使用异步编程进行类型转换错误 [英] Type conversion error with async programming

问题描述

有人可以告诉我为什么它不起作用吗？

Can anyone tell me why this is not working?

我有一个看起来像这样的方法：

I have a method that looks like this:

public virtual async Task<IList<User>> GetAll()
{
    return this.db.Users.Where(model => model.CompanyId.Equals(this.companyId, StringComparison.OrdinalIgnoreCase)).ToListAsync();
}

当我尝试编译代码时，出现错误提示：

when I try to compile my code I get an error stating:

错误10无法将类型'System.Threading.Tasks.Task>'隐式转换为'System.Collections.Generic.IList'。存在显式转换（是否缺少强制转换？）C：\Users\Jaymie\Documents\GitHub\Skipstone\Skipstone.Web\Repositories\UserRepository.cs 70 20 Skipstone.Web

Error 10 Cannot implicitly convert type 'System.Threading.Tasks.Task>' to 'System.Collections.Generic.IList'. An explicit conversion exists (are you missing a cast?) C:\Users\Jaymie\Documents\GitHub\Skipstone\Skipstone.Web\Repositories\UserRepository.cs 70 20 Skipstone.Web

但在其正下方，我有此方法：

Yet directly under it I have this method:

public Task<User> FindByIdAsync(string userId)
{
    return this.db.Users.Where(model => model.Id.Equals(userId, StringComparison.OrdinalIgnoreCase)).SingleOrDefaultAsync();
}

效果很好。

我认为我看不到树木的木头，所以需要别人的眼睛来帮助我：）

I think I can't see the wood for the trees so need someone else's eyes to help me out :)

推荐答案

ToListAsync 方法返回 Task< List< T>> 对象，该对象转换为 Task< List< User>> 对象，但方法的返回类型为 Task< IList< User>> 。

The ToListAsync methods return a Task<List<T>> object, which translate to a Task<List<User>> object in your case, but your method's return type is Task<IList<User>>.

此处的问题是 Task< T>中的 T 不支持协方差。 。

The issue here is that covariance is not supported for T in Task<T>.

因此，您可以将方法的返回类型更改为 Task< List< User>> ，或者您自己编写代码来进行转换：

So, either you change the method's return type to Task<List<User>>, or you write the code to make the conversion yourself:

return this.db.Users
    .Where(model => model.Id.Equals(userId, StringComparison.OrdinalIgnoreCase))
    .ToListAsync()
    .ContinueWith<IList<User>>(t => t.Result, TaskContinuationOptions.ExecuteSynchronously);

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