我刚刚被玩弄异步/等待，并发现了一些有趣的事情。看看下面的例子：

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);
}

考虑例3和4。唯一的区别在于，使用3异步/等待关键字。 3建立正常，但是4提供了有关一个列表的隐式转换为IEnumerable的错误：

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>>'  

它是什么，异步/等待关键字在此更改？

What is it that async/await keywords change here?

推荐答案

任务＆LT; T＆GT; 根本就不是一个协变型

Task<T> is simply not a covariant type.

尽管名单，其中，T＆GT; 可以被转换成的IEnumerable＆LT; T＆GT; ，任务＆LT ;列表＆LT; T＆GT;＆GT; 粗野，被转换为任务＆LT; IEnumerable的＆LT; T＆GT;＆GT; 。而在＃4， Task.FromResult（医生）返回任务＆LT;列表＆LT; D​​octorDto＆GT;＆GT;

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>>.

在＃3，我们有：

return await Task.FromResult(doctors)

这是一样的：

Which is the same as:

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

这是一样的：

Which is the same as:

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

这工作，因为名单，其中，DoctorDto＆GT; 可转换的IEnumerable＆LT; D​​octorDto＆GT;