Why async / await allows for implicit conversion from a List to IEnumerable?

问题

I've just been playing around with async/await and found out something interesting. Take a look at the examples below:

// 1) ok - obvious
public Task<IEnumerable<DoctorDto>> GetAll()
{
    IEnumerable<DoctorDto> doctors = new List<DoctorDto>
    {
        new DoctorDto()
    };

    return Task.FromResult(doctors);
}

// 2) ok - obvious
public async Task<IEnumerable<DoctorDto>> GetAll()
{
    IEnumerable<DoctorDto> doctors = new List<DoctorDto>
    {
        new DoctorDto()
    };

    return await Task.FromResult(doctors);
}

// 3) ok - not so obvious
public async Task<IEnumerable<DoctorDto>> GetAll()
{
    List<DoctorDto> doctors = new List<DoctorDto>
    {
        new DoctorDto()
    };

    return await Task.FromResult(doctors);
}

// 4) !! failed to build !!
public Task<IEnumerable<DoctorDto>> GetAll()
{
    List<DoctorDto> doctors = new List<DoctorDto>
    {
        new DoctorDto()
    };

    return Task.FromResult(doctors);
}

Consider cases 3 and 4. The only difference is that 3 uses async/await keywords. 3 builds fine, however 4 gives an error about implicit conversion of a List to IEnumerable:

Cannot implicitly convert type 
'System.Threading.Tasks.Task<System.Collections.Generic.List<EstomedRegistration.Business.ApiCommunication.DoctorDto>>' to 
'System.Threading.Tasks.Task<System.Collections.Generic.IEnumerable<EstomedRegistration.Business.ApiCommunication.DoctorDto>>'

What is it that async/await keywords change here?

回答1:

Task<T> is simply not a covariant type.

Although List<T> can be converted to IEnumerable<T>, Task<List<T>> connot be converted to Task<IEnumerable<T>>. And In #4, Task.FromResult(doctors) returns Task<List<DoctorDto>>.

In #3, we have:

return await Task.FromResult(doctors)

Which is the same as:

return await Task<List<DoctorDto>>.FromResult(doctors)

Which is the same as:

List<DoctorDto> result = await Task<List<DoctorDto>>.FromResult(doctors);
return result;

This works because List<DoctorDto> can be converted IEnumerable<DoctorDto>.

回答2:

Just think about your types. Task<T> is not variant, so it's not convertible to Task<U>, even if T : U.

However, if t is Task<T>, then the type of await t is T, and T can be converted to U if T : U.

回答3:

Clearly you understand why List<T> can at least be returned as IEnumerable<T>: simply because it implements that interface.

Also clearly, the 3rd example is doing something "extra" that the forth one isn't. As others have said, the 4th fails because of the lack of co-variance (or contra-, I can never remember which way they go!), because you are directly trying to offer an instance of Task<List<DoctorDto>> as an instance of Task<IEnumerable<DoctorDto>>.

The reason the 3rd passes is because await adds a big bunch of "backing code" to get it to work as intended. This code resolves the Task<T> into T, such that return await Task<something> will return the type closed in the generic Task<T>, in this case something.

That the method signature then returns Task<T> and it works is again solved by the compiler, which requires Task<T>, Task, or void for async methods and simply massages your T back into a Task<T> as part of all the background generated asyn/await continuation gubbins.

It is this added step of getting a T from await and needing to translate it back into a Task<T> that gives it the space it needs to work. You are not trying to take an existing instance of a Task<U> to satisfy a Task<T>, you are instead creating a brand new Task<T>, giving it a U : T, and on construction the implicit cast occurs as you would expect (in exactly the same way as you expect IEnumerable<T> myVar = new List<T>(); to work).

Blame / thank the compiler, I often do ;-)

来源：https://stackoverflow.com/questions/25182011/why-async-await-allows-for-implicit-conversion-from-a-list-to-ienumerable

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