Interface with method returning Task error

Asked Feb 01 '22 at 09:45

Active Feb 01 '22 at 10:10

Viewed 99 times

public interface IRepo<out T>
{
    Task<IEnumerable<T>> DbFetch();
}

I want the method to be a task because DbFetch() will be slow.

error: Invalid variance: the type parameter 'T' must be invariantly valid on 'IRepo.DbFetch()'. 'T' is covariant.

I understand the reasoning for the error but how to get around it so I could await on that method. Note: cannot remove out T because of design requirement elsewhere.

edited Feb 01 '22 at 10:10

Camilo Terevinto

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asked Feb 01 '22 at 09:45

Tiju John

2

`IAsyncEnumerable`? – canton7 Feb 01 '22 at 09:47
1

'Task : Task' is the issue. it is not 'out T' – Tiju John Feb 01 '22 at 09:54
3

This is not possible since you're saying that `T` can only be specified in output-positions, whereas in order to provide it to a `Task` it also has to be able to be specified in an input-position. – Lasse V. Karlsen Feb 01 '22 at 09:54
I cannot use IAsyncEnumerable as the wait is not on each item but the entire DbFetch() is one REST call. – Tiju John Feb 01 '22 at 10:15
1

Well, you *can* use it -- just the first await will take a while, and the ones afterwards will be synchronous. It also gets around your issue, as `IAsyncEnumerable` is covariant – canton7 Feb 01 '22 at 10:41
If covariance doesn't work, fallback is using the base type. – shingo Feb 01 '22 at 10:52
1

https://stackoverflow.com/questions/12204755/can-should-tasktresult-be-wrapped-in-a-c-sharp-5-0-awaitable-which-is-covarian – Stephen Cleary Feb 01 '22 at 14:45

Interface with method returning Task error

0 Answers0