What is difference between Task.FromResult and Task.CompletedTask?

public Task Test1()
{
    return Task.CompletedTask;
}

public Task Test2()
{
    return Task.FromResult(0);
}


    

CodePudding user response：

Your Task.FromResult(0) really returns a Task<int>, not (just) a plain Task.

So this would compile fine

 public Task<int> Test2()
 {
     return Task.FromResult(0);
 }

You couldn't use a Task.CompletedTask here

CodePudding user response：

The CompletedTask has no result field, and no generic type parameter for that matter. You can use it to just skip taking system resources to do a job.

The FromResult variant returns a generic Task, with the argument's type being the generic type parameter. You can use this variant to avoid wasting resources to just return a value, in contexts where caller expects asynchronous behavior and hence requests a Task back.

Please note that generic Task<T> is deriving from non-generic Task, and hence both methods are equally valid in contexts where the caller makes a fire-and-forget call, or just doesn't care to read the result.