Task.FromResult() can not infer nullable types directly

[marwalsch]

I hope this is the right place for this type question.

Assume the following method signature:

public Task<Guid?> Foo()

Let some inheritance or interface compliance require this method to return Task but lack the need to be async.
Given that, the following can be returned without error:

Guid? emtpyId = Guid.Empty;
return Task.FromResult(emtpyId);

However, returning Task.FromResult(Guid.Empty) does not work as Guid.Empty is not nullable. The same applies to Task.FromResult(null)

Why is that? Shouldn't the compiler be aware that assigning a Guid or null to a Guid? are valid operations?

[Joe4evr]

Why is that? Shouldn't the compiler be aware that assigning a Guid or null to a Guid? are valid operations?

No it does not. For one thing, Guid and Guid? are completely different types, and type arguments on classes and structs must be exact because they can't be co-/contravariant. Even when using reference types, you can't convert a Task<string> to a Task<object>, and you can't convert a Task<Guid> to a Task<Guid?> for the same reason.
On the other hand, the statement Guid? emtpyId = Guid.Empty; simply uses the implicit conversions defined on Nullable<T> (which can't be used for inferring type arguments) and so doesn't produce the same error.

Note, by the way, that a similar restriction holds with Nullable Reference Types as well, albeit with a warning rather than an error. This is one thing I'm hoping can be addressed at some point so generic inference and NRTs can play together better, but it's probably not a high priority.

[marwalsch]

I wasn't aware of the strict distinction between RTs and NRTs. Thanks for elaborating.

[alrz]

This relates to #92

return Task.FromResult(null) could be resolved as Task.FromResult<Guid?>(null) based on the target-type, however for FromResult(Guid.Empty) it won't work because the type inference has already succeeded with T=Guid.

[marwalsch]

Task.FromResult<Guid?>(null) is what I was looking for. I initially stumbled over the issue as I simply returned null directly thinking it would infer to Task, which it doesn't (which wakes sense looking at it now).

Related Issue for any Googlers.