Why is no covariant Task<TResult> possible? #123574
Description
We're currently migrating something from sync to async calls and need to do it one class after another, so I thought we'd just extend the common interface, which has an <out T> generic parameter, with an async method that returns a Task<T> instead of T directly. Unfortunately, this is not allowed.
Did we misunderstand something about covariance, or is this just a limitation of how Task<T> is implemented that might be adapted? I would guess that this is a common scenario:
// Common DTO
public interface IDto { }
// Common interface
public interface ICommon<out TDto> where TDto: IDto
{
// Current sync usage
T GetData();
// Async method with default impl for easy one-by-one migration
Task<T> GetDataAsync() => Task.FromResult(GetData());
}
// Implementation
internal class Impl : ICommon<SpecificDto>
{
// Current usage
public SpecificDto GetData() => LoadSpecificDtoFromNetworkBlocking();
// Planned usage
public async Task<SpecificDto> GetDataAsync() => await LoadSpecificDtoAsync();
}