from interfaces and then you use said interface types instead of concrete types

Accord•17mo ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ Attempting to create a Result<T, E> type, running into issues with subclasses